Cc latest files concat as of Oct. 2003 [cct]



CHAPTER 1: IS THE EARTH WORTH SAVING?

DM draft (2/16/97), CRC edit (23 February 1997)

Killer asteroids and comets are out there. And someday, one will be on a collision course with Earth. Of all the species that ever crawled, walked, flew, or swam on Earth, an estimated two-thirds became extinct because of impact from space. Mankind may yet meet that fate, too. But we're the only species that can even contemplate it and, just maybe, do something to prevent it. -- Melinda Beck and David Glick, in "Doomsday Science", Newsweek cover story, November 23, 1992.

This book could not have been written a decade ago. Ancient peoples feared the heavens, but modern society has been complacent about any danger from cosmic impacts. While no person has been struck and killed by a meteorite, so far as we can be sure, new research demonstrates that a real danger from impacts exists. Indeed, cosmic collisions are the only known natural hazard that could end human civilization forever. A key step in recognizing this threat came in late July 1994, when astronomers and TV viewers around the world witnessed what well could have been Armageddon. Fortunately for us, the target planet was Jupiter, not Earth.

Scientists had long thought of the solar system as obeying the laws of nature with humdrum, machine-like precision. There was little room in our cosmology for sporadic violent collisions. Even after telescopes and spacecraft had revealed the cratered surfaces of the Moon and planets, testifying to bombardment aeons ago, scientists assumed that large impacts were of little contemporary concern. Yet we found ourselves watching just such a spectacle as the fragments of Comet Shoemaker-Levy 9 (S-L 9) plunged into Jupiter and exploded one after another. After a week of celestial fireworks, the gaping, black devastation zones on Jupiter were visible to ordinary folk with small telescopes, erasing forever the illusion that planets are safe from catastrophic collisions.

The drama of S-L 9 came just as geologists were already re-evaluating the forces that shape our world and had begun to realize the dramatic effects of asteroids and comets. For three decades, planetary spacecraft had explored our neighbors in space, and found almost all of their surfaces cratered by impacts. The Earth could hardly remain untouched by such cosmic bombardment. Since the late 1950's, when Arizona's Meteor Crater was proved to be an impact scar, geologists had redoubled their search for other evidence of the continuing bombardment of our planet. More than 150 eroded craters have now been found. The largest of all -- 125[?]-mile-wide Chicxulub in Mexico -- was first mapped by Mexican oil geologists in the 1950s, but only recognized as the dinosaur-killing crater in 1991.

A decade-long search for the "smoking gun" of the great mass extinctions 65 million years ago began in 1980, after Luis and Walter Alvarez and their colleagues at the University of California at Berkeley proposed a bold hypothesis in the pages of Science magazine: based on a thin layer of rock enriched in rare metals, which they found in a road-cut in central Italy, they suggested that the ebb and flow of evolution of species had been brutally changed forever by collision of a 6-mile-wide asteroid with Earth. Not only Chicxulub, but the rise of mammals and of the human species itself, was the result. Now we, too, must face the risk of mass mortality from an as-yet-undiscovered celestial projectile.

Before publication of the Alvarez paper, asteroids, comets, and impact craters were of interest only to a few geologists and astronomers. What the Alvarez team understood was that impacts of even modest-sized asteroids -- which must have happened many times since plants and animals evolved 500 million years ago -- can transform the environment so severely that the course of biological evolution is profoundly altered. Theorists now believe that asteroids and comets are common leftovers from the general processes that form stars and planets. Thus the history and future of life on Earth -- and on planets elsewhere in the universe -- may be intimately coupled to these inevitable, sporadic impacts that persist long after the formation of the planetary habitats required for life to evolve in the first place. Indeed, the prevalence of intelligent life in the universe may depend on whether advanced species and civilizations can evolve rapidly enough, in the intervals between catastrophic impacts, to the point that they can defend themselves. A major question is whether human beings yet have that capability ourselves. Can and will we defend ourselves, or will we allow ourselves to be as defenseless before the cosmic threat as were the dinosaurs?

This book deals with an emerging new paradigm: that life has evolved on Earth in an environment punctuated by impact catastrophes. We have learned that our planet lives in a "bad neighborhood," with occasional outbursts of incredible violence. We will evaluate the contemporary hazard posed by impacts, and discuss ongoing political and scientific debates about ways to deal with it. We have the responsibility to decide whether or not to defend our planet against catastrophic collisions. We must also decide whether the defense should lie in civilian or military hands, and whether such programs should be undertaken unilaterally or internationally. Ours is the first generation in the history of the human species to have the option to protect ourselves from the ultimate environmental disaster of a large impact. This book describes the options available to deal with this challenge.

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When Comet Shoemaker-Levy 9 was discovered in spring 1993, it already consisted of more than 20 fragments, having been ripped apart eight [?] months earlier during a very close encounter with Jupiter. Each bit of celestial flotsam was on a collision course for Jupiter. Over the course of a week in July 1994, the fragments hit, but at sites just over Jupiter's horizon as seen from the Earth. Astronomers could not image the actual entries and explosions, but they were able to monitor the huge plumes of debris ejected thousands of kilometers into space, above the impact sites. Within minutes following each explosion, the impact sites were carried into direct view by Jupiter's rapid rotation so that astronomers could study the after-effects.

Comets have surely run into Jupiter before, but nobody was looking. This time, with more than a year's warning, astronomers were ready with the greatest array of modern telescopes ever used to observe the same celestial event. Scientists worried that the impacts and their aftermaths might be difficult to observe. After all, Jupiter is far away, the comet fragments were fairly small, and the explosions would happen behind the jovian horizon. They feared that the whole comet crash might turn into a disappointing "cosmic fizzle," despite all the advanced hype by the news media. The explosive potential was so enormous -- perhaps millions of millions of tons of TNT -- that it was difficult for the scientists to contain their enthusiasm, so of course the media had trumpeted the impending celestial light show.

When comet fragment "A" hit on the evening of July 16th, 1994, the world was watching. Telescopes on every continent (including at the South Pole, as well as the Hubble Space Telescope orbiting above) were equipped with sensitive, advanced instruments with the best chances for detecting the impacts, including infrared cameras that would sense heat from the distant explosions. Dramatic telescopic pictures of the ejected plumes and devastation zones were immediately pumped into the arteries of the information superhighway -- including the fledgling World Wide Web -- and were televised around the world well before the second comet fragment hit a few hours later. In a way unprecedented in the history of science, computer hobbyists, cablevision viewers, and students could peer over astronomers' shoulders from their living rooms and classrooms. And when it was discovered that the impacts were leaving huge, long-lasting bruises on Jupiter, many people saw with their own eyes the damage done to Jupiter's stratosphere.

The dramatic consequences of the jovian impacts have direct ramifications for us. Given that each jovian "bruise" was the size of our whole planet, one naturally wonders about the consequences of a comparable impact on Earth. Each of the larger S-L 9 fragments apparently penetrated not far beneath the giant planet's cloud deck, exploding with a total energy of several hundred thousand megatons -- much greater than the Earth's entire inventory of nuclear weapons. Most of the energy was directed back upwards, along the original path of the incoming projectile, expanding into an enormous plume of hot gas that rose nearly 4000 km above the Jupiter's clouds. Pulled back down by the giant planet's enormous gravity, the plumes of ejected material soon collapsed. Re-entering Jupiter's atmosphere at high speeds, the incorporated debris flashed into incandescence, scorching the jovian skies with blazing visible and infrared (heat) radiation. By this time, 20 minutes after impact, the target regions had rotated into view from distant Earth, and the jovian firestorms were widely documented by telescopes with infrared cameras. An analogous meteor storm would occur on Earth following a large impact, but in our case the debris would re-enter the atmosphere around most of the globe. The furnace-like heat from a red-hot sky would ignite forests and grasslands around the world. Just such a conflagration probably accounted for the permanent extinction of the dinosaurs and many other species of terrestrial flora and fauna 65 million years ago.

The plume material that cascaded back down into Jupiter's stratosphere remained suspended there, darkening regions hundreds of millions of square kilometers in area (***ck). Through our telescopes from afar, these devastation zones looked like smudges or bruises on the face of the planet. A comparable impact on Earth would produce a debris layer in Earth's stratosphere of similar extent, but in the case of our smaller planet, it would extend around the entire globe, blocking incoming sunlight everywhere. However, much more of the impact energy would be channeled into dust production as the impactor smashes into Earth's solid ground. The resulting dust cloud could plunge the entire world into profound darkness, bringing freezing temperatures on land, even in summer latitudes, and a breakdown in the oceanic food chain. Presumably it was the dust cloud that led to the marine deaths that define the mass extinction that terminated the Cretaceous period, 65 million years ago. On Jupiter, the dark clouds persisted for many months, and darkness would last on the Earth at least that long.

Something else seen on Jupiter by astronomers bears on the terrestrial effects of impacts. Enormous atmospheric waves spread away from each of the larger impact sites. Analogous but much more destructive waves occur on Earth whenever an impactor hits in the ocean (as most will). If an object with the energy of one of the S-L 9 fragments struck in the North Atlantic, tsunami waves as high as a 30 story building would strike both North America and Europe, devastating the coasts and obliterating many of the world's great cities.

None of the S-L 9 impacts damaged Jupiter as a whole; its orbit and rotation were unaffected. Yet important changes took place in its atmosphere, producing new features larger than the planet Earth. Neither would an impact of a mile-sized object on Earth change its orbit or the solid globe itself, but it would temporarily damage the ecosphere -- the atmosphere and oceans and the entire biosphere -- in ways that could be catastrophic for life. It is the sensitivity of the atmosphere to impacts, and the sensitivity of life to changes in the atmosphere, that place impact issues squarely in the public policy domain.

Both comets and asteroids can strike the Earth. It is easy to estimate the numbers of such cosmic impacts. Since the Earth is hit, of course, by the same population of asteroids and comets as the Moon, which orbits around the Earth, we can safely assume that the Earth has been struck over time as often as the cratered surface of the Moon -- actually *more often* because the Earth is a bigger target and has stronger gravity, which tends to focus projectiles toward our planet. According to age-dating of Moon rocks returned by Apollo astronauts, the lunar surface has been collecting craters for more than 3 billion years. We can also estimate the contemporary impact rate on the Earth from a telescopic census of existing Earth-crossing asteroids and comets, yielding a result that is similar to the average rate recorded on the Moon over the last several billion years. It makes little difference whether an impact is from a comet or asteroid; what counts is the power of the blow, not the composition of the hammer.

We can then evaluate the danger posed by impacts of different sizes. Of particular interest are two threshold sizes: the threshold for penetration though the atmosphere (smaller projectiles burn up and cause little damage), and the threshold above which impacts not only produce local and regional damage, but damage the global climate as well.

The atmosphere protects us from smaller projectiles. On average, we know that an impact event with the energy of the Hiroshima nuclear bomb occurs every few months, while a megaton event (the size of a typical hydrogen bomb) is expected at least once per century, somewhere on Earth. Obviously, however, such common meteoric explosions have not been destroying cities or killing people during the last century. Even at megaton energies, most projectiles break up and explode very high in the atmosphere, and they have no affect on the ground below -- except for a brilliant flash in the skies. In order to reach the ground, a typical asteroid or comet would have to be larger than a football field. Even to make it down to the lower atmosphere, it would have to be the size of a large house. Smaller objects pose no danger, although their high-altitude explosions are routinely detected by military surveillance satellites.

If the projectile is large enough and strong enough to penetrate below about 15 km altitude before it explodes, the resulting airburst can be highly destructive. History provides us a concrete example of such an event. This is exactly what happened over the Tunguska region of Siberia in 1908, when a stony asteroid about 60 m in diameter penetrated to within 8 km of the surface before exploding. The yield of the Tunguska blast has been estimated at 15 megatons, and it destroyed an area the size of a city like Atlanta or Phoenix. If a heavily populated area were struck, the results would be catastrophic. However, a Tunguska-size impact takes place over the land area of the Earth only about once per millennium, and there is no historical example of the destruction of a city by such an impact. Since not one city has ever been devastated by an impact, our risk from cosmic impacts is obviously much lower than from common natural disasters such as earthquakes and severe storms, each of which destroys or badly damages several cites somewhere on Earth within the span of a human lifetime.

Sufficiently energetic impacts have devastating global consequences. The Cretaceous impact of 65 million years ago, which ravaged the global ecosystem, caused a terrible mass extinction and provided the opportunity for mammals to thrive and evolve. This impact of a 15-km object released more than 100 million megatons of energy and excavated an enormous crater (Chicxulub in Mexico). Among the environmental consequences were devastating wildfires and dramatic short-term cooling of the climate produced by fine dust injected into the stratosphere. We know from the fossil record that major mass extinctions of species occur at intervals of many millions of years. The chances of such an event taking place within, say, the next century are extremely low.

However, even projectiles substantially smaller than 15 km across can affect the global climate by injecting dust into the stratosphere, producing climate changes sufficient to reduce crop yields and precipitate mass starvation and disruption of human economies (but not a mass extinction). Based on recent research results, we estimate that an impact by an asteroid or comet with an energy of a million megatons (diameter of about a mile) would produce a global calamity that might kill more than a billion people. Using this value and the known impact rate, we calculate that there is about 1 chance in 4000 that such a globally catastrophic impact will take place in the next century and that, for an average individual, the chances of dying as a result of an impact are about 1 in 20,000. Phrased in terms of annual risk of death for one individual, this amounts to a little less than one chance in a million per year, or about the same as the chances of a traveler dying in one round-trip commercial airline flight.

As we will explain in more detail in later chapters, we have found that the total impact risk is dominated by objects a mile or two in diameter, near the threshold for global agricultural collapse; smaller objects pose less risk, even though there are many more of them. The total impact hazard approaches that associated with other natural disasters, such as earthquakes or severe storms, suggesting that it could be serious enough to inspire public (and governmental) concern. Further, there is the qualitative difference between a globally catastrophic impact and all other natural dangers. Only impacts have the potential to kill billions and destabilize civilization. This unique distinction separates the impact hazard from other natural dangers and justifies special measures to deal with it.

How a person reacts to the risk estimates given above varies greatly from person to person, depending on their own psychology, economic status, and life experiences. It is especially difficult to come to grips with such mind-boggling numbers, since the impact hazard represents such an extreme combination of low probability together with high consequence. It is beyond our personal or historical experience. Since no one is known to have been killed by a large impact in all of recorded history, it is easy to dismiss the risk as negligible and to regard those who express concern as alarmist. Further, the calculated annual risk of about one in a million is similar to risks of ***____ and ***____, which many people consider risks to be a wholly negligible hazard. On the other hand, modern industrial societies spend large sums to protect people from even less likely hazards, ranging from hurricanes to terrorist attacks to trace quantities of carcinogenic toxins in food and water.

For other natural hazards, risk reduction or mitigation strategies can deal mainly with the consequences of the disaster. Thus, for example, we cannot stop an earthquake or even reduce its force, but we can mandate higher standards in building construction and develop plans to treat casualties and restore public services after such a disaster. If impacts could be predicted weeks or months in advance, similar approaches could be taken, including evacuation of the populace from the target area. In addition, however, the possibility exists of avoiding the impact entirely by deflecting or destroying the cosmic projectile before it hits. Impacts are the only natural catastrophes that can be so effectively avoided.

Although scientists often discuss the probabilities of a large impact, in reality this is not a Las Vegas game of chance. Either there is an asteroid or comet out there aimed at the Earth or there is not. Any approach to this problem must therefore first consider the search for potentially hazardous asteroids and comets. We can't fight an unseen and unknown enemy. Plans to augment current survey efforts have been presented, but funding is slow. As a result, only a handful of astronomers are actively engaged in the search for potentially catastrophic asteroids or comets. In fact, the total workforce devoted to this task on the entire planet is smaller than the staff of one McDonald's restaurant. Given that the survival of our civilization (including McDonald's) is at stake, our priorities should perhaps be reconsidered.

A survey for threatening objects is justified because we can do something to avert a collision if one is predicted. If the warning time is several years or longer, as is most likely, it appears to be within out current technology to mount a defense, either by deflecting the object or destroying it.

The most straightforward way to deflect an asteroid is to give it a push to change its orbital period. If the push is applied several years before the threatened collision, only a very small velocity change is required. Engineers who have examined this problem believe that the optimum way to push such an asteroid, without risking accidental disruption, is a stand-off neutron-bomb explosion. Bombs of the appropriate yield exist within current nuclear arsenals, and in many examples that have been studied where warning time is ample, only a megaton or so of energy is required.

The alternative of blowing up a projectile requires *much* more energy. In order to avoid making the situation worse by converting the incoming object from a cannon ball into a cluster bomb, we would have to do more than simply disrupt it. We must hit it hard enough to literally pulverize it (ensuring that no fragment is large enough to survive atmospheric entry) or to disperse all of the fragments so that none strikes the Earth. Current research in both the United States and Russia is examining ways such defense systems might operate.

Proposals to develop defensive systems raise troubling issues, both philosophical and political. At the most basic level, we must decide if we wish to interfere with a natural process that shapes the evolution of life and that, in the form of the impact 65 million years ago, was essential to our own existence. Most people would agree with us that efforts at self protection and self defense are justifiable. But what kind of defense system is appropriate to such a low-probability hazard?

There is consensus among experts, but not yet among politicians, that a survey for potential impactors is the first step. The philosophy is to look first and move on toward constructing an expensive defense system only if and when a dangerous object shows up. A survey such as the Spaceguard Program (detailed in a later chapter) is a form of cost-effective insurance that protects our civilization against most cosmic threats. But not all of them. What should we do about the risk of a new comet, descending into the inner solar system from great distances and aimed to strike the Earth with a warning of only a year or two? How much should we spend for the extra insurance rider to cover this additional contingency?

There are no clear answers to these questions, which is the focus of much of the current policy debate. There are those, among them Edward Teller (the "father of the H Bomb"), who advocate the immediate development and testing of nuclear deflection technology, leading toward the deployment of a planetary defense system early in the next century. Several Russian aerospace firms have proposed a specific "Space Shield" system for initial deployment before the turn of the century. Many other people oppose building a defense system, questioning its cost-effectiveness (how can we afford to spend billions of dollars on a defensive system that is unlikely to be used?) or its potentially harmful side-effects. After all, such a defensive system might pose risks from accident or misuse that are greater than the low-probability impact danger it is designed to mitigate. This ongoing debate is likely to intensify as more individuals and constituencies are drawn into it. For example, environmental activists have not yet joined this discussion. Would they give more weight to protecting our precious blue planet from the ultimate environmental catastrophe of a large impact, or might they instead be more concerned with protecting us from the incremental but more immediate risks of nuclear accidents associated with deployment of a defense system?

Even as the public debate unfolds, actions are being taken to initiate planetary defenses. On February 17, 1996, NASA launched the first spacecraft mission to a near-Earth asteroid. Although this mission is motivated by basic science rather than defense, it will provide invaluable information on these potential Earth-killers. At the same time, the Air Force and the Livermore National Laboratory are starting construction on an asteroid mission called Clementine 2 which is to intercept three near-Earth asteroids and fire a high-velocity interceptor spacecraft into each, using technology that was developed for the "Star Wars" missile defense systems. Although this too is billed as primarily a science mission, its applications to future defenses against asteroids are obvious.

This book tells the story of impacts in our solar system and how we came to appreciate their significance as a shaping force in the evolution of life -- and as a potential threat, however remote, to the continuation of civilization as we know it. In the chapters that follow, we discuss what has been learned about the comets and asteroids, and about the physics of mighty impacts far larger than any ever studied before. We outline the policy options that confront us as we try to comprehend this most extreme example of a truly catastrophic natural hazard. The decisions made in the next few years will determine whether planetary defense is an international or national effort. It will be decided whether it will be carried out by civilian agencies such as NASA or by military organizations such as the U.S. Air Force Space Command. Or, whether by governmental decision or by simple inaction, we may decide to blindly let nature runs its capricious course. Nothing less than the long-term survival of human civilization is at stake.

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CHAPTER 2: THE COSMIC SHOOTING GALLERY

Life on Earth has frequently been interrupted by frightful events. -- Nineteenth-century paleontologist Baron George Cuvier.

Some ideas are important, even revolutionary. One such idea is that the Earth is a cosmic target, vulnerable to global catastrophe. In the previous chapter we outlined the case for impacts in Earth history, but let us now step back to the situation about 1980. To many scientists at that time, the concept of Earth's vulnerability to impacts appeared to contradict common sense and the lessons we were taught about global history. The Earth is 4.5 billion years old, and astronaut photos emphasize its stability -- the blue planet, an ancient world of temperate climate with clouds drifting by. For almost 200 years, scientists have thought of geological and biological change as gradual, resulting from the modest action over time of the same forces that we see around us every day. They believed that the balance of nature rules, at least until disturbed by human presence. This viewpoint of science and philosophy -- uniformitarianism, they called it -- leaves little room for catastrophe, cosmic or otherwise.

Uniformitarian philosophy makes much sense. Most natural processes are cyclical. Rivers empty into oceans, yet the oceans don't overflow. In the words of the Old Testament, there is "nothing new under the Sun". Clearly nature must be stable for the familiar Earth to be so ancient. That life has been sustained for billions of years seems to mean that ecological changes have been minimal. We judge the past by our own experiences: the present is the key to the past. We can hardly imagine that processes or events far beyond our experience might have been important for the history of our planet. A catastrophic impact from a cosmic projectile seems as remote and unfathomable as a thunderbolt hurled by Zeus or an invasion by space aliens.

Yet evidence of cosmic impacts is there for those who look. In this chapter we describe the Earth's cosmic neighborhood, and why we know that we have indeed been bombarded by some of our smaller neighbors in space.

We begin our quest for a cosmic perspective on the Moon. Because the Earth's surface is constantly reworked by erosion and geological change, it preserves a poor record of its past. Even "ancient" mountain ranges are scarcely older than 5% of our planet's age. The Moon, in contrast, is a geologically dead world, whose mountains and craters were formed aeons ago. No continents press into one another on the Moon, and volcanoes no longer erupt. There is no air, no water, and no wind erosion. The surface of the Moon is a window onto its past, preserving four billion years of history.

When Galileo and his successors looked at the Moon through their telescopes, they saw a surface covered by the shallow circular depressions they called craters, after the Greek word for cup. Craters are the quintessential lunar landform. With a modest telescope we can count thousands of them, ranging from numerous cavities 1 km across up to Clavius, fully 250 km in diameter. Still larger impact features are called basins rather than craters. Some of the huge ringed basins are filled with dark lava plains, forming the familiar spots we call the "Man in the Moon." From lunar orbit, spacecraft have photographed millions of craters, down to mere hollows the size of a swimming pool. Returned moon rocks bear pits down to microscopic dimensions. How curious it is that these ubiquitous features, and the lessons they hold for our own planet, were misunderstood and ignored for centuries.

We now know to use the Moon to infer the history of the Earth -- the perspective taken in this chapter. But the opposite approach was applied during the 19th century and much of the 20th, when our world served as a model for the Moon. On Earth, craters are rare (in the United States, they are often in national parks), and most of them are volcanic. Some are the familiar cinder cones like Sunset Crater in Arizona, or on the summits of high mountains like Mauna Loa in Hawaii. Others are pit craters, collapse craters, and even volcanic explosion craters. By analogy, lunar craters were assumed to be volcanic, although few terrestrial volcanoes have the shallow, bowl-shaped appearance of most features on the Moon. By circular reasoning, scientists assumed that the Moon must be like the Earth, so we were unlikely to learn anything very revolutionary about the Earth from looking at the Moon.

A few geologists suggested a century ago that the ubiquitous lunar craters might have an impact origin, but the concept made real sense only after scientists calculated the energy of impacts by cosmic debris. Interplanetary velocities of asteroids and comets are so great -- tens of kilometers per second -- that each ton of projectile typically carries 100 times more kinetic energy of motion than the chemical energy of a ton of high explosives. So an impacting ton of asteroid explodes with the energy of 100 tons of TNT, and impact craters are, in effect, explosion craters. More geologists were converted to the impact hypothesis after seeing aerial photos of the Moon-like, crater-scarred battlefields of World War I. In 1924, British astronomer Charles Gifford wrote "The fact which has not been taken into account hitherto in considering the meteoric hypotheses [for formation of the lunar craters] is that a meteor, on striking the Moon, is converted, in a very small fraction of the second, into an explosive compared with which dynamite and T.N.T. are mild and harmless." Final proof that the lunar craters were formed by impacts, however, awaited the Apollo explorations that revealed the lunar surface to be an extremely ancient, battered landscape.

Knowing that each lunar crater was made by an explosive impact, our perspective is fundamentally changed. The Earth and the Moon have moved together through the solar system since their birth. If the Moon has been battered by cosmic debris, the Earth must have been as well. Only the smaller components of this cosmic rain burn out as they pass through Earth's atmosphere: any projectile large enough to make one of the telescopically visible lunar craters would have had a similar effect on the Earth. Were it not for the continual erosion and reworking of the Earth's surface, which fills in and erases craters, our planet would be as densely cratered as the Moon.

The United States space program of the 1960s and 70s played a critical role in stimulating scientific interest in cratering. As exploratory spacecraft like the Mariners and Voyagers ventured to the other planets, we found that craters are the common coin of planetary geology. Mercury, Venus, Mars, the moons of Jupiter and Saturn -- all have cratered surfaces. Impacts are a part of planetary history, competing with internal forces such as volcanism in sculpting geologic landforms. Only on the Earth and Venus (and a couple of volcanically active moons) do the internal forces dominate. Elsewhere, as on the Moon, impacts have generally shaped most of the surface features we see.

Today we cannot look at the Moon without recognizing the lesson it teaches us about our own planet's history of bombardment from space. But this is a modern viewpoint, a product of the planetary exploration programs of the past three decades.

To read the records of lunar and terrestrial history, we need a clock. The crater-scarred Moon shows us that the Earth must have been struck repeatedly by cosmic debris, but it does not immediately tell us when these impacts happened. Perhaps, we might suggest, the era of bombardment was confined to the distant past, when the planets were young and the Earth's crust was still molten and could not retain craters. If so, the lunar record has little relevance to the recent history of the Earth. Can we test this hypothesis?

Before the Apollo expeditions to the Moon, we could not determine the age of the lunar surface just from looking through a telescope. Maybe it is extremely old, and craters were formed in the early era of planetary accretion. Maybe we are safe today from such impacts. However, as soon as the first moon-rocks were returned to Earth by the Apollo astronauts, the age of the lunar surface could be measured. Most of these rocks are igneous, formed in volcanic eruptions that formed the dark "Man in the Moon" lava plains. The ages when the molten lava solidified and became rocks were measured from the decay of radioisotopes to be 3.3 to 3.9 billion years old -- very old, but up to a billion years younger than the age of the Moon itself (and the Earth), which is 4.5 billion years. The lava plains of the Man in the Moon had erased all previous topography, including craters, but they are no longer smooth today. Even a small telescope reveals that craters have formed in the plains due to continuing bombardment that has persisted beyond the accretionary era. Indeed, it is still continuing.

How do we read the cosmic scorecard represented by these fresh lava plains? We can see just five craters on the plains that exceed 50 km diameter -- that's one or two such craters formed every billion years (on average) since the plains were formed. Since the Earth is struck by the same cosmic debris as the Moon, we can calculate that it was probably hit about 150 times during the same period of 3½ billion years, since our planet has approximately 30 times more land area than the lunar maria. That's one new 50-km crater formed on land every 20 million years [check]. Indeed, as we will discuss later, this is just about the rate of crater formation rate inferred from geological studies of ancient terrestrial impact craters.

These cratering rates assume that the cosmic rain has been steady during the past 3½ billion years. We don't know how correct this assumption is. Probably there have been a few heavy showers interspersed with long dry spans, during which fewer impacts took place. Yet measured ages for both lunar and terrestrial craters tell us that at least the larger craters have formed throughout geological history. Some astronomers think that the smaller, more frequent impacts come in storms, which would be particularly scary if we were unlucky enough to be in a rare storm right now. These interesting ideas are speculative, however, and there is no compelling evidence for such variations over time.

Each impact crater on the Earth or Moon is the result of a huge explosion. Something smashed into the surface, releasing a burst of energy and excavating a large cavity in the ground. From the size of the crater we can estimate the energy of the projectile, but little else. Just what are these cosmic bullets?

One reason 19th century astronomers and geologists failed to realize that lunar craters were made by impacts was the lack of a known source for the projectiles. At that time they did not know that there were asteroids loose in the solar system that could run into the Earth or Moon. Asteroids are small rocky or metallic bodies ranging up to about 1000 kilometers in size. They were all thought to be safely stored in the asteroid belt, out beyond the orbit of Mars. Not until 1932 was the first small asteroid found on a path that crossed the Earth's orbit. This little rock, only a few kilometers across, was named Apollo (for the Greek Sun god) because it approached the Sun so closely. Another of the early asteroids that came near the Earth and Sun was called Icarus for the same reason. Since then, about 200 more Earth-crossing asteroids have been discovered.

The Earth-crossing asteroids are not so numerous as their larger main-belt siblings, but their irregular, elongated orbital paths bring them close to the Earth and other planets. Occasional close encounters with the gravity fields of planets send them veering off into unpredictable paths that continue to pose a threat. It has been estimated that one in four Earth-crossing asteroids will eventually score a direct hit on Earth.

The Earth-crossing asteroids are all too small to be photographed from Earth in any detail, even with the Hubble Space Telescope. However, the new techniques of planetary radar have allowed us to probe the shape and physical nature of several of these mysterious objects. The use of radar to obtain images has been pioneered by Steven Ostro of the Jet Propulsion Laboratory (JPL) in Pasadena, California. Very powerful radar pulses are transmitted from big radio antennas on Earth. A few minutes later, the pulses intercepted by the little asteroid are reflected back toward Earth and, some minutes after that, faint echoes are received -- either by the radio telescope that transmitted the pulses, or another dish aimed at the object and precisely tuned to the correct frequency. Detailed computer analysis of the returned signals enables Ostro to infer the shape and rotation of the object. He has successfully used radar instruments operated by NASA as part of its Deep Space Net of tracking antennae, as well as the giant 1000-foot (300-m) radar telescope, the largest in the world, operated by Cornell University at Arecibo in Puerto Rico.

If the asteroid is unusually close so that the returned radar signals are strong, they can be processed into actual pictures of the asteroid. By the mid 1990s, Ostro had reconstructed excellent images of three asteroids: Castalia (500 m across), Toutatis (5 km across), and Geographos (2 km across). Each is an elongated, highly irregular object, as might be expected for a fragment spawned in some ancient collision between two asteroids. Unexpectedly, however, 2 of these asteroids appear to be double objects, perhaps created when two roughly spherical objects collided at low speed to create a binary. Castalia, in particular, is unmistakably double. Today there is speculation that many of the small Earth-approaching asteroids may be binaries or multiple-component "boulder piles".

To learn more about these mysterious objects, two spacecraft have been launched on exploration missions. The first attempt was made by the U.S. Department of Defense as a part of its growing interest in defending the Earth from impacts, a subject we discuss in detail in a later chapter. The Air Force Space Command and the Naval Research Laboratory collaborated in 1994 to send a spacecraft called Clementine, built using some of the technology developed for the "star wars" Strategic Defense Initiative, toward the Earth-crossing asteroid Geographos via the Moon. Unfortunately the spacecraft failed soon after leaving lunar orbit, and no asteroid images were obtained.

A larger spacecraft called NEAR (for Near Earth Asteroid Rendezvous) was built by NASA as part of its planetary exploration program and launched from Cape Canaveral on February 17, 1996. It passed through the main asteroid belt in June 1997, taking the first-ever close-up images of one of the common class of black asteroids -- this one is called Mathilde. NEAR's chief target is the large Earth-approaching asteroid Eros, which will be reached by the spacecraft in 1999. If successful, NEAR will orbit Eros for more than a year, returning a great deal of data to the Earth, before finally being sent to crash-land on the asteroid's surface.

As the first space mission ever dedicated to studying asteroids, NEAR carries a payload of instruments specifically designed to learn about the nature, including the chemical composition, of Eros. Eros will be mapped in exquisite detail, as only an orbiter can do it. (When a spacecraft only flies past an object, en route to something else -- as Galileo flew past the main-belt asteroids Gaspra and Ida in the early 1990s -- we are always left with the nagging question: What might have been on the other side?)

NEAR, which is about the size of a Volkswagen Beetle, will be watching in case Eros holds some surprises for us, like some moons or unexpected geological activity. Even if Eros turns out to look just like a giant example of a meteorite we already have samples of here on Earth, we can reflect on the fact that this object -- about 20 km across -- could well be the next major extinctor of life on this planet. Computer calculations show that Eros has about a 50-50 chance of ending its existence by colliding with Earth. Although we know such a collision cannot possibly occur during our lifetimes, it could well do so in the next few millions of years, and it will make the K/T boundary dinosaur extinctor look paltry by comparison.

So as NEAR scrutinizes Eros for clues about its origin, or for practical information that might facilitate eventual mining, this enormous "flying mountain" with the name of "love" might instead be seen as the Sword of Damocles hanging over the heads of Earth's species incapable of establishing themselves elsewhere when the potential destruction happens.

There are indirect, and cheaper ways, of studying the physical and chemical nature of asteroids. For instance, we can learn about them by studying meteorites, which are asteroidal fragments that have survived passage through our atmosphere. From these rocks we learn that some asteroids are "primitive", meaning that they represent the original unmodified material that formed the building blocks for the planets. The primitive meteorites and their asteroidal parents are composed of rocky minerals mixed with fine metallic grains. Other asteroids were somehow heated to their melting points, permitting their rocky and metallic parts to separate. The meteorites that originate from the once-molten cores are nearly pure iron-nickel alloy, while the samples of their mantles and crusts resemble terrestrial rocks. Although metallic meteorites are common in museums, their presence results from selection effects: they make it through the Earth's atmosphere almost undamaged and, once fallen, iron-nickel meteorites are much easier to identify than stony meteorites. In fact only about 2% of the meteorites that fall are metallic. This and other evidence indicates that metallic asteroids must likewise be quite rare, so the great majority of asteroidal projectiles are rocky objects.

Comets, like asteroids, are small, solid bodies that orbit the Sun. They differ from asteroids by having ices (including water ice and dry ice) near their surfaces. When a comet approaches the Sun, the heat vaporizes the ices to drive out an enormous dusty atmosphere (the comet's "head"), which is often swept away from the Sun into a tenuous "tail", typically tens of millions of kilometers long. Most of the light we see from a comet is reflected from the thin gas and dust of the head and tail. In addition there is most certainly a solid nucleus of rock and ice that is the source of the visible atmosphere. The existence of solid nuclei was generally recognized in the 1940's but not really proven until 1986 when the European Giotto spacecraft took pictures of a very substantial object about 10 km across in the middle of the head of Halley's Comet. Unfortunately, we do not have cometary samples in the form of meteorites, since their icy remnants burn up in the atmosphere, but we are pretty sure that if we were struck by a cometary nucleus, it would be very nearly like being hit by a rocky or metallic asteroid of the same mass and incoming velocity.

We need a simple term for these cosmic projectiles, including both Earth-crossing asteroids and comets. They are now usually called NEOs, for Near Earth Objects. By coincidence, this terminology emphasizes the newness of the perceived threat of impact from NEOs. . . the idea of impacts by NEOs could even be called neo-catastrophism (pun intended).

Although many of the NEOs will eventually strike the Earth, it is still a long time between such cosmic collisions. The target (Earth) must be in exactly the same place at the same time as the NEO. Since interplanetary space is very big, the chances of such co-location are very small. Typically, an NEO will cross the Earth's orbit many millions of times, with the Earth elsewhere, before there is an actual hit. It may be many tens of millions of years before an NEO hits the Earth, Venus, or the Moon, or is gravitationally ejected by near-misses. Thus there is plenty of opportunity to spot these objects and track them before they strike our planet.

Comets and asteroids can be seen with ordinary telescopes (otherwise we would not know that NEOs exist), but they are not easy to find. With rare exceptions, astronomers on their mountain-tops do not spend their time "scanning the skies." Instead, they focus on particular problems involving single stars or galaxies. Besides, the great telescopes of the world (and the Hubble Space Telescope in orbit) are designed to peer intently at patches of the heavens so small that thousands of such fields-of-view would be required to fill the bowl of the Big Dipper. Thus astronomers don't necessarily know when something changes in the sky. When one of the countless stars of the firmament explodes, it does so unwatched. Maybe it becomes brilliant enough eventually to be noticed, but astronomers always come on the scene in the aftermath, like police after the burglar has fled.

There are a few telescopes of different design. Instead of magnifying a small piece of the heavens, they capture a wide part of the sky. During the 1980s and early 1990s just two telescopes on Earth were routinely used to watch for new objects in the solar system. One was the 18-inch wide-field telescope on Palomar Mountain, in southern California -- the mountain made famous for the 200-inch Hale telescope, once the world's largest. The other is a 36-inch telescope operated by the University of Arizona in a search called Project Spacewatch. The few users of these telescopes, many of them unpaid volunteers, have been the only astronomers about whom it could properly be said that they "scan the skies."

In 1995 the 18-inch wide-field telescope on Palomar discontinued its survey program, although two new automated search telescope are taking its place in 1997 and 1998. Even with this modest increase in telescopic power, however, only a small portion of the sky will be scanned each month, and there is no assurance that an incoming projectile will be found before it hits. It is entirely possible that when the next big one hits the Earth, the first we will know about it is when we feel the ground shake and watch the fireball rising above the horizon.

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Let's conclude this chapter by introducing one of the pioneers in asteroid hunting -- and one of the leading characters in our impact saga. He is Gene Shoemaker, now best known as co-discoverer of Comet Shoemaker-Levy 9, which crashed into Jupiter in July 1994. Long before the great comet crash, Shoemaker had established himself as the world's leading expert on impacts and impact craters, and his work had already earned him the Presidential Medal of Science and membership in the U.S. National Academy of Sciences. Shoemaker, a charming and energetic scientist who recently retired from the U.S. Geological Survey, was trained as a field geologist. He founded the field of planetary geology (which he originally called astrogeology) and helped train the astronauts for the Apollo Moon landings. Later he participated in the Voyager robotic mission to the outer planets and helped interpret pictures of the cratered moons of these distant planets. In the midst of these activities, Shoemaker also took up a new career as an observing astronomer.

Shoemaker was one of the first scientists to calculate the average rate of impacts on the Earth from the lunar data, at the same time going "into the field" to discover and explore the impact scars on our planet. But he was not content with such historical research on past impacts; he also wanted to know the current cratering rate, as well as the nature of the projectiles that produced these craters. Very few astronomers cared about the NEOs, and no one in the 1970s was carrying out a systematic effort to discover and catalogue these interplanetary wanderers. Characteristically, Shoemaker decided that the best way to remedy this situation was to undertake the search himself.

Through his association with the California Institute of Technology (Caltech) as a part-time Professor of Geological and Planetary Science, Shoemaker got access to the small 18-inch telescope on Palomar Mountain. In collaboration with JPL scientist Eleanor Helin and later with his wife Carolyn Shoemaker, he made the personal commitment to spend one week out of each month photographing the sky with this small telescope. Each region would be photographed twice and the two images compared to look for any objects that were moving against the background of stars. This is a formidable task, because each individual photographic plate contained XX,000 [check] stellar images. The simplest way to pick out the moving asteroids was to view the pairs of plates through a stereo microscope. When aligned properly, all the star images appear to be in the same plane, while any moving object appears to "float" above the background stars. Carolyn Shoemaker proved to be particularly adept at recognizing these floating objects, and she is the person who actually discovered most of the team's new asteroids and comets.

The Palomar NEO search began in 1973. In 1982 it split into two efforts, each using the 18-inch telescope for one week per month. One team was headed by Eleanor Helin and the other by the Shoemakers. In the early years about one NEO per year was found by each group, but in the 1990s the pace accelerated. When the program began only xx of these objects were known, but by the middle of 1995 xx were known, xx of them discovered at Palomar. These discoveries form the basis for estimating the hazard of impacts and developing the strategy for dealing with them, as we will discuss in later chapters of this book. Gene Shoemaker himself has played a leading role in all of these activities, and he is truly the father of modern NEO and impact research, as well as something of a father figure to many younger scientists. The "Mom and Pop" team of Gene and Carolyn Shoemaker have probably done more to save the Earth from impact catastrophe than anyone else alive, and they will reappear frequently throughout this narrative.

* * * * * * *

One of the most important products of the searches for NEOs is an estimate of the number of comets and asteroids with orbits that cross the orbit of the Earth, and hence an estimate of the frequency of impacts by objects of various sizes. For those interested in the impact issue, whether one is an astronomer, geologist, insurance actuary, or civil-defense planner, it is essential to know this impact rate. Thus we conclude this chapter with a quantitative estimate of this cosmic rain that pelts down continuously on our planet.

From the NEO surveys carried out by Gene Shoemaker and his astronomer colleagues, we can estimate the number of near-Earth asteroids in various size ranges. The largest such asteroids, named Ivar and Betulia, have diameters of about 8 km, roughly half the size of the object that killed off the dinosaurs when it struck the Earth 65 million years ago. We are confident that there are no undiscovered near-Earth asteroids larger than these two, but once we move down to smaller sizes, our catalogues are less complete. The numbers of these objects must therefore be estimated statistically. Our telescopes are watching only a part of the sky for a part of the time, so many objects are missed. To calculate the true numbers, we correct our incomplete observations, in much the same way you could calculate the total number of trucks that pass along a highway during the course of a day even if you only actually counted the trucks for 5 minutes out of each hour.

Combining the data from astronomical surveys with the record of lunar craters (which represent a long-term average), Shoemaker has estimated that there are 400 Earth-crossing asteroids larger than 2 km in diameter, 2000 larger than 1 km, 10,000 larger than 500 m, and roughly 300,000 larger than 100 m across. These numbers are good estimates for the larger objects, but become increasingly uncertain as we move to smaller sizes, where only a very small fraction of the NEOs have actually been discovered.

Probably the most meaningful way to present these statistics on the NEO population is to make a plot of the average frequency of impacts of a given size or larger over the whole Earth. Shoemaker first derived such a plot in 1981, and an updated version is illustrated in the Figure. In addition to asteroid size, this figure shows the energy (in megatons of TNT equivalent) for each impact. Thus, for example, we can see on the plot that Earth is struck by an NEO with energy of 100 megatons or more (diameter 100 m or more; the size of a large city office building) approximately once per millennium. (It is purely coincidence that this particular set of values comes out in such round numbers, but it makes them easy to remember). Larger impacts are much rarer: the interval for one million megatons (2 km diameter) is approximately once every one million years (another number that is easy to remember).

These estimated average impact rates will come up repeatedly in the chapters that follow. To evaluate the implications of these impacts, we must know many additional factors, especially concerning the physical and environmental consequences of impacts of various sizes, but this impact rate curve remains essential.

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CHAPTER 2: THE COSMIC SHOOTING GALLERY

SUMMARY

Some ideas are important, even revolutionary. One such idea is that the Earth is a cosmic target, vulnerable to global catastrophe. In the previous chapter we outlined the case for impacts in Earth history, but we now step back to the situation about 1980 for a more detailed view. To many scientists at that time, the concept of Earth's vulnerability to impacts appeared to contradict common sense and the lessons we were taught about global history. In many ways their uniformitarian philosophy makes much sense; it is difficult to imagine that processes or events far beyond our experience might have dominated the history of our planet. Yet, as we recount in this chapter, the discovery of ubiquitous impact craters on many planetary bodies reintroduced the ideas of catastrophism into geology. We use the record of the Moon to derive the surprisingly violent cratering history of our own planet. Next we describe the comets and asteroids, which are the culprits in this cosmic bombardment, including the results of recent radar studies and space missions. Geologist Gene Shoemaker enters the story as one of our chief characters, and we describe how Gene and his colleagues (including his wife Carolyn) search for asteroids and comets. The chapter concludes with the primary product of these searches, a description of how often the Earth is struck by comets and asteroids of various energies, from less than a kiloton up to more than a million megatons of TNT.

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CHAPTER 3: TARGET EARTH

"While there are no definite data to reason from, it is believed that an encounter with the nucleus of one of the larger comets is not to be desired." -- Nineteenth-century Astronomer Herbert Howe in his 1897 textbook

Is the Earth really being struck by comets and asteroids? The Moon, Mercury, Mars, and even Venus are heavily cratered, but there are few impact features readily found on our planet. Why is this? Once it was thought that the atmosphere protected us from impacts, but now we know enough about meteors to recognize that only smaller impacts -- those by asteroids or comets less than about the size of a football field -- are filtered out by the atmosphere. If there were any doubt as to this conclusion, the mapping of Venus by the Magellan radar satellite in the early 1990s dispelled it.

The atmosphere of Venus is about 100 times denser than that of the Earth, yet Venus has many impact craters. None of the craters on Venus is smaller than about 10 km across, however. Scientists have studied the process of meteoric breakup in an atmosphere to interpret the cratering record on Venus, concluding that the ability of an atmosphere to fragment an incoming projectile is roughly proportional to the amount of gas in the atmosphere. The atmosphere of Venus filters out rocky asteroids with a diameter of less than about 1 km (and hence a cross-section of about 1 square kilometer); these are the objects that produce the 10-km craters. The Earth's atmosphere, being a hundred times less massive, shields us from impacts of objects only if their cross-section is less than 1/100 square kilometer, corresponding to diameters of about 100 m, or the size of a football field. The atmosphere protects us from most small objects, but not large ones; thus the paucity of large craters on Earth cannot be an atmospheric shielding effect.

What most distinguishes our planet from the Moon, Mercury, Mars and Venus is the Earth's higher levels of geologic activity, including volcanism and plate tectonics. These effects, together with water and ice erosion from the continents and sedimentation in the oceans, effectively fill in, erode, and erase impact craters almost as quickly as they are made. Depending on where a crater is formed, it may remain visible for only a few million years, a mere blink of the eye compared with the 4.5-billion-year age of the planet. We should expect to see only the most recently formed terrestrial craters, and even for them it is more productive to look in areas of geological stability and low rates of erosion, such as mid-continental deserts. Only in the past few decades have geologists begun to study these recent craters and learned to use more subtle clues to locate the faint ghosts of older, more heavily eroded impact scars.

Tourists driving east across Arizona on Interstate 40 pass through some magnificent scenery. First there are the dark twisted mountains south of Lake Meade, then the high plateau cut by the Grand Canyon of the Colorado River. Near Flagstaff the road skirts the snowcapped peaks of the San Francisco volcanoes, fringed with recent cinder cones and fresh lava flows near Sunset Crater. Proceeding eastward, a driver descends across a broad treeless plain with distant views of the Painted Desert and, eventually, the Petrified Forest National Park. About midway between Flagstaff and Petrified Forest, however, is another natural wonder, Meteor Crater. Many tourists zip past, perhaps failing to distinguish the signs announcing the privately-owned Meteor Crater from the commercial rock shops and dinosaur exhibits that are also advertized along this lonely stretch of highway. But if they do pause, they can visit the best preserved and most accessible impact feature on our entire planet.

Seen from the air, Meteor Crater is a striking circular bowl, looking very much like one of the smaller lunar craters. From the ground, it is barely visible as a low (50 m) mesa (originally called Coon Butte) until you reach the rim and look down into its cavernous interior. The circular crater is almost one mile across (1.2 km) and 570 feet (175 m) deep, with a steep trail descending to the crater floor. Surrounding it for many miles are scattered small fragments of nickel-iron alloy, some of it melted and mixed with the local limestone rock.

Meteor Crater was first noted by local sheepherders in the 1880s, and they were followed by prospectors who collected the nickel-iron fragments from the surface. By the 1890s some geologists were already convinced that this was an impact structure from the apparent meteoritic nature of the metal samples. However, the uniqueness of the feature and its proximity to the San Francisco volcanic features confused others. Even Grove K. Gilbert, the Chief Scientist of the U.S. Geological Survey and one of the first geologists to advocate an impact origin for lunar craters, concluded that this terrestrial crater had probably been formed by a volcanic steam explosion.

One man who believed passionately in the meteoritic origin of the Coon Butte feature was mining engineer Daniel M. Barringer, who began studies of the crater early in the 20th century and eventually purchased the land that contains the feature. Barringer was motivated in part by his conviction that the bulk of the meteorite was buried within the crater, and that this would prove to be an economically attractive mining investment. He brought in drilling rigs, and by 1908 he had drilled 28 bore holes in the crater floor, all to no avail. Later Barringer drilled holes around the crater thinking that perhaps the body had impacted at an oblique angle. By the time of his death in 1929, Barringer had spent nearly a million dollars, a huge fortune by the standards of the time, on his fruitless search for the iron-nickel mass. Eventually Barringer's heirs gave up on the search and began to develop the property as a tourist attraction.

Shortly before his death, Barringer hired as a consultant the British astronomer and mathematician Forrest Ray Moulton. Moulton was one of the first scientists to calculate the energy of a body impacting at more than 15 km/s speed, and to estimate the amount of rock that would be melted or vaporized by the release of this energy. He recognized that an impact crater would be much larger in diameter than the size of the incoming projectile, and he calculated that the projectile itself would be largely destroyed in the impact. Writing in 1931, Moulton noted that "the energy given up in a tenth of a second would be sufficient to vaporize both the meteorite and the material it encountered -- there would be in effect a giant explosion that would produce a circular crater, regardless of the direction of the impact." Although bitterly disappointing to those who hoped to find a large body of iron ore in Meteor Crater, these conclusions pointed the way toward a proper understanding of the impact process on both the Earth and the Moon.

It was not until the 1950s that another major scientific study was carried out of Meteor Crater. Gene Shoemaker, then a young field geologist, had been studying the craters formed by nuclear explosions. At that time the United States was developing its arsenal of fission bombs, and these devices were tested in the deserts of Nevada. Shoemaker had the bright idea of comparing Meteor Crater with these man-made features. He was the first geologist to map the crater systematically and to compare its structure with that of lunar craters as well as bomb craters, and his studies of Meteor Crater eventually formed his doctoral thesis. Even today he leads groups of fellow scientists and students on fascinating (and energetic) field trips into the crater, where he appears to know every rock like an old friend. Shoemaker's work clearly established that the crater is of impact origin. It also set the stage for detailed studies of lunar craters carried out by the Apollo astronauts, many of whom Shoemaker had trained in Meteor Crater. Shoemaker estimated the energy of the impact at 15 megatons, and later one of his students determined what is now accepted as the age for Meteor Crater, 50,000 years.

No humans lived in North America when the half-million-ton mass of iron and nickel (probably in form of an irregular lump 40-50 m across) came screaming down into the Arizona desert; the ancestors of the Native Americans had not yet crossed the land bridge from Asia. Because iron is so dense and strong, the atmosphere had little effect on the incoming projectile, which probably struck the ground with a speed of more than 15 km/s. The resulting explosion must have raised huge clouds of dust and spread a blanket of debris for hundreds of miles. Originally, the crater was nearly twice as deep as it is today, but subsequent erosion has partially filled the interior and obliterated most evidence of the ejected material, except near the crater rim. Even in its current state, however, Meteor Crater remains one of the best examples on our planet of a relatively fresh impact feature. The tourists who do stop to visit on their drive across Arizona witness the most characteristic of planetary landforms, rare on our planet but actually common elsewhere in the solar system.

Meteor Crater is a spectacular a hole-in-the-ground, but it is extremely young geologically speaking. Arizona is a place of considerable geological upheaval, as can be judged from the recent volcanic peaks that are visible from Meteor Crater itself. An impact crater will not survive for many millions of years in such an environment, but there are other places on our planet where the weather is less stormy, subterranean forces are mild, and change is slow. Such a place is the Outback of central and western Australia. For hundreds of millions of years, there has been no faulting, no mountain-building, and no volcanism. Erosion works slowly in a level landscape where anything over 50 m high is called a "mountain", making central Australia one of the Earth's best scorecards for recording extraterrestrial impacts.

Many years ago Gene Shoemaker was attracted to this bleak, unchanging landscape to hunt for impact craters alongside Australian geologists. As his retirement approached, Shoemaker and his comet-hunting wife, Carolyn, began to spend two months every year camping in the Outback, away from the hubbub of America, researching the craters of Australia.

In September 1990, springtime in Australia, the Shoemakers decided to show off nine of their favorite craters to 50-odd fellow geologists and astronomers who were coming to Australia for a scientific conference. One of the largest tour groups to venture into the Outback, their caravan (using an especially constructed 6x6 army-green outback bus) traversed 6,000 km in just 3 weeks. Besides a few emus and patches of thorny spinafex grass, there was little to see but craters, and most of them were difficult to see as well. One crater was barely larger than a camel wallow, but some of the biggest ones were even more difficult, requiring a trained geologist to pick out.

Camping in the desert night after night, hundreds of miles from the nearest town, we could visibly connect the sky with the land. With the Southern Cross off to the side and newly-discovered Comet Levy shining overhead, accompanied by meteors that streaked across the dark sky, it was easy to imagine that celestial objects can plummet to the ground to wreak devastation on the Earth.

There are many lessons to be leaned from this, the most densely cratered terrain on Earth. The smaller craters, a mile across and less, were (like Arizona's Meteor Crater) all formed by metallic meteorites, remnants of which could still be found in the soil near the craters. This fact proves that the much more common stony boulders from the sky that would otherwise make craters of such sizes (boulders less than about the size of a football field) are torn apart during their atmospheric plunge. Only the stronger iron meteorites make it through to form mile-wide (or smaller) craters. Australia even offers one example -- the Henbury crater field near Alice Springs -- where an incoming stony object began to break up in the atmosphere and formed a tightly-packed cluster of craters.

A two-part side trip to Lake Acraman and the faraway Flinders National Park was particularly dramatic. Acraman is a vast waist-deep lake where a huge crater formed millions of years ago. Several hundred kilometers away, in the low but colorful Flinders Range of hills, Gene Shoemaker showed the travellers a layer in the rocks where ejecta from Acraman Crater was preserved in the geological strata. The composition of this ejecta layer is identical to the target rock in the Acraman area -- with a little bit of extraterrestrial iridium mixed in.

Returning from Flinders, the Shoemaker party stopped at a winery, reached civilization at Adelaide, then flew to Perth in time for the annual meeting of the international Meteoritical Society. There Shoemaker and his colleagues addressed one of the most important scientific issues that inspires the study of Australian craters: what is the rate at which asteroids and comets strike the Earth? From dating the time of formation of the Australian craters, Shoemaker concludes that craters larger than 10 km in diameter (produced by an asteroid just under 1 km across) are produced on the land area of the Earth about once every million years -- a value in good agreement with the impact rate deduced from counting asteroids or evaluating the numbers of craters on the Moon (as we discussed at the end of the last chapter). So we conclude that there is nothing special about the Earth or about our particular time -- we are currently pelted by cosmic debris just like any other planet, and at about the average rate.

Only a small fraction of the impact structures on Earth are as readily visible as those in Australia. Usually, the surface expression of a crater is obliterated by erosion and sedimentation within a few million years of its formation. The underlying structures may still be detected, however, using the modern techniques of geophysical exploration. One byproduct of the global search for oil and ore deposits has been the discovery of buried impact structures.

Paradoxically, as a large crater erodes it tends to evolve from a depression into a hill. An example is Gosses Bluff, west of Alice Springs -- the crater is gone, but its central peak remains. The inner part of the crater rebounds to fill in the original cavity formed by the explosion. Many of the larger craters on the Moon have similar central peaks. These anomalous upthrusts or hills have in the past been called crypto (meaning hidden or secret) volcanic structures. One example is in the flat prairies of Iowa, where the so-called Manson cryptovolcanic structure has recently been confirmed as the remnant of the central peak of a crater 35 km in diameter that formed 70 million years ago. Other cryptovolcanic structures in the U.S. Great Plains include Kentland in Indiana and Serpent Mound in Ohio. This outdated terminology is another example of the confusion of impact structures with volcanoes that has characterized terrestrial geology until recently.

When a geological feature is badly eroded, arguments about its origin are inevitable. What geologists need are objective criteria for distinguishing an impact structure from one that has been formed by volcanism or other geological forces. Two such geological clues have been discovered, both of them signatures of the explosive shocks associated with impact, producing pressures that far exceed anything that can be generated in a volcano.

Silica (silicon dioxide) is a compound found in many rocks. It has several mineral forms, the most common of which is quartz. One very rare form of silica, originally produced in laboratory experiments in 1953, has a very dense and highly stable structure that can be formed only at very high pressures. The chemist who discovered this mineral was named Loring Coes, and the mineral is called coesite. Gene Shoemaker discovered natural coesite in 1960 in samples from Meteor Crater, and subsequently in association with other known impact craters. Another shocked mineral was found in 1961, called stishovite after one of its Russian discoverers, S.M. Stishov. In addition, ordinary quartz grains subjected to shock can develop characteristic internal striations visible under a microscope. Today, shocked silica is generally accepted as the most convincing fingerprint of an impact, a fingerprint that survives long after the crater itself has faded into invisibility.

A second indication of the high pressures of impact is found in structures called shatter cones. As a shock wave passes through the target rock, it fractures the rock in a characteristic cone-shaped pattern that is visible to the trained eye of the geologist and requires no detailed laboratory confirmation. Shatter cones have been identified in the underlying rock of all of the younger terrestrial impact craters.

Armed with these tools and using space-age remote sensing from aircraft and orbiting satellites, geologists have discovered more than 150 impact scars on the Earth. In Chapter 6 we will discuss the discovery of the 200-km Chicxulub Crater in Mexico, but here let us mention two other giant scars, the Bushveldt-Vredefort complex in South Africa and the Sudbury basin in Canada.

The Transvaal region of South Africa contains some of the Earth's most ancient rocks, more than two billion years old. The features that interest us here are the Vredefort Ring, a half-circle of rock about 75 km in diameter, and a nearby region about 300 km long called the Bushveldt Igneous Complex. This area interests geologists not only for its age and complexity, but also for its concentration of valuable ore deposits. In the 1960s investigators found numerous shatter cones in the Vredefort ring as well as abundant shocked quartz throughout the region. By the 1970s, some geologists were interpreting the entire Bushveldt-Vredefort complex to be the result of four nearly simultaneous impacts producing overlapping craters, the largest of which was about 250 km in diameter. Unfortunately, the leading geologist studying this problem was killed in an accident in 1975, and to date there remain many unanswered questions about his interpretations of the observations.

The Sudbury basin, on the north shore of Lake Huron in Canada, contains the word's largest deposit of nickel and one of the most productive areas for mining iron as well. Like Bushveldt-Vredefort, Sudbury is an extremely complex region containing violently shattered rocks. There are also huge deposits of rapidly cooled lava, in spite of the absence of supporting evidence of extensive volcanism elsewhere in this part of the Canadian Shield of ancient rocks. In 1964 Arizona geologist Robert Dietz, who was the leading proponent of impact geology at the time, proposed that the Sudbury basin was an impact structure 1.7 billion years old. Presumably the meteorite was of iron-nickel composition, and it is the source of the rich nickel deposit in the area. A single metallic asteroid 4 km in diameter could provide all the iron-nickel ores present. Shatter cones were found in 1962, followed by the identification of impact minerals. Although controversy remains, today it seems likely that this feature, which has been so important to the economy of Canada, is one more example of our planet's long history of cosmic impacts.

Impacts were especially important for the Earth during the first half-billion years after it formed. Like the Sun and the rest of the solar system, our planet was born approximately 4.5 billion years ago in the collapse of a cloud of gas and dust. This collapse, triggered perhaps by the explosion of a nearby star, produced a spinning disk called the solar nebula. The central part of this solar nebula became the Sun, while the building blocks of the planets formed from the small solid bodies -- called planetesimals -- that formed out of the dust in the spinning disk.

The first solid matter in the solar nebula probably consisted of tiny grains of dust that condensed from the cooling gas like raindrops forming in a rising cloud of water vapor. Bits of dust clung together to form larger rocks and boulders, and these collided to build up still larger objects. The Earth is thought to have formed in this cauldron of swirling debris through a process of accretion of these planetesimals. As the planet grew, its gravity attracted more and more material, and the accreting material struck with higher and higher speed. Energetic impacts generated so much heat that the upper layers of the planet melted to form a global ocean of liquid rock. No craters date from that period, of course, since the Earth had no solid surface to preserve them.

The Earth was formed without a Moon. At some point during the period of accretion, however, it is likely that our planet was struck by another coalescing world about the size of Mars today -- that is, with a mass about 10% that of the Earth. The result of such an impact between worlds was catastrophic on a scale that, fortunately, is no longer conceivable within the solar system. The smaller, Mars-sized planet was completely destroyed, and even the larger Earth was shattered to its core. The explosion formed a massive atmosphere of hot rock vapor and ejected gargantuan quantities of molten and vaporized rock into space. Some of that ejected material continued to orbit the Earth as a giant ring, which cooled and collapsed to form our Moon. Scientists have deduced this scenario from a detailed comparison of the composition and structure of the Earth and Moon, which testifies to just such a catastrophic birth for our satellite. Understanding the role of impacts in the formation of the Moon is one of the many products of the Apollo expeditions and the priceless moonrocks returned for laboratory analysis.

If the Moon-forming impact had been just a little larger, the Earth itself would have been disrupted beyond repair. Several planetary collisions of comparable magnitude probably occurred during the early days of solar system history, but any direct evidence is long gone. We do see planetary peculiarities, however, that are best understood as the product of random collisions among planetary-scale bodies. Venus spins in the opposite direction from its orbital motion about the Sun, probably as the result of a late collision that struck it a glancing blow and reversed its direction of rotation. The small planet Mercury appears to be just the metal-rich remnant of a larger parent, stripped of most of its rocky mantle in another giant collision. It is largely a matter of luck that the final product of this chaos was the four inner planets we have today: Mercury, Venus, Earth, and Mars, plus the Moon.

Calculations predict that all four of the inner planets should have been made from the same building blocks of rock and metal. Because these building blocks were formed in the inner solar system, they lacked the liquid or solid forms of water and other volatile materials. The temperatures in the inner part of the solar nebula were too hot for these vapors to condense. Where, then, did the atmosphere and oceans of the Earth originate?

In the outer parts of the solar nebula, far from the Sun, temperatures were much lower (as they are today among the outer planets). Here water ice was abundant, as well as other frozen gases such as methane, ammonia, carbon dioxide, carbon monoxide, and even ethyl alcohol. The volatiles on Earth were probably derived from this distant reservoir in the outer solar system.

The most likely way to bring water to the Earth was in the form of comets. Comets (as well as the distant asteroids) are rich in volatiles and organic compounds. During the first half-billion years of solar system history, gravitational forces scattered many comets from their place of origin inward toward the Sun. This comet bombardment took place at least one hundred million years after the birth of the Earth and Moon, after both bodies had cooled and formed solid crusts. When the early comets and volatile-rich asteroids crashed into the Earth, their water and organics and other exotic compounds vaporized. Part of the vapor was blasted back into space by the force of the explosion, but part was retained to gradually built up a thick hot atmosphere. The same thing presumably happened to Venus, which is nearly the twin of the Earth in size and mass. However, the Moon and Mercury were too small to retain their new atmospheres, which escaped to space, leaving a dry surface exposed directly to the vacuum of space. Mars, being intermediate in size, retained a part of its impact-derived atmosphere, but less than did Earth or Venus.

The bombardment of the inner solar system by volatile-rich remnant planetesimals was critical for the history of the Earth. Most of the material of the biosphere -- and of our own bodies -- is probably comet-stuff, derived from the outer solar system. Were it not for cometary ice and carbon compounds, our planet might well be as dry and lifeless as the Moon. Life is a gift from the comets. But the gift did not come without a price to pay.

As the rain of cometary materials persisted, the Earth (and presumably Mars and Venus as well) built up a thick atmosphere of carbon dioxide and other compounds, and it developed shallow oceans of liquid water, rich in dissolved organic materials. Such an environment is exactly what chemists think was required for the origin of life. The first self-replicating molecules must have formed in such early seas, perhaps on all three planets.

If all of the impacting planetesimals were small, this environment might have approximated the "warm little pond" hypothesized by Charles Darwin for the origin of life. However, the evidence preserved in the densely-packed craters of the lunar highlands suggests otherwise. At least a few of the impactors from that first half-billion years of solar system history were hundreds of kilometers across -- the size of the largest asteroids and comets of today. Kevin Zahnle of the NASA Ames Research Center and his colleagues realized a few years ago that such large impacts were capable of drastically altering the terrestrial environment.

What happened when a several-hundred-kilometer asteroid or comet smashed into the early Earth? Zahnle calculated that the energy of such an impact would melt and vaporize so much of the crust near the point of impact that the planet would acquire a temporary atmosphere of rock vapor at a temperature of about 1000oC. Under this terrible red-hot blanket, the oceans would boil away, killing any life forms that might have arisen. In effect, such impacts sterilized the planet. After a few decades the hot rock would have cooled and the oceans recondensed, but the clock would have been reset to zero for the origin of life. In an obvious understatement, this is called the "impact frustration" of the origin of life.

Zahnle has calculated that the Earth probably experienced a handful of such sterilizing impacts during its first half-billion years. Venus probably took as many hits, while Mars, being smaller, may have escaped such catastrophe. The last such impact probably took place about four billion years ago, as determined by extrapolating from the lunar cratering record. It is interesting -- and perhaps not coincidental -- that the earliest chemical evidence of life on Earth dates from not too long afterwards, at about 3.8 billion years ago. It appears that our ancestors appeared very soon after the end of this period of "frustration". It is not too great an extrapolation from this evidence to suppose that life had formed several times previously, only to be wiped out by a sterilizing impact.

We are thus led to a remarkable picture of the role of impacts in the history of the Earth, a role recognized by scientists only during the past decade. Our planet was formed by the accretion of impacting debris; the Moon was blasted from the Earth's surface in a giant impact not long after the planet formed; the rain of comets from the outer solar system subsequently carried life-giving water and organic compounds to the inner solar system, but at the same time subjected the Earth to a terrible bombardment of projectiles, the largest of which episodically boiled away the oceans and sterilized the surface. Although life may have arisen several times early in our planet's history, it was only after this heavy bombardment declined that life was able to survive. However, the role of impacts in biological evolution was not over. As we shall see in the following chapters, impacts continued to play a major -- perhaps dominant -- role in the evolution of terrestrial life, even up to the present era.

4800 words (3/26/97)

CHAPTER 3: TARGET EARTH

SUMMARY

If the other planets in our solar system are heavily cratered, why do we see so few craters on the Earth? What most distinguishes our planet from the Moon, Mercury, Mars, and Venus is the Earth's higher levels of geologic activity, including volcanism and plate tectonics. These effects, together with water and ice erosion from the continents and sedimentation in the oceans, effectively fill in, erode, and erase impact craters almost as quickly as they are made. Nevertheless, a few craters can be found. We discuss Meteor Crater in Arizona is considerable detail, including the early 20th century excavations by D.M. Barringer and the space-age research by Gene Shoemaker, and explain the reasoning that led to the crater's identification as the product of explosive impact by an iron asteroid the size of a small office building. The scene then moves to Australia, where we accompany Shoemaker through the Outback in his continuing search for terrestrial impact craters. At Sudbury, Ontario, the world's largest nickel mine is the product of an impact 1.7 billion years ago. And as we note at the end of the chapter, the water and carbon in our own bodies are directly related to impacts, brought to our planet by comets striking Earth more than 4 billion years ago.

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CHAPTER 4: DEATH OF THE DINOSAURS

"To suggest that the dinosaurs were not wiped out by the comet impact is something like saying that the 1500 victims of the sinking of the Titanic in 1912 did not die in the collision with the iceberg. Technically, that is correct. No one died in the collision. Everyone who died did so later, as the result of drowning, hypothermia, heart attacks, falling to their death, or being crushed by swinging lifeboats. In the same way, virtually no species went extinct at the moment of impact. It is what happened afterward that took its toll. That was when habitat was destroyed and entire species disappeared because they burned, drowned, froze, or starved to death." -- Astronomer Gerrit Verschuur in Impact: The Threat of Comets and Asteroids (1966)

More than 99 percent of the species that have lived on the Earth are extinct. An extinction occurs when all organisms comprising a species die out so that the species ceases to exist. Extinction is a part of biological evolution, just as death is a part of an individual's life. Roughly speaking, for every new species that appears, one disappears from the world. However, as University of Chicago paleontologist David Raup has noted, naturalists have usually concentrated on the origin of species rather than on their extinction. Darwin's famous book is The Origin of Species, not The Origin and Extinction of Species. Only in the past few years has the study of extinctions moved into the mainstream of natural science.

We have abundant fossils only from the most recent 15% of Earth's history, the past 600 million years. Throughout this span, pollens, shells, bones, and so on have been deposited on the bottoms of lakes and oceans. The muds became rocks, and the layers of embedded fossils -- when exposed by a geologist's rock hammer -- provide windows onto the evolving diversity of life. Occasionally a period is well documented by a particularly fine set of fossils, while the biota of other periods is less well understood. We see general patterns in the origin, modification, and extinction of species, but it is not easy to pinpoint the exact time that a particular species first appears on the scene or when its last members die out.

Despite such uncertainties, the fossil record shows that the formation of new species and extinction of old ones are discontinuous processes. Many paleontologists used to think that the abrupt changes in observed fossils reflected gaps in the geological record rather than sudden changes in the evolution of life, but we now know that this is not generally true. Many and probably most extinctions have happened during a few brief intervals of time. These great killings, when a large fraction of the normally well-preserved marine species suddenly disappear from the face of the planet, are called mass extinctions. They are among the most provocative features of the history of life on Earth.

What sort of global catastrophe could cause such dramatic dying? There had been hypotheses of volcanic cataclysms, sudden ice ages, even supernovas exploding near the solar system. Still, no consensus had been reached as the 1970s drew to a close. Meanwhile, several Berkeley scientists were collecting rock samples in the foothills of the Umbrian Apennines, from a road-cut in a gorge about a mile up the highway from Gubbio, Italy. Their work had a revolutionary effect on scientific thinking about the history of our planet and its relationship to the cosmos.

The largest recent mass extinction occurred 65 million years ago, at the end of the Cretaceous era of geological history. Although the Cretaceous extinction is better documented than earlier mass extinctions, rocks formed from marine sediments of the Cretaceous are still difficult to find. Many remain beneath the oceans; others lie buried far beneath the ground. Some of the most accessible exposures are near the medieval, walled town of Gubbio, Italy. You can board a small aerial tram in Gubbio and ascend to hill-top overlooks of the agricultural valley below and the Bottaccione Gorge to the north, through which the road to the east ascends. Near Gubbio, ocean-floor rocks spanning 150 million years are exposed in tilted strata. One can walk out through an ancient gate from the town and up the road, passing rocks that are progressively older. Soon, you reach a site where the rocks are 65 million years old. For decades, geologists have used their rock hammers to chip out hand samples near a dark layer of lithified or petrified clay, only about as thick as a pencil's width.

To the trained eye, a magnifying glass reveals -- below the dark band -- fossilized calcite shells of tiny, single-celled animals that floated in the surface waters of the ancient oceans. Above the dark band, the rock looks lifeless at low magnification. However, microscopes reveal a suite of entirely different, tinier fossils that are absent from the part below. Similar abrupt changes are found in 65-million-year-old rock samples collected from around the world. Nineteenth-century geologists identified this band as the boundary between the Cretaceous (abbreviated K) and the Tertiary (abbreviated T), the name given to most of geologic time since the changeover. The intervening band -- obscure to a layperson's eye until a geologist points it out -- marks the K-T boundary. This is an especially interesting boundary because dinosaur bones are found in Cretaceous rock beds but are absent in Tertiary rocks. Evidently something profound happened 65 million years ago to end the age of the dinosaurs.

The Berkeley father/son team of Luis and Walter Alvarez and their coworkers realized that the enigmatic, intervening layer of lithified clay might hold clues to what happened at the time of the K-T boundary. Their idea was to measure how fast sediments had accumulated before, during, and after the clay layer was deposited. They reasoned that meteoritic dust -- microscopic remnants of "shooting stars" -- falls to Earth at a steady rate. This makes sense, since the rate at which Earth intercepts cometary and asteroidal dust doesn't vary much, no matter what is happening to our planet's climate or geology. By measuring the meteoritic contamination in sedimentary rocks, they hoped to infer how fast the silt had been accumulating in the ocean bottoms where these sediments formed. When silting was very slow, the ocean floor clays would be relatively rich in meteoritic dust; rapid sedimentation would swamp the slowly accumulating cosmic dust. They hoped that sedimentation rates, measured this way, might provide clues about how the Earth's climate changed at the end of the Cretaceous period.

There isn't much cosmic dust; we hardly need to sweep it away from our driveways. How does one detect minute quantities of this extraterrestrial material? Nobel-Prize-winning physicist Luis Alvarez had a way. A sensitive modern chemical technique called neutron activation analysis permitted the Alvarez team to search for certain elements in the rocks at concentrations of less than 1 part in a billion. Cosmic dust contains several metals that are almost absent from the crust of the Earth. Metals are quite common among the raw materials of which planets are made, including the Earth -- but not in the crust. Early in our planet's history, iron, nickel, and much rarer metals like platinum, iridium, and osmium, sank to form the Earth's core. Less than a thousandth of the Earth's complement of such metals remains in the crust, providing a challenge to miners. So the amount of iridium in a rock provides a sensitive measure of extraterrestrial dust. If other rare "cosmic" elements can be measured in addition to iridium, and all show similar concentrations, the chemist has virtually a fingerprint for meteoritic material.

What the Alvarezes found was startling. First from K-T boundary rocks at Gubbio, and then from another K-T site in Denmark, the research team measured more than a hundred times as much iridium in the boundary layer as in the rocks immediately above and below. Could it mean that oceanic sedimentation rates dropped by a factor of 100? That was inconceivable. The alternative hypothesis was a sudden influx of iridium-bearing cosmic dust at the end of the Cretaceous period. Moreover, since the amounts of other elements that contaminated the boundary layer were in proportions similar to what's found in meteorites, extraterrestrial material was clearly implicated in the K-T mass extinction.

The Alvarez research team had sampled only two sites, both in Europe, but they made a bold extrapolation. Suppose the K-T boundary layer were enriched in iridium and cosmic elements around the entire Earth? The layer is only 1 cm thick, and the enriched concentration of iridium is a millionth of a percent. But the Earth is big, so the total extraterrestrial iridium in the K-T boundary would be 100,000 tons. In meteorites and other cosmic material, iridium makes up only one atom in a million, so the presence of 100,000 tons of iridium implies the presence of a million times as much total extraterrestrial material, or 100 billion tons. A global iridium-enriched boundary layer would require a rocky impacting object fully 10 kilometers across. The Alvarezes stopped thinking of micrometeoritic dust and began to read up on asteroids. They soon learned that Gene Shoemaker and other scientists had estimated that 10-km sized asteroids strike Earth every 50 to 100 million years. The 65-million-year-age of the K-T boundary fit perfectly.

Based on this chain of reasoning, Luis and Walter Alvarez offered a radical, even revolutionary hypothesis in a paper submitted to Science magazine in autumn 1979: the Cretaceous-Tertiary extinctions were caused by the impact of a 10 km-asteroid. Dust ejected into the stratosphere from this impact would spread around the globe, dimming sunlight worldwide and triggering an ecological catastrophe. Having concluded that an impact explained the K-T mass extinctions, the Alvarezes boldly went on to suggest that the other major mass extinctions in the fossil record could be impact catastrophes, as well.

That simple suggestion began the biggest change to our view of the evolution of life since Darwin. If the Alvarezes were right, most species became extinct not because they lost out struggling with competitors within a relatively static environment, but because asteroids from outer space capriciously struck our planet. The opening of ecological niches encouraged new species to arise and multiply. The fittest were not necessarily the biggest or fastest or smartest species, but rather those that happened to be able to survive a global impact catastrophe. The Alvarez paper, after review and revision, was published in the June 6, 1980, issue of Science, by which time a K-T iridium anomaly had been found in New Zealand, as well. They acknowledged that their data did not yet totally prove their theory, that it deserved to be tested further. How true. In fact, their theory provoked a storm of debate that continues today.

For astronomers and planetary scientists, much of the Alvarez hypothesis seemed logical and almost self-evident. Of course the Earth is struck by asteroids from time to time. Of course they would create enormous damage and distribute ejecta far and wide -- just look at the Moon! It was reasonable that the stratosphere might carry the dust around the globe, block sunlight, and cool the climate. They recalled reading about the "year without summer" in 1816 following volcanic contamination of the stratosphere, when crops failed across New England and there was famine in Asia. The Alvarez hypothesis seemed so natural, it was immediately embraced by planetary scientists, many of whom probably wondered why they hadn't thought of such a theory themselves. In a way, actually, the idea had been published before: Nobel Prize-winning cosmochemist Harold Urey had said as much in a short article in Nature magazine published in 1973. But Urey had lacked the measurements of iridium in the K-T boundary layer that made the Alvarez paper so compelling.

The Alvarez hypothesis also sounded good to Stephen Jay Gould, the Harvard evolutionist and popular columnist for Natural History magazine. He had promoted the "punctuated equilibrium" concept for evolution, a variation of Darwin's scheme in which long periods of comparative stability within an ecosystem were interrupted by times of crisis, during which most new species arose. Mass extinctions are basically the punctuational style of evolution extrapolated to a global scale. The causes of global ecological crises had been cloudy. Now Gould and the evolutionary geologists and biologists who agreed with him had a natural -- indeed inevitable -- mechanism for creating such crises.

The Alvarez hypothesis was less warmly received in other quarters. Indeed, among the geologists and paleontologists who had spent their careers studying the fossil record, trying to reconstruct the environments of the Cretaceous era, it was so much poppy-cock. Everyone knew, they asserted, that the dinosaurs and other species that became extinct at the end of the Cretaceous had declined gradually, over millions of years. A catastrophic killing based on darkness, devastation, and extinction within just a year or so was ridiculous.

The proponents of gradual hypotheses for mass extinctions each thought that their own theories remained valid. Those who had previously concluded that the extinctions were due to gradual climate change during an era of increased volcanism that peaked 60 to 70 million years ago still liked their own idea. Why look (they said) for ad hoc, out-of-this-world causes for the extinctions, when we had the answer right here in our back yard? To astronomers the idea of cosmic impacts may have seemed natural, but to most geologists it was an alien idea, inconsistent with their education and professional experience.

Resolution of the debate, it became clear, would require interdisciplinary discussions spanning a broad array of sciences. Rarely has there been any reason for astrophysicists to debate volcanologists, for cosmochemists to confront paleontologists, or for zoologists and botanists even to meet with planetary scientists. In this case, however, the issues were too profound, too close to the fundamental assumptions of these fields of science, for the issue to be swept under the rug. Traditional geologists were unwilling to let the radical new hypothesis go unchallenged, while the physicists and planetary scientists were equally unhappy to hear one of the fundamental discoveries of the Space Age -- the impact history of the solar system -- characterized by opponents as "ad hoc" (not a nice thing to say to a scientist!).

In October 1981, the first interdisciplinary meeting on the K-T boundary took place at the Snowbird ski resort, near Salt Lake City. More than a hundred scientists crowded into the small meeting room to listen to talks by leading experts on topics in diverse fields. They mingled during lunch breaks and got to size each other up. The organizers from the National Academy of Sciences and the Lunar and Planetary Institute had encouraged many of the major players in the controversy to attend, including the Alvarezes and Eugene Shoemaker, plus paleontologists like David Raup from the University of Chicago and Dale Russell of the Canadian National Museum.

By the time this meeting convened, evidence for iridium-enriched layers at the K-T boundary had been extended beyond Europe. Six research groups, in addition to the Alvarez team, had found iridium enrichment in K-T boundary-layer rock extracted from drill cores in the Atlantic and the Pacific, in North America, and elsewhere. There was no question that the Alvarez's intuitive extrapolation -- that the iridium layer had been deposited world-wide -- was by now established beyond dispute. Furthermore, microscopic glassy spherules (called microtektites), thought to be droplets from molten ejecta, were found embedded in the boundary layer rocks. And evidence accumulated that major species of microscopic plankton had died out at precisely the time the iridium and microtektites had been deposited.

However, some contradictory evidence had emerged, as well. In northern New Mexico, at least one dinosaur bone had been found well above the iridium-enhanced boundary layer. If the dinosaurs had survived into the Tertiary era, then whatever had caused the iridium enrichment and affected oceanic plankton had perhaps not been so catastrophic after all. The argument was made that even one dinosaur bone found above the K-T boundary disproved the impact hypothesis.

The question of the death of the dinosaurs, although not essential to the Alvarez impact hypothesis, is a fascinating topic to paleontologists and laypersons alike. However, it is difficult empirically to locate the exact stratum in the geological record where the last dinosaurs appear. There are relatively few surviving dinosaur bones, and the skeletons themselves are large with respect to many of the layers dated by geologists. The impact proponents replied that a few bones have been displaced by subsequent soil motion or even by being dug up and moved long after the animal's death, so we expect a scattering of the recorded locations where the bones are found. There have never been any complete skeletons of dinosaurs found above the K-T boundary, only the odd isolated bone.

It is worth noting just how rare fossils really are. Imagine a single farmer's field, which is the home to hundreds of field mice and dozens of rabbits that are born and die each year. Over the past million years, hundreds of millions of these animals would have died on this plot of land, yet it is very unlikely that the farmer will ever dig up a mouse or rabbit fossil. Each fossil found on Earth represents untold millions of creatures who lived and died, dust to dust. Indeed, it is estimated that far more species have existed on the planet than are identified by fossils, even for such large animals as dinosaurs.

One of the outcomes of the Snowbird Conference was to challenge paleontologists to test the hypothesis of abrupt extinction at the K-T boundary. Careful statistical tests were applied to define the stratigraphic location of the last occurrence of many common Cretaceous species, including some of the dinosaurs. The result of these studies is that the data are statistically consistent with a sharp extinction for many species, taking place precisely at the K-T boundary. Thus, while we cannot prove that no dinosaurs survived beyond the impact, we also cannot prove from existing data that the decline was gradual. In spite of an occasional misplaced bone, the reign of the dinosaurs appears to have ended abruptly.

The Snowbird Conference probably changed few minds at the time, but the 120 attendees (who included one of us: Chapman) departed Utah aware that the Alvarez hypothesis would be no passing fad. The interdisciplinary clash was a profound challenge to paleobiology. Indeed, it heralded another revolution in the Earth sciences as profound as the 1960's establishment of continental drift and plate tectonics, when century-old dogma of continental stability was swept away by irrefutable geophysical data. That was the view expressed by Snowbird Conference organizer Lee Silver, who said that the planetary lessons of the Space Age were finally coming home to roost on planet Earth.

Not long after the Snowbird Conference, public and scientific interest were raised even higher by the Nemesis Hypothesis. The Alvarezes had speculated that impacts might have caused other mass extinctions, but searches for tell-tale iridium at other extinctions had yielded equivocal results. Now, however, two paleontologists presented fossil (not chemical) evidence that virtually all mass extinctions had an extraterrestrial cause.

While still a graduate student at Harvard, studying under Stephen Jay Gould, Jack Sepkoski had begun assembling a huge compendium of information about marine fossils. In November 1982, after a decade of combing through the paleontological literature and databases, he published the stratigraphic ages of the first and last occurrences for more than 3,500 different fossil types. By then, Sepkoski was at the University of Chicago, working with the eminent theoretical paleontologist David Raup. Raup, a pioneer in applying mathematics to paleontology, helped Sepkoski subject the compendium to statistical tests. Almost immediately, an extraordinary pattern leapt from their graphs: major extinctions happened every 26 million years! Such a periodicity could hardly result from the complex interplay of climate and evolutionary processes, Sepkoski reasoned. It must have an extraterrestrial cause, because astronomical orbits have long-term, cyclical periodicities.

Sepkoski announced his results at a 1983 geological meeting on mass extinctions. Ordinarily, results presented at such a specialized conference would never be heard by astronomers, but the K-T debates had risen to such a pitch that many science journalists attended. Their reports, published in general science magazines and newspapers, alerted astronomers, who quickly came up with a theory to explain periodic extinctions: periodic comet showers. The idea that the history of life on Earth was shaped dominantly by the regular impacts of comets was too much for paleontologists to take. A New York Times editorialist said it for them: "Terrestrial events, like volcanic activity or changes in climate or sea levels, are the most immediate possible causes of mass extinctions. Astronomers should leave to astrologers the task of seeking the cause of earthly events in the stars."

At another Snowbird meeting in 1988, Jack Sepkoski heaped more data onto his paleontological colleagues. He had subdivided his 1983 compendium of 3,500 fossil families to trace the durations for 30,600 genera, a finer taxonomic division. The 26-million-year periodicity was even stronger. Rubbing salt in paleontologists' wounds, Sepkoski's colleague Raup formally suggested that impacts might be the sole cause of extinctions, even of minor extinctions. Few scientists would be so daring as to undercut the philosophical foundation of their own field, but as Raup wryly told the Snowbird audience, "I have tenure!"

Throughout the 1980s, a few diehards held out against any large K-T impact at all, and they raised a debater's point: Where was the crater? Failure to find it was hardly a fatal flaw to the impact theory, for if the crater had formed on the ocean floor, it might well have been destroyed by movement of the Earth's crust during the intervening 65 million years. But the Alvarez supporters sorely longed for a crater to point to. In the next chapter we recount the story of how geologists homed in on the K-T crater called Chicxulub.

3650 words (3/28/97)

CHAPTER 4: DEATH OF THE DINOSAURS

SUMMARY

More than 99 percent of the species that have lived on the Earth are extinct. Extinction is a part of biological evolution, just as death is a part of an individual's life. Roughly speaking, for every new species that appears, one disappears from the world. However, as University of Chicago paleontologist David Raup has noted, naturalists have usually concentrated on the origin of species rather than on their extinction. Darwin's famous book is The Origin of Species, not The Origin and Extinction of Species. Dramatic changes displayed in the fossil record prove that the formation of new species and extinction of old ones are discontinuous processes. In this chapter we focus on the 1980 discovery that an impact triggered the mass extinction of 65 million years ago, in which three quarters of all species (including the dinosaurs) went extinct. This simple suggestion has brought about the biggest change in our view of evolution since Darwin, and we explore its many ramifications. We also turn to the conflicts these ideas generated among scientists, recounting in detail the scientific meeting (in 1981) in which biologists, geologists, astronomers, and physicists first confronted each other to debate the role of impacts in mass extinctions. Although most of the scientific community was soon converted to the idea of impact catastrophism, a few hold out even today against these revolutionary ideas.

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CHAPTER 5: THE SMOKING GUN

Needs opening quote

The Spanish conquerors of Mexico arrived when the great Mayan civilization was in decline. In the Yucatan region, Mayans lived in small villages, and their great city, Chichén Itzá, was being reclaimed by the jungle. The fabled metropolis had been built in the 6th century A.D. near the only source of water in the region, two huge natural wells. "Chichén" in Mayan means "mouths of wells." From the same word is derived the term cenote, which refers to these and numerous other sink holes in the Yucatan formed by the collapse of underlying limestones.

The sixteenth-century Mayans regarded Chichén Itzá as a sacred place and told their conquerors of the cult of the cenote, in which sacrifices of human beings and valuables were made to the goddess of rain, thought to reside in the wells. Archaeologists have since dredged the cenotes and confirmed the legends. Yet only recently has Earth-orbiting satellite photography revealed a larger pattern to the cenotes. Many lie almost precisely along an immense, semi-circular arc, more than a hundred miles long. Most cenotes in the Yucatan, including those at Chichén Itzá, lie outside the ring. A map of the Yucatan shows that inside the cenote ring is a remarkably circular province, nearly 200 kilometers across, that is virtually devoid of cenotes. That is Chicxulub.

It is wondrous to realize that cenotes, these central features of Mayan life, are the only modern-day surface manifestation of one of the most stupendous events to happen on Earth in the last 600 million years. The ring of cenotes, we now realize, marks the outline of the crater that provided the proponents of the Alvarez hypothesis with their smoking gun. The crater is named Chicxulub for the Mayan village Puerto Chicxulub located near its center. Although Chicxulub is one of the largest impact scars on Earth, it was not easy to locate. Even satellite photos reveal nothing except the enigmatic ring of cenotes. Nor would such an impact, the equivalent of hundreds of millions of megatons of TNT, affect our planet as a whole. As a world, planet Earth would be left very much intact. There would be no change in its orbit, nor would its axis of rotation be tipped. The fossil changes at the K-T boundary arose not from any damage to the bulk of our planet, but from damage to that precious, thin, supersensitive layer of water and air that we call the ecosphere. After the publication of the Alvarez paper and the intense public interest in the "dinosaur-killing impact", several geologists began looking for the crater, and the scientific sleuthing began.

They could hardly be sure that the K-T crater still existed. A few large impact structures have been found in the older centers of continents, such as the Sudbury Basin and the Vredefordt Ring discussed in Chapter 3. However, eastern Canada and the Transvaal are among the most ancient and stable parts of the Earth's crust, like the Australian Outback. These regions have escaped much of the wrenching episodes of mountain-building, faulting, and volcanism that continually reshape the margins of the continents and other boundaries of Earth's ever shifting crustal plates. If the K-T boundary impact had struck near a continental boundary -- or, worse, on a part of the spreading ocean floor since consumed within the Earth -- there would be no chance of finding the impact scar. Some geologists were not deterred. When a murder has been committed, any good prosecutor knows that a jury may be hard to convince without a smoking gun.

By the mid-1980's, researchers were beginning to find evidence about where the K-T impact might have occurred. Some chemical clues from the boundary layer implicated the oceanic crust. Other evidence, such as impact-shocked grains of continental minerals, pointed to an impact on land. Some thought that the projectile hit on a continental margin, where continental and oceanic rocks were adjacent or overlapped each other. But such a location reduced the chances for finding a crater, since continental margins, more often than not, are geological war zones where the Earth's crustal plates grind into each other, changing topography even in our lifetime (as along the San Andreas fault in California).

One searcher was a University of Arizona graduate student, Alan Hildebrand. A Canadian, he had already spent years as a field geologist before he returned to academic studies in planetary science. Perhaps, as a geologist from a country where many of the world's largest and most ancient impact craters are preserved, he had a more open mind concerning impact as a geological process than geologists from elsewhere. In any case, Hildebrand began a hot pursuit for the K-T boundary crater.

Some scientists from the U.S. Geological Survey had focused on the southern part of North America (including Central America and the Caribbean) as a likely site for the K-T impact because of the prevalence of large shocked mineral grains in K-T boundary exposures in that general area. By 1988, Hildebrand had zeroed in on the Caribbean; he reported at the second Snowbird conference that he was intrigued by a basin northeast of Central America. The Geological Survey scientists shortly afterwards suggested western Cuba as a possible impact site, although few American geologists had visited Cuba during the preceding quarter century of hostility between the countries.

One of the most significant talks at the 1988 Snowbird conference was by Joanne Bourgeois, an expert on the geological effects of tsunamis. The Japanese word tsunami is the term scientists use for so-called tidal waves -- those shallow swells in the ocean surface that become towering, devastating waves when they run ashore. Elaborate alarm systems, including beachside sirens, warn residents of Hawaii and other coastal regions to seek higher ground when far-flung ocean detectors sense an onrushing tsunami. Most tsunamis are created when earthquakes joggle the ocean floor. But an occasional tidal wave, we are realizing, may result from an errant asteroid striking the ocean.

Bourgeois had studied ancient tsunami deposits in the geological record. She had also travelled to Chile and elsewhere to look at the effects of recent disasters. She had found evidence, from a K-T boundary region exposed at the Brazos River in Texas, of enormous tsunami deposits indicating waves 50 to 100 meters high. They meant that huge waves had sloshed back and forth in the Caribbean basin one day 65 million years ago. Tidal waves propagate poorly beyond the body of water in which they are generated, so it became clear that the K-T boundary impact must have occurred in the region we now call the Gulf of Mexico.

Alan Hildebrand was not the first K-T boundary researcher to focus on the Yucatan peninsula, but he was the first to promote it seriously as the K-T impact site. Hildebrand's determination was fueled by pursuit of his Ph.D. degree and by a race with his U.S. Geological Survey competition and other researchers who were also scouring the Caribbean for a crater. After several trips south of the border, gathering geological evidence about the impact, he finally found crushed rock samples in the Yucatan that contained shocked grains of quartz, proof of a nearby impact. It was Hildebrand who named the buried crater Chicxulub.

Hildebrand announced his conclusion that Chicxulub was the missing crater at the Geological Society of America annual meeting in 1990. It is an ancient crater at least 200 km across, buried beneath more than a kilometer of limestone sediment. Impact melt rocks, found in cores drilled through the sediments into Chicxulub, soon showed the crater to have the same age (65 million years) as the K-T boundary. Not only that, but the unusual chemistry of the rocks in which it is embedded matches that measured for the spherules and other ejecta deposits located in nearby Caribbean exposures of the K-T boundary, such as in Haiti. The fact that Chicxulub ejecta was found precisely at the K-T boundary clinched the relationship between the crater and the extinction: no age measurements by radioactive techniques was needed. Finally, melt rock from boreholes into the crater was found to be rich in iridium, the tell-tale signature of an extraterrestrial source.

A traveler to the Yucatan can see no hint of the buried crater, except for the cenotes. Today, the Yucatan coastline cuts across its center, but over its history it has sometimes been completely submerged or completely on dry land, depending on encroachments and recessions of the sea due to modest variations in sea level (the coastal margin is very shallow). To map out the crater, geologists had to probe beneath the sediments that had periodically been deposited on top of the crater.

Given the psychological importance of a "smoking gun," a skeptic might wonder why the world's largest crater was found only after the Alvarez hypothesis suggested that it might exist. Actually, Chicxulub was found three decades earlier -- found and then forgotten, in a strange tale of missed opportunities.

Much as we like to suppose that scientists work for the sake of pure knowledge, most research is funded for eminently practical reasons. The Mexican government-run petroleum company, Pemex, has long supported geological studies of Mexico to search for oil. Chicxulub was first discovered as a spin-off of oil-exploration research. About 1950 Mexican geoscientists, collaborating with the Houston geophysicist Glen Penfield, discovered an anomalous region on the northern coast of the Yucatan peninsula -- a region in which both the detailed direction of the local gravity and the strength of the magnetic field were different from surrounding areas. Though mentioned from time to time in the geological literature through the 1970's, the anomaly's significance lay dormant until 1981. In the mean time, Chicxulub began rewarding the company that funded its discovery. Subterranean rocks smashed by the great impact have held vast quantities of oil. Chicxulub may be responsible for more than a quarter of the oil reserves that have enabled Mexico to become one of the major petroleum producing countries in the world.

Glen Penfield remained intrigued by the possible crater in the Yucatan. When the Alvarez paper was published, he wondered if there was a connection. At the 51st annual international meeting of the Society of Exploration Geophysicists, Penfield collaborated with Mexican petroleum geologist Antonio Camargo to propose that the anomalies in the Earth's magnetic and gravity fields could be best explained as the subterranean signature of an enormous impact crater. The paper might have been ignored, because no one with a professional interest in impact craters or the K-T extinction heard Penfield's talk. Academic scientists involved in the K-T extinction debate, like Gene Shoemaker and Walter Alvarez, rarely attend meetings of their oil-exploring colleagues, who normally would not be studying impact craters. However, a Houston science writer who was familiar with the Alvarez work -- which had been regularly reported at scientific meetings on his beat, at NASA's Johnson Space Center in Houston -- wrote a news item about the purported Mexican crater. This story stimulated Sky & Telescope, a magazine widely read by astronomers, to publish an interview with Penfield. Soon news that there might be a large crater in the Yucatan began to circulate among the geologists studying the then-new Alvarez hypothesis.

But Chicxulub was prematurely discounted. Walter Alvarez himself considered the limited evidence about the buried crater, but he soon accepted the viewpoint of his geologist colleagues -- like Buck Sharpton, of the Lunar and Planetary Institute in Houston -- that the Yucatan structure was of volcanic origin. It is easy, with hindsight, to say that Alvarez, Sharpton, and others screwed up. But the reality of science is that one must pursue the most rewarding clues. There are too few geologists and too little time to study everything. Very little was known in 1981 about the buried Mexican crater, and more interesting K-T impact candidates were under study, including a region near the Bering Sea and another near Iceland. Besides, all of the drill-core samples from Chicxulub were thought to have been destroyed by a fire that gutted the building in which they were stored, precluding study of Chicxulub without a major new expedition.

Thus the Yucatan crater slipped through the fingers of the K-T geologists for seven more years, until other evidence shifted attention back toward the Gulf of Mexico as the most likely site for the K-T impact. But how to check the Penfield idea that there was a large crater in the Yucatan? If only the critical rocks had not been destroyed in that unfortunate fire! Finally, Alan Hildebrand found bore-hole samples that had survived the fire and were lying forgotten in a dusty desk drawer in New Orleans. These rocks contained impact-melted (and iridium-enriched) rock as well as fragments of shocked quartz, providing the critical evidence needed to clinch the identification of Chicxulub as an impact crater. In addition, the layering of the cratering ejecta confirmed that the impact was the source of the K-T boundary clay, and this temporal coincidence of the crater with the extinction clearly vindicated the impact extinction hypothesis.

How big is the Chicxulub crater? We would certainly like to know, because then we would know how large an impact must be to produce the devastating mass extinctions that ended the Cretaceous epoch. But we can't just go and measure the diameter of the cenote ring. Pictures of the Moon and other planets tell us that the largest impacts don't produce simple craters with unique rims. Instead, the rebounding waves produced by the impact freeze into a "multiring basin," with concentric rings inside the main crater rim and a succession of circular scarps outside, extending far beyond the crater rim, like a giant bulls-eye. While the primary rim is evident in pictures of an uneroded basin on the Moon's surface, it is far more difficult to decipher, from geophysical measurements of topography buried beneath hundreds of meters of subsequent rock deposits, just which circular outline marks the true rim.

Lately a debate has raged between Alan Hildebrand, who has his doctorate and is back in Ottawa working for the Canadian Geologic Survey, and Houston's Buck Sharpton. Sharpton belatedly agreed that Chicxulub is an impact structure, not a volcano. Not only was he convinced, he soon reported evidence in Science magazine indicating that Chicxulub was not just 200 km across, but about 300 km! If true, Chicxulub is so big that it could be the very biggest crater to have formed on the Earth in the last billion years, not merely the biggest in the last hundred million. That would make the K-T boundary event an exceptional event, indeed, in Earth's history. However, Hildebrand continues to argue for the 200-km crater size, and he appears to us to have a slight edge in interpreting the existing evidence. New results by a third team of geologists, reported in 1997, seem to indicate a diameter of 250 km, right between the values favored by Hildebrand and Sharpton.

* * * * * * *

In 1994, historian-of-science William Glen published a book on the mass extinction debates. He nicely summarized the evidence about Chicxulub, but he also included chapters by paleontologists who continue to argue that whatever may have happened in the Yucatan 65 million years ago had little to do with the K-T mass extinction. One contributor to Glen's book, Florida paleontologist John Briggs, even claims that there are no such things as "mass extinctions" at all. They are artifacts, he believes, of the lack of uniformity in the depositional history recorded in rock strata.

One might imagine that the multi-disciplinary detective work described in this chapter, identifying the Chicxulub crater and linking it unambiguously to the K-T boundary layer, would constitute irrefutable proof that an impact triggered the mass extinction 65 million years ago. We certainly consider the mystery solved, and the case closed. But science is not really the wholly logical endeavor it pretends to be. Instead, it is a thoroughly human enterprise. The training and beliefs of a geologist acquired throughout a lifetime career are hard to negate. So the news media are correct when they report that the K-T debate isn't wholly finished. Consensus has been achieved, but not unanimity; a few hold-out jurors remain. Meanwhile, the rest of the scientific community is moving on to ask just how -- not whether -- the Chicxulub impact wiped out species around the globe.

2750 words (5/5/97)

CHAPTER 5: THE SMOKING GUN

SUMMARY

As the 1980s drew to a close, the biggest problem faced by the impact hypothesis for the extinction of the dinosaurs was the absence of a specific crater associated with this event of 65 million years ago. The scientific detective story of the successful search for this crater is the topic of Chapter 5. It is a convoluted tale, with several false starts, crucial data lost and then relocated, and ultimately a journalist who bridged the gap between different scientific communities to close in on the crater itself, called Chicxulub. Once located, this crater turned out to be the largest impact structure on Earth, and today there is no question of its association with the mass extinction that ended the Cretaceous period. There is still an ongoing scientific debate concerning the size of the crater, however, which is the subject of the concluding paragraphs of this chapter.

With Chapter 5 we end the first part of this book, devoted to the discovery that impacts have been important for the geological and biological history of our planet. Next we turn to the issue of what actually happens in a giant cosmic impact.

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CHAPTER 6: FIRE AND ICE

DM draft (2/15/97)

[In the K-T impact,] a world-immolating fire burned vegetation to a crisp all over the planet; a stratospheric dust cloud so darkened the sky that surviving plants had trouble making a living from photosynthesis; there were worldwide freezing temperatures, torrential rains of caustic acid, massive depletion of the ozone layer, and, to top it off, after the Earth healed itself from these assaults, a prolonged greenhouse warming ... It was not a single catastrophe, but a parade of them, a concatenation of terrors. Organisms weakened by one disaster were finished off by the next. It is quite uncertain whether our civilization would survive even a considerably less energetic collision. -- Carl Sagan, in Pale Blue Dot (1994)

The empirical evidence is that the Chicxulub impact killed off the dinosaurs and most other Cretaceous forms of life, but how, exactly, was the killing accomplished? What really happened, and why did it annihilate some species while others survived? These are important scientific issues. Unless we can provide answers, the connection between impacts and extinctions remains an empirical correlation without a sound basis in understanding. Our position would be analogous to a that of a public health official who had established a statistical link between smoking and lung cancer, for example, but did not know how smoke damaged the lungs or induced premature carcinoma.

There is another reason that we want to understand the environmental effects of impacts, related to self-interest. We know that big mass extinction events are rare, coming at average intervals of tens of millions of years. The chance of such an impact taking place in our lifetimes, or of our children or grandchildren, is very small. But there must be smaller and more frequent impacts that, while falling short of a mass extinction, would still make things very difficult for life on our planet. Suppose that an impact killed only 50% of each species. There would be no mass extinction, but the calamity would still be far greater than any other natural catastrophe we can imagine. If we are to estimate the frequency of such events we require a better understanding of the physical, chemical, and biological mechanisms by which an impact can disrupt the environment. Evaluating the environmental effects of impacts of various sizes is a prerequisite to rational public policy debate about what to do, if anything, about the impact hazard today.

We have never witnessed a large impact on Earth (but we did on Jupiter, as we will describe in later chapters), nor would we wish to carry out such an experiment, but there are relevant data. Direct evidence from the K-T extinction is preserved in the fossil record. In addition, theoretically-minded scientists can carry out calculations of the environmental effects of impacts using the same models and computational tools that are employed in estimating the effects of global warming or atmospheric ozone depletion, both issues of considerable contemporary interest. We will describe in this chapter how these lines of evidence are beginning to provide some answers, although much remains to be done.

The best direct evidence on the effects of impacts comes from the K-T boundary layer itself. This band of lithified clay consists of material blasted from the Chicxulub crater as well as finer dust that was suspended in the atmosphere and later diffused downward to fall to the surface. A careful examination of this layer tells us several things about events that happened 65 million years ago, in the immediate aftermath of the Chicxulub impact. The most significant layer is quite thin, about the width of a pencil. This has been called the fireball layer or the ejecta layer; it contains most of the iridium and appears to consist primarily of the mass blasted to great heights in the initial plume of debris ejected from the explosion site.

We have referred to some of the direct effects already, but it is useful now to summarize the whole sequence of events. In the Western Hemisphere, and particularly around the Caribbean Sea, some of these effects are quite dramatic. On Haiti there are deep layers of sediment at the K-T boundary that have been churned by powerful waves and currents, presumably in the immediate aftermath of the impact. Near the Gulf Coasts of Mexico and Texas, submarine gravels were carried to heights as great as 100 m, indicating that the coast was washed by huge tidal waves or tsunami. Wherever the K-T boundary layers have been preserved in the Americas, there are abundant mineral fragments that indicate the rock near the target area was shocked and shattered in an explosion of remarkable ferocity. These sedimentary layers bear mute witness to the tremendous explosion and massive tsunami that were associated with the impact itself.

Also in the North American sediments from 65 million years ago, scientists find abundant spherules of glass called tektites. This glass was formed when solid or molten fragments of ejected material plunged into the atmosphere and were melted by the heat of entry. The tektites provide evidence that some of the material was ejected far beyond the atmosphere and then fell back to Earth and re-entered at high speed -- presumably creating an awesome display of shooting stars that filled the entire sky.

A remarkable result has come from analysis of samples of the K-T boundary layer deposited on land sites all over the world. A team of researchers from the University of Chicago found that nearly all land samples contain bits of carbon that appear to be soot from massive fires. They have estimated the total amount of carbon in the boundary layer and hence the quantity of burned vegetation that was required to generate so much soot. The startling conclusion is that the greater part of the Earths biomass -- its forests and grasslands -- must have been consumed in a global conflagration.

Finally, there is the material in the boundary layer that is in the form of fine dust particles, which could have remained suspended in the stratosphere for months before falling to the surface. These are particles no more than one micrometer -- a millionth of a meter -- in size. To date there have not been many careful measurements of the fraction of the boundary layer that is made up of such particles, but indications are that it is a few percent. We can then perform a "thought experiment" and ask ourselves what would have been the consequences if all of these micrometer-size particles had been suspended in the atmosphere following the impact. The answer is that this much dust would have blocked sunlight completely from the surface, bringing on a darkness deeper than that of the darkest night. The presence of these fine particles together with an estimate of their residence time in the atmosphere leads to the idea, as suggested in the original Alvarez paper, that the impact induced a period of profound darkness and cold that lasted several months.

Armed with this information, several scientist have set out to understand how these effects came about and to estimate how they depend on the size of the impactor. In the following discussion we lean heavily on the recent work of two young atmospheric physicists at NASA Ames Research Center, Brian Toon and Kevin Zahnle. Toon is handsome, dresses soberly, and could pass as a banker as easily as a scientist. Zahnle is tall and gangling, prematurely balding, and prefers loud t-shirts and long hair; no one could mistake him for a banker. Both brought to their work great experience in using computers to model complex physical and chemical systems.

When a comet or asteroid smashes into the Earth, its kinetic energy -- the energy of its motion -- is instantly converted into shock waves, which pulverize the rock and generate massive earthquakes, and into heat, which vaporizes the projectile and a part of the target rock and blasts it upward in an immense explosion. Beyond the limits of the crater itself, the most immediate damage results from shaking of the ground and formation of an atmospheric blast wave moving at hurricane speeds. We have some experimental data on such effects from the testing of large surface or subsurface nuclear explosives.

The most important measure of the impact is simply its energy or, in military terms, its yield. Other considerations, such as the nature of the projectile or of the target area, are secondary. When you are hit by a hammer, the important thing is the strength of the blow, not the composition of the hammer. Energy is measured in megatons, or millions of tons of TNT. The Hiroshima nuclear bomb had an energy of 0.015 megatons (15 kilotons); standard-issue fusion bombs such as those carried on intercontinental missiles are about 1 megaton; and the largest nuclear explosion in history, set off by the Soviet Union in 1959 [check], had a yield of approximately 60 megatons. Among natural impact events, we have seen that the impact that produced Meteor Crater in Arizona had an energy of about 15 megatons.

Explosion of a 15-megaton nuclear weapon in the lower atmosphere (an airburst) or on the surface will knock down frame and brick buildings to a distance of about 30 km. This result is also consistent with the 1908 Tunguska impact (which we will discuss in Chapter 9), when a rocky asteroid about 60 m across plunged to within 8 km of the surface and exploded with an estimated yield of 15 megatons. This explosion flattened trees over an area of 2000 square kilometers. As we scale up to larger energies, the radius of devastation increases approximately in proportion to the cube root of the energy. As impacts become extremely large, however, they are less efficient at causing blast damage, as more of the energy is blasted upward into space. No impact can wipe out an area much larger than a country like France or Britain by direct blast effects.

A part of the energy of impact shakes the ground, causing damage even farther from the point of impact. For energies up to a few thousand megatons, the area in which structures are shaken down by these explosion-induced earthquakes is about the same as the area in which they are blown down by the blast wave, but for yields greater than a few thousand megatons, earthquakes dominate in causing surface damage.

More than half of all impacts strike in the oceans, and here we must also consider the damage done by tsunami waves, which can travel for thousands of kilometers across the ocean to devastate distant coastlines. The potential for destruction and casualties is very great because humans have historically settled on the coast or in river estuaries that can focus the incoming waves even more.

We can illustrate these effects of blast, earthquake, and tsunami waves with a hypothetical example. Imagine an explosion taking place in New York City, at the southern end of Central Park. If it is the size of the Hiroshima bomb, 15 kilotons, buildings will be destroyed to a distance of about 3 km, primarily by the blast, devastating most of Manhattan between the Empire State Building and Columbia University. An explosion of 15 megatons (equivalent to Tunguska or a large nuclear weapon) would extend the destruction to 30 km, taking out all five boroughs and much of industrial New Jersey across the Hudson River. At 15,000 megatons (the yield of an asteroid half a mile across), the blast damage would reach to Philadelphia and New Haven, and the earthquake would severely damage structures as distant as Boston or Baltimore. If this 15,000 megaton blast took place a few hundred kilometers to the east in the Atlantic Ocean, there would be little direct damage but the resulting tsunami would strike cities along the entire U.S. East Coast with waves more than 50 m high and would even endanger low-lying coastal areas of Europe.

A large blast blows a hole in the atmosphere and ejects pulverized and melted rock into space. The ejected material may skyrocket to altitudes of thousands of kilometers, but most does not escape entirely from the Earth, and within a few minutes it falls back into the atmosphere over a wide area. The tektites, which are found in many areas of the planet in association with recent impacts, are examples of ejected material that was heated as it re-entered the atmosphere.

In the case of the K-T impact, much of the material in the global boundary layer appears to consist of such ejecta, which was so widely distributed that it rained down over much of the planet. Calculations show that this material was accelerated to high speed as it fell back, so that it was heated to thousands of degrees in the atmosphere. Countless shooting stars or meteors could have been seen at any one time if the dinosaurs chose to look. The sky turned a brilliant red-orange as a tremendous pulse of heat struck the surface. This global heat pulse was so great that the forests and grasslands ignited, and presumably most of the exposed animals perished as well.

This meteoric heat pulse was first suggested in 1990 by planetary scientist Jay Melosh of the University of Arizona as one of the major causes of the dinosaur extinction. With three colleagues, Melosh submitted a scientific paper on this subject to the British journal Nature entitled "Broiled Alive! An incendiary approach to the Cretaceous/Tertiary extinction". However, the editors thought this title was too provocative, and when they published the paper they insisted on retitling it "Ignition of global wildfires at the Cretaceous/Tertiary boundary". Nevertheless, Melosh's original title accurately captures the implications of this event.

According to Zahnle, any impact with energy greater than about 10 million megatons is likely to generate a firestorm of at least continental dimensions. In the case of the K-T event, with an energy of more than 100 million megatons, the heat pulse and subsequent fires were probably the primary immediate killing agents on land. If these calculations are correct, we now have an answer to the question: how long was required for the extinction of the dinosaurs? For most species, the answer is probably about an hour -- just long enough to be cooked by the global broiler. For animals who survived this holocaust in caves or burrows, there was broiled dinosaur meat to eat for a while, but soon everything rotted and there was nothing but charred death across the land.

Since shock waves in air are known to generate nitric acid, we might expect global acid rain in the aftermath of a major impact. Sulfuric acid would also be produced in the atmosphere from the vaporization of a comet or asteroid, both of which are relatively rich in sulfur compounds. Brian Toon, Kevin Zahnle, and others have calculated how much and where such acids would have been produced following a major impact. For impacts even as large as the K-T event (roughly 100 million megatons), the atmospheric acid is expected to be confined largely to regions near the impact and to be washed out rather rapidly by rain. This acid rain could damage surviving land plants, but there is probably not enough acid produced to destroy the chemical balance of oceans or lakes, and the problem passes quickly. Compared with other environmental damage, acidification of the atmosphere and waters of the Earth may not be a dominant concern.

There is a more serious problem with nitric acid, however. This compound is capable of reacting with ozone and destroying the Earth's protective ozone layer if sufficient quantities are formed in an impact. According to calculations, as little as one part in 10 million of acid mixed into the stratosphere would be sufficient to destroy more than 75% of the ozone layer, rendering the ozone screen biologically ineffective against solar ultraviolet light. This much nitric oxide could be injected into the stratosphere by an impact of only a few million megatons, substantially smaller than the K-T impact. Following a catastrophe as large as the K-T impact, the temporary loss of ozone would make little difference since the sunlight (ultraviolet as well as visible) would be blocked anyway by stratospheric dust (as we describe in more detail below). For impacts in the millions of megatons, however, less dust is produced, and the ozone loss could be an important contributor to the extinction of land plants and animals.

While the larger fragments of rock ejected in an explosion fall back promptly to the ground, dust grains smaller than about a micrometer in size (like the particles in cigarette smoke) would remain suspended in the stratosphere for weeks or months. Unlike most of the impact effects discussed above, we have direct experience with injection of dust into the stratosphere. Large volcanic eruptions such as Mt. St. Helens (1980) or Pinatubo (1992) produced stratospheric dust clouds that quickly spread over the entire planet, producing beautiful red sunsets and blocking up to 1% of the incoming sunlight. Earth-observing satellites have tracked these dust clouds, and sensitive measurements of temperature have detected their effects on global climate. It is not too difficult to scale up from these volcanic examples to estimate the effects of injecting much larger quantities of dust from a large impact.

Brian Toon concludes from the K-T data that at least 100,000 tons of fine dust are generated for each megaton of energy in a large impact. For the K-T event at 100 million megatons, the quantity of stratospheric dust was thus about ten trillion tons. This is sufficient dust to turn off the sunlight completely, plunging the world into darkest night. According to Toon's calculations, an impact of about one million megatons would be sufficient to reduce the brightness at noon to the level of a moonlit night, and an impact of 100,000 megatons (equivalent to an impact by a 1-km asteroid) would reduce noon light levels equivalent to a heavy overcast. All of these examples represent considerably more dust than that produced in any historic volcanic eruption.

When sunlight is blocked, surface temperatures fall and photosynthesis ceases. The atmospheric circulation changes too, as the sunlight that would otherwise have heated the surface is absorbed instead by the stratospheric dust at altitudes of 20-30 km. Understanding how the atmospheric circulation responds to such changes is a difficult scientific challenge. Fortunately, however, a good deal of work on this topic had been done in the early 1980s in the context of nuclear winter.

In 1980, at about the same time the Alvarez team was deciphering the story of the K-T impact, another group of scientists was investigating the environmental effects of a large-scale nuclear war. Any nuclear exchange would be terrible to contemplate, of course, with the direct death of hundreds of millions of people in the targeted cities and military complexes. Military and civil defense planners were well aware of these effects. What they had not considered previously, however, was the possible effect of nuclear war on the global climate.

The scientists who explored the global consequences of nuclear war had been researching the effects of volcanic eruptions on the Earth and of the global dust storms that occasionally envelop the planet Mars. The late planetary scientists Carl Sagan and James Pollack were both members of the NASA Viking Team that studied the effects of martian dust storms in the late 1970s. They joined with atmospheric scientists Richard Turco, Thomas Ackermann, and Brian Toon to study the climatic effects of nuclear war. The initials of these five authors, when arranged in the proper order, form the memorable acronym TTAPS.

The TTAPS authors concluded that the atmospheric injection of smoke from burning cities could reduce temperatures and lead to widespread crop failures throughout the world, affecting non-combatants as well as the nations which were actually targets in a nuclear exchange. They called this phenomenon nuclear winter. Because of the potential importance of their result (which said, in effect, that all nuclear war was suicidal), many other scientists became involved, and a number of their studies were directed toward understanding the effects on the atmosphere of stratospheric dust and soot. When questions about impact-injected dust arose in the 1990s, the computer programs written to model nuclear winter were available to analyze the effect on the atmosphere and climate. By analogy with nuclear winter, the climatic consequences of a large impact are sometimes called impact winter.

Nuclear winter was a controversial concept, but the application of these ideas to the impact scenario is more straightforward. Calculations of the global effects of a nuclear war depend on assumptions about how much smoke is produced by burning cities and how much of the soot reaches the stratosphere, neither of which is well known. In the case of impacts, however, there is little question about the production of pulverized rock and the injection of this dust into the stratosphere. As Comet Shoemaker-Levy's impact into Jupiter vividly demonstrated, ejecta really does rain down onto a planet's atmosphere following a large impact.

Even though the impact dust cloud would persist for only a few months, this would be sufficient following a multi-million megaton blast to lower continental temperatures by at least 10oC, leading to intermittent killing frosts at most latitudes. The damage would be greatest, of course, in the tropics and in the summer hemisphere of the planet at the time of the impact. Plants hit during the sensitive part of their growing season would be stunted or killed by the cold. In the oceans the temperature would remain close to normal, but photosynthesis would cease and the plankton that forms the base of the marine food chain would die, leading to the collapse of marine ecosystems. Only the bottom-feeders and scavengers from the deep ocean depths might be expected to survive undamaged.

As the dust settled and the atmosphere cleared, normal circulation patterns would be re-established. However, impact-induced changes in atmospheric chemistry might persist. The addition to the atmosphere of carbon dioxide and other greenhouse gases might lead to an enhanced greenhouse effect, causing the temperatures to rebound to unusually high values. Such global warming might persist for centuries before the planet fully recovered.

The many environmental effects enumerated above would combine to create a global catastrophe of unimaginable magnitude. We have described them in rather neutral scientific language, but we can imagine the horror that marked the end of the Cretaceous period on Earth.

For up to a thousand kilometers from ground-zero, no living thing is likely to have survived the initial blast, and the earthquake waves radiating from the impact site would have knocked down trees and shaken mountains over the entire Western Hemisphere. Within a few minutes walls of water bearing loads of gravel swept over the low-lying coasts and penetrated hundreds of kilometers inland. Approximately half an hour after the strike, the back-falling hot debris turned the skies of the entire planet as hot as fresh pig-iron from a blast furnace, causing the forests and prairies to burst into flame and broiling any exposed land animals. For days the conflagration contributed soot and smoke to the dust injected into the atmosphere by the blast. (Imagine the oil-field fires in Kuwait after the Gulf War expanded to global proportions). Within a week the entire world was shrouded in blackness, and the pulse of death begin to spread through the oceans. The marine food cycle collapsed, even as the scorched continents languished under a dismal blanket of ice and soot.

In such a world, it is surprising that any land species survived. And indeed, most did not. While every species of dinosaur went extinct, about half the species of mammals did also. The survival of life itself was in no danger, as sheltered environments in the deep oceans or terrestrial burrows were almost untouched, but clearly most individual living things succumbed, and the direction of evolution was profoundly altered.

When we look at the evidence for an impact catastrophe at the end of the Cretaceous, it is hard to understand how strongly this hypothesis was resisted by the majority of paleontologists. As we have already seen, desperate efforts were made to argue that volcanism or some other more familiar geological event was responsible for the death of the dinosaurs. If we judge the importance of an idea by how strongly it is resisted, then the Alvarez theory was truly revolutionary. It has forced the scientific community and the public as well to face the possibility of catastrophic change. And it has raised the interesting question of whether other mass extinctions are also due to impacts. Perhaps impacts have played a major role throughout the history of life on our planet.

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CHAPTER 7: HISTORIC IMPACTS

Early in the ninth hour of the morning of June 30, in the Village of Nizhne-Karelinsk [near Lake Baikal, Siberia], the peasants saw a body shining very brightly with a blue white-light. As the luminous body approached the ground, a vast cloud of black smoke formed, and a loud explosion, as from heavy gunfire, was heard. All the buildings shook, and at the same time a forked tongue of flame burst upward through the cloud. All the inhabitants of the village ran into the street in terror. Old women wept; everyone thought that the end of the world was upon them. -- Report in the Irkutsk newspaper Siberia, July 2, 1908, as translated in Lewis: Rain of Iron and Ice, 1996.

Collisions with asteroids and comets have played an important role in Earth history, as we have seen in previous chapters. On the long time scales of geology, impacts have repeatedly produced ecological catastrophes and influenced the course of biological evolution. We, as mammals, owe our dominant position on this planet to the impact 65 millon years ago that eliminated the dinosaurs, our chief rivals among the large land animals.

We know that, statistically, large impacts like the one that terminated the Cretaceous era occur at intervals of tens of millions of years. That is the typical interval between mass extinctions in the fossil record as well as the expected average interval between impacts by objects larger than 5 km in diameter. Most of the larger terrestrial craters discussed in Chapter 2 are also tens of millions of years old. But it is also clear to astronomers who study comets and asteroids that there are a great many more small objects than large ones, so the frequency of smaller impacts must be much higher. In Chapter 2 we showed the standard curve for estimating the typical intervals between impacts of various sizes. According to that graph, the Earth should experience an impact with the energy of the Hiroshima bomb (15 kilotons) every few months, and every few centuries we should expect to be hit by an impact with the energy of the largest nuclear weapons (15 megatons). Do these predictions make sense?

If the Earth is regularly subject to impacts the size of nuclear bombs, we should know about it. There must be historical records of such impacts, or current evidence of massive explosions in the high atmosphere for objects that break up before reaching the surface. If no such evidence exists, then the entire impact hypothesis would be in question (and our publisher would, correctly, have refused to release this book). The subject of this chapter is the contemporary and historical evidence for the continuing cosmic bombardment of our planet.

We begin with the records of recorded history and their prehistorical antecedents in mythology and oral tradition. We must approach this subject with caution, however. The ancient records are easily misinterpreted, and some investigators have gone astray when trying to base scientific conclusions on such sources. Many religious documents, in particular, were written and preserved for their religions message and not as accurate representations of historical events.

The most notorious effort in this century to extract scientific fact from ancient manuscripts is that of the Russian-American psychologist Immanuel Velikovsky. In his 1950 book Worlds in Collision, Velikovsky argued that the Earth has experienced many catastrophes in historic times caused by interplanetary collisions. Based on an extensive but uncritical reading of the ancient records, Velikovsky concluded that catastrophic myths (such as the plagues of Egypt and parting of the Red Sea in the Jewish tradition) were synchronized in widely separated cultures, thus proving that the events were global in scale. He attributed these global catastrophes to several hypothetical events: the ejection of Venus from Jupiter about five millennia ago, its subsequent near-collisions with the Earth in the second millennium BCE, and several close encounters among Venus, Mars, and Earth in the first millennium BCE. He concluded that these violent events had stopped the rotation of the Earth and reversed its direction of spin, had altered its orbit about the Sun, and had melted the surface of the Moon, among other things -- all effects that he justified from his interpretation of myth.

Velikovsky's ideas were unanimously rejected by scientists but received wide attention from the public and news media. The scientific community was accused of trying to suppress Velikovsky's ideas, but if so they were unsuccessful, since Worlds in Collision became a best seller. Only after the direct exploration of the planets by space probes contradicted Velikovsky's predictions did interest in his ideas subside, although there remains a small cult of true believers who still defend Velikovsky and even publish a journal devoted to carrying on his work. For nearly half a century, however, scientists have been suspicious of anything that smacks of "Velikovskyism".

While Velikovsky's idea of planets bouncing about the solar system in historic times is absurd, we cannot be sure that there were no cosmic catastrophes in the history of human civilization. We now know that catastrophes can be produced by the impact of relatively small asteroids and comets. Thus it may well be that information on long-ago impacts or meteorite falls can be extracted from the mythological and historical record.

Among the longest and most continuous such records are those kept by the Chinese imperial courts, which date back nearly three thousand years. Taken at face value, some of these documents appear to describe large meteorite falls, of a scale sufficient to destroy a village or even inflict major casualties on an army. In several cases whole armies were struck down by falling stones, perhaps like the Biblical account of the destruction of the Assyrian army of Sennacherib, so eloquently described in the poem by Byron which begins "The Assyrian came down like the wolf on the fold / His cohorts all gleaming with silver and gold". We know that stones (meteorites) do fall from the sky even today, and that they frequently fall in showers that result when an incoming projectile breaks up under the stress of atmospheric deceleration. The main reason to be skeptical of the Chinese reports is their uniqueness. There are no other records of similar meteorite falls in other cultures. We must ask why were so many ancient Chinese armies hit by falling stones, while nothing like that happened to larger and more recent armies, such as those deployed across Europe during the Napoleonic Wars or the World Wars of the 20th century? Is there something about ancient Chinese armies that attracted meteorites? Or are we perhaps misinterpreting the records? We do not know.

There are also descriptions in some ancient documents that might correspond to major impact explosions rather than meteorite falls. The Biblical account of the destruction of Sodom and Gomorrah can be interpreted in these terms, but since the sites of Sodom and Gomorrah have never been identified, there is no way to acquire confirming physical evidence. Some historians have associated the destruction of Atlantis in Greek myth with impacts, but the Atlantis story seems more closely allied to volcanic events, which were familiar to the inhabitants of the Mediterranean in the second millennium BCE.

A much broader look at the legends of the past has been undertaken recently by British astronomer Victor Clube and his collaborators. Clube asks how the idea arose in so many cultures during the second millennium BCE that the gods lived in the sky, and that events in the sky influenced human lives? This was the period in which astrology arose in both Eastern and Western Asia, a religion based on the notion that our lives on Earth are influenced by the positions of the stars and planets. Today this is obviously an absurd idea, but apparently astrologer-priests searched the sky for omens and portents in many cultures of the first and second millennium BCE. What was going on then?

Clube answers these questions with a sweeping hypothesis that our ancestors lived in a world where destructive impacts were much more common than today, and where events in the sky really did have profound influence on the terrestrial realm. He suggests that the breakup of a giant comet in the inner solar system created thousands of large interplanetary projectiles in addition to streams of meteorites and cosmic dust, and that the collision of some of these projectiles and dust streams with the Earth had important influences on climate, history and culture.

Unfortunately for Clube, there is little specific historical or physical data to support his theory. Interest in the sky was not universal in ancient cultures; for example, the Egyptian civilization, probably the most advanced at the time, apparently had limited interest in the sky beyond worship of the Sun. Further, the time period in which rather reliable record keeping began in China and the Mediterranean, about 2500 years ago, seems to coincide with the end of the era of cosmic catastrophes that Clube postulates. These more reliable records do not support the idea of constant danger from celestial events, which is based primarily on earlier myths. According to Clube, we "remember" this earlier catastrophic era through the cultural-religious legacy of astrology, heavenly angels (which he says were originally comets), and planets that were identified as gods like Zeus and Aries. But the fact that the catastrophes apparently stopped just when record-keeping improved is suspicious.

Clube and his collaborators (primarily British astronomers Bill Napier, Mark Bailey, and Duncan Steel) have been widely quoted in newspaper stories and television documentaries, in part because they predict an especially intense period of impacts about a thousand years in the future. Remnants of the giant comet that they believe broke up some hundred centuries ago can still be identified with a dust stream that intersects the Earth's orbit in June and November to produce the Taurid meteor showers. They propose that denser concentrations of dust plus many large objects are imbedded in this dust stream, and that the "Taurid complex" threatens the very survival of our civilization a few centuries hence when the Earth will encounter this material. If they are correct, we are currently experiencing a lull between the intense bombardment of three millennia ago and another period of chaos soon to come. The idea of an extensive Taurid complex is not accepted by most astronomers, however, and the pre-historical interpretations are vague, leaving us with little hard evidence for impacts in ancient history.

Although the ancient historical records are frustratingly ambiguous, our situation becomes much clearer when we turn to the evidence from the 20th century. The first unequivocal example of a destructive cosmic impact dates from 1908. Although it happened in a sparsely populated wilderness, this event was witnessed directly, recorded by scientific instruments worldwide, and subsequently investigated by a number of scientific expeditions.

The scene on that morning of June 30, 1908, is set in the taiga forests of central Siberia, in an outlying part of the Empire of Tsar Nicholas II. The nearest populated areas were along the newly completed Trans-Siberian Railway, which passed through the cities of Krasnoyarsk and Irkutsk, about 1000 km to the south and south-east of the impact site. The site itself was located along the Podkamennia (Stony) Tunguska River, a tributary of the mighty Yenesei River. The impact area is generally known by its shortened form as Tunguska.

The incoming object was first seen by trappers and hunters near the northern end of Lake Baikal, as a brilliant white fireball traveling in a north-west direction at high speed. Shortly after the fireball disappeared over the horizon, distant observers witnessed an explosion and "flame" and "smoke" near the apparent point of impact. At the settlement of Varanova 60 km south of the impact point, a flash of intense, burning heat was followed by an earthquake and blast that shattered windows, shook goods off the shelves in the general store, and knocked one man from his chair, sending him sprawling on the ground. A native hunter about 20 km west of "ground zero" was thrown against a tree with such violence that he was pierced by the branches and died from his wounds. While there are no other records of direct human casualties, it was later reported that several small herds of reindeer were killed and that nomad shelters near the impact were destroyed by the heat and explosion.

The sound of the Tunguska explosion was heard in Krasnoyarsk and Irkutsk, and the earthquake was recorded at the Irkutsk Magnetic and Meteorological Observatory. The attenuated atmospheric blast wave itself was picked up on a variety of sensitive instruments called microbarographs, which had recently been installed in Germany, England, and Russia. The Potsdam microbarograph station near Berlin recorded both the direct wave traveling from Tunguska and a second wave that arrived from the opposite direction nearly 24 hours later after circumnavigating the world. Most of these records went unnoticed at the time and were not located until the 1930s, after the Tunguska event was more widely recognized. From the measurements of these pressure waves, scientists estimated that the energy released into the atmosphere was at least 10 megatons.

One of the phenomena associated with the Tunguska event was the bright night skies seen over Europe following the impact. At several locations in Russia, it was reported that a newspaper could be read at night without artificial light. These strange white nights were widely reported at the time, and the effect seems to have persisted in diminished form for more than a month after the impact. The exact cause is still unknown, but it may be related to dust injected into the stratosphere and carried rapidly westward by motions in the upper atmosphere.

The Tunguska impact took place in a remote wilderness area, and it was not until many years later that the first scientific expedition was dispatched. In 1927, an expedition of the USSR Academy of Sciences, led by L.A. Kulik, reached the site of flattened forest known previously only from rumors. They discovered that the trees were uprooted over a huge area, all pointing away from the presumed location of the explosion. There was no obvious crater, but Kulik suspected that a large marshy depression 7-10 km across might be the remains of an impact structure. Forced by bad weather and lack of supplies to retreat after a cursory inspection, Kulik returned with larger expeditions in 1928 and 1929-30. Since this permafrost country becomes marshy and mosquito-infested in summer, most of the expeditions were made in cold weather; contemporary pictures show a heavily bearded Kulik wrapped in fur-lined skins, while his baggage train of loaded reindeer stretches across the snow-covered land. You had to be tough to undertake scientific expeditions in Siberia then!

Kulik's primary objective was to recover the presumed meteorite, much like the objective of Daniel Barringer, digging at the same time at Meteor Crater in Arizona. The Soviet scientists excavated in the central marshy depression and sank several boreholes in other locations, but found only peat and permafrost. No meteorite was recovered, nor any convincing evidence of craters. Kulik concluded that the explosion must have taken place before the meteorite hit. Later estimates from the distribution of downed trees suggested that the height of the explosion was about 8 km, and the energy released was about 15 megatons. (Published estimates of the force of the Tunguska blast range from 5 to 50 megatons, but we will stick to the standard value of 15 megatons, about the same as that of the Meteor Crater blast 50,000 years earlier in Arizona).

From its energy, the mass of the Tunguska impactor can be estimated at hundreds of thousands of tons, but the fate of this meteoric material has remained a mystery. Presumably it was distributed over a wide area by the airburst, and if an expedition had reached the site immediately a great deal of this fine meteoric dust could have been collected. After 19 years of rain and weathering, however, the 1927 expedition had no hope of recovering the material. It was not until 1994 that the first microscopic fragments of the Tunguska projectile were extracted from resin in trees that were exposed to the explosion.

The basic facts about the Tunguska event are rather simple: an object entered the atmosphere at high speed and descended at an angle of about 45 degrees, radiating intense heat and ending its path with an explosion at 8 km altitude equivalent to a 15-megaton nuclear weapon. The explosion knocked down trees but left no trace that could be recovered 19 years later. What could have caused this unprecedented event?

Several astronomers suggested in the 1930s that the Tunguska explosion was caused by the collision of a small comet with the Earth. At the time, comets were generally thought to be loose agglomerations of icy rubble like a flying gravel pile. It seemed reasonable to suppose that such an insubstantial body might break apart during its fiery plunge through the atmosphere. The ice and other volatile materials would be vaporized by the heat of entry, and the shattered fragments of dust or rock dispersed. Although not supported by any detailed calculations, this seemed like a reasonable scenario, and it is still to be found in most astronomy texts or other accounts of the Tunguska events.

However, there is something about Tunguska that inspires less prosaic explanations. The absence of recovered fragments augmented by the remoteness of the site has endowed the Tunguska event with an aura of mystery. In Russia even today, there are many amateur and professional scientists who disbelieve the official stories and speculate on various exotic causes of the event. It has even been noted that a disproportionate number of scientists in Soviet mental asylums seemed preoccupied with the Tunguska event.

One bizarre explanation for Tunguska surfaced after the 1947 "discovery" of unidentified flying objects by the American news media. Some people suggested that the Tunguska event was due to a crashed UFO that lost control and was destroyed upon entering the atmosphere. In some variants of this idea, the explosion was caused by the detonation of the nuclear power source of the alien spacecraft. As recently as 1994, we heard a member of the Russian Academy of Sciences present a paper in which he argued that the recovery of the forest at Tunguska showed biological anomalies similar to those of the ecology near former nuclear test sites, darkly hinting that a nuclear explosion of some sort was implicated.

Another Tunguska theory, although perhaps not a serious one, was that the impacting object was a mini-black hole. In the 1970s British physicist Steven Hawking had postulated the existence of such black holes not much larger than single atoms but containing substantial mass and energy. If such exotic things exist (and there is no evidence they do), then perhaps the impact of such an object travelling at nearly the speed of light could precipitate an explosion while leaving no physical trace of its passage.

What appears to be the correct explanation was proposed in 1992 by NASA scientists Chris Chyba and Kevin Zahnle, together with Paul Thomas of the University of Wisconsin. Chyba, who later was selected as a Presidential Fellow and honored by Time magazine as one of the 50 most promising young Americans for 1994, had investigated in his doctoral thesis the effects on the Earth of its early bombardment by comets, seeking to determine how much water and organic material comets might have brought to our planet. Zahnle, for his part, was working on mathematical models for the breakup of large meteorites. They were bothered by one part of the comet impact theory: if Tunguska was due to a comet impact, then why do we not see evidence elsewhere on Earth of recent impacts from the much more common stony projectiles? Why weren't there lots of small craters due to impacts by such mini-asteroids?

Chyba, Thomas, and Zahnle calculated what would happen if projectiles of different composition and strength, but each with 15 megatons of energy, entered the atmosphere. They developed a quantitative model for the effects of atmospheric drag on a projectile, showing how it would fragment and disperse from the stress of hypersonic entry. Depending on its strength, any object will begin to break up when the air resistance is high enough; smaller objects then disperse explosively, while larger ones hang together until they penetrate deeper or strike the surface. These calculations showed that 15-megaton comets (loosely bound volatiles and dust) come apart very high in the atmosphere, disintegrating at altitudes near 50 km. At the other extreme, 15-megaton iron objects are only slightly slowed by the atmosphere and crash, whole, into the ground. Stony materials, however, disintegrate at altitudes of 8-10 km, producing airbursts very similar to that observed at Tunguska. They concluded, therefore, that the Tunguska impactor was a common stony asteroid with a mass of half a million tons and a diameter of about 60 meters -- about the size of a 20-story building. Another team of scientists from Los Alamos National Laboratory reached the same conclusion and published their similar calculations at the same time.

The new asteroidal theory for Tunguska was also consistent with other evidence. It predicted that comets with 15 megatons of energy would burn up at such high altitudes that there would be no measurable blast wave at the ground. The more common stony projectiles would, like Tunguska, penetrate to the lower atmosphere where the explosions could do great harm, but they would make no craters. At 15 megatons, only the very rare irons would form craters. In fact, all the small craters that have been studied were formed by iron projectiles rather than stony objects. An example is Meteor Crater in Arizona, which was caused by an object with the same energy -- 15 megatons -- as Tunguska, but one made of iron rather than stone. In order to reach the surface and make a crater, a stony projectile must to be more than 100 m in diameter (energy greater than 100 megatons).

Tunguska is the prototypical contemporary impact, the only major destructive impact explosion witnessed in modern history. According to the standard frequency curve for impacts (see Chapter 2), we can expect a 15-megaton impact on Earth about once every 300 years, which translates to an average interval of nearly a millennium between events of this size on land. We are thus fortunate to have witnessed such an impact in the present century, and it is no surprise that Tunguska remains unique in history. Smaller impacts occur more frequently, however, and we should also see some evidence of these.

Probably the most dramatic impact event since Tunguska took place on February 12, 1947. Once again the target was Siberia. (The Russians often point out that they have a special interest in impacts, since their country seems to be a preferred target.) This strike occurred in the Sikote-Aline mountains of western Siberia, about 400 km from the port of Vladivostok, and it is referred to as the Sikote-Aline meteorite fall.

The projectile entered the atmosphere in broad daylight and was witnessed by many people, who reported that it was brighter than the Sun. The fireball left a dark column of dust or smoke that remained visible for many minutes after its passage. After the disappearance of the fireball, loud detonations were heard, followed by rumblings and roarings like distant thunder. Russian scientists reached the site of the impact a few days later, finding a total of 122 small craters distributed over an area about 1 km wide and 3 km long. The four largest craters were each about 20 m across, or the size of a 60-foot house lot. The craters were irregular in shape and did not resemble explosion pits, and the surrounding forest was largely untouched.

The Russian scientists recovered 12 tons of iron meteorites of all sizes, including one individual object weighing nearly 2 tons. From the appearance of the recovered fragments, they concluded that many of the meteorites had broken apart upon striking the ground and had not been disrupted in flight. They inferred that the original mass of meteoritic metal was close to 100 tons at entry, which would correspond to an energy of about 10 kilotons.

From all the evidence, we can conclude that Sikote-Aline was an iron meteorite that had only begun to break up at the time it hit the surface. Being so small, no bigger than a truck in size, the incoming body had been slowed by atmospheric friction so that it hit the ground at relatively low speed. Since nearly all of the 10 kilotons of its original energy had been dissipated in the atmosphere, there was no explosion, and the damage was largely confined to the direct points of impact of the various fragments.

A stony asteroid of comparable energy entered the atmosphere over Revelstoke National Park in Canada in 1965. As we might expect, the incoming projectile broke up in the atmosphere, spreading small fragments over a wide area. Since it was winter and fresh snow had recently fallen, scientists recovered many of these small stony fragments from the ground. From microbarograph measurements, the energy of the Revelstoke object was measured to be 12 kilotons, but only a tiny fraction of the material from the projectile was ever recovered.

In addition to these and other recorded meteorite falls, there was one impactor that, like the proverbial fish, "got away". On August 10, 1972, tourists in the Teton-Yellowstone National Parks were startled to see a bright point of light moving across the daytime sky from south to north trailing a luminous column like the contrail of a jet. Hundreds of people photographed this strange, silent intruder, and one beautiful series of 8-mm movies was also obtained. The object was tracked for hundreds of miles as it headed across the border into Canada, and it was also observed by a U.S. Defense Department satellite in space. Apparently, however, it never fell to the ground, but passed out of the atmosphere having lost only a little of its energy to atmospheric friction. Various estimates suggest that the diameter of the projectile was about 10 m (energy several hundred kilotons), and that it never got lower than about 60 km altitude. Had it come lower, it would presumably have broken up in the atmosphere and might have produced a meteorite fall like Sikote-Aline or Revelstoke.

Upper atmosphere explosions of kiloton or greater yield should be visible from space, and the standard frequency curve for impacts predicts many of these. As long ago as 1972, there were surveillance satellites in space capable of detecting an intruder like the Teton fireball. Since then these capabilities have steadily increased. During the 1991 Gulf War it was reported that surveillance satellites could detect the firing of individual scud missiles from distances of more than 1000 km. Surely these systems must also detect the atmospheric entry of fireballs, and such data can be used to check our theories about the frequency of such events and the way these objects break up in the atmosphere. However, it has not proved easy for scientists to gain access to this information.

The U.S. surveillance satellites are operated by the Air Force Space Command from locations in Colorado and California. The main complex for these activities is buried deep within Cheyenne Mountain outside Colorado Springs, Colorado, with a secondary control center at Onizuka Air Force Base in Sunnyvale, California. These are among the most sensitive and secure elements of the national defense system. Onizuka Air Force Base, known locally as the Blue Cube, is in a 9-story windowless building constructed to withstand earthquakes and natural disasters and protected from terrorist attacks by a wall and ditch that resemble the defense works of a medieval castle. Cheyenne Mountain is, as the name implies, constructed in caves dug out of the mountain and supposedly safe even against a direct nuclear hit.

The nerve center for this intelligence operation is at Falcon Air Force Base, located in the prairies east of Colorado Springs. Here, again, the level of security is daunting to the visitor. The complex is surrounded by double chain-link and razor-wire fences, and the space between the fences is swept by microwaves to detect the motion of any intruder. Heavily armed soldiers guard the gates, which are specially constructed so that no vehicle could crash through, and the staff are screened by the most advanced electronic identification systems, including one that reads the pattern of veins in the eye's retina for positive identification of each individual who enters. Behind these secure boundaries, the data from a constellation of surveillance systems are scanned for evidence of nuclear attack against the United States or its allies.

Both visible-light and infrared sensors continually scan the Earth from space, and these systems "see" all of the large meteors or fireballs that enter the atmosphere. However, the software that screens and analyzes these data has been constructed to remove signals that are not relevant to the defense mission of the orbital surveillance systems. From the military perspective, natural events such as meteors are "noise" that should be filtered out. Therefore, no systematic records have been kept of fireball data. Nevertheless, individual operators over the years have noted bright fireballs, and gradually a fair amount of information has accumulated on these events.

The first public discussion of these results did not come about until 1993, after the collapse of the Soviet Union, and the first technical papers were not published until 1994. Between 1975 and 1992, it is reported that 136 atmospheric impacts were recorded worldwide by either the nuclear burst monitoring system (visible light) or a scanning infrared sensor, both operated by the U.S. Department of Defense. Since 1992, the rate of reporting for such detections has increased. The largest event so far took place on February 1, 1994, when a cosmic projectile entered the atmosphere over the Western Pacific, within the territory of the island nation of Micronesia. The object, with an energy later estimated at about 100 kilotons (more than 5 times larger than the Hiroshima bomb), began to break up at an altitude of 35 km, and several fragments were tracked by 6 different optical and infrared sensor systems in space. The largest of these fragments penetrated to 21 km before exploding. Two eyewitnesses were later located from the island of Palau, and these fishermen reported that the final explosion was "brighter than the Sun" but that no sound or concussions were noticed. Chyba and Zahnle applied their model for the Tunguska event to this fireball and concluded that its behavior was consistent with a stony object -- basically a miniature Tunguska, consisting of a house-sized boulder only about 10 m in diameter rather than 60 m like Tunguska.

The space surveillance records are incomplete, and it is estimated that the majority of the impacts might have gone unreported by the Air Force. In 1996 a second independent record of atmospheric explosions was located in previously classified documents, this one based or arrays of low-frequency sound detectors that had been placed around the globe to "hear" nuclear test explosions. As expected, these sonic records include more than twice as many "events" as the satellite data, and they are interpreted to indicate at least one atmospheric explosion per month of yield 10 kilotons or greater.

Putting all of this information together, we can formulate a consistent picture of the current impact environment of the Earth. The most common impacts are by stony objects, with airbursts like Tunguska expected somewhere on the land area of the planet about once per millennium. The fact that history records only one Tunguska-scale event is consistent with the time and area of the Earth covered by historical records. Smaller impacting bodies (except for the rare metallic projectiles) do not penetrate to the lower atmosphere but explode harmlessly at high altitudes, and these events are being detected by surveillance satellites even though most go unnoticed by ground-based observers. Based on the data that have been released, the Earth experiences a Hiroshima-scale high-altitude atmospheric explosion several times per year, consistent with predictions by the astronomers. There is no evidence that we live in "special" times of cosmic onslaught, and the current impact rate appears to be similar to the long-term average derived from observations of the lunar craters.

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CHAPTER 7: HISTORIC IMPACTS

SUMMARY

We have seen that collisions with asteroids and comets have played an important role in Earth history. On the long time scales of geology, impacts have repeatedly produced ecological catastrophes and influenced the course of biological evolution. We, as mammals, owe our dominant position on this planet in part to the impact 65 millon years ago that eliminated the dinosaurs, our chief rivals among the large land animals. In this chapter we move from geology to history, to see the evidence that the Earth may still be subject to a cosmic bombardment. This search begins with the records of recorded history and their prehistorical antecedents in mythology and oral tradition. We look at the ideas of Immanuel Velikovsky and Victor Clube, but conclude that their interpretations of history are ambiguous and suspect. We are on more solid ground as we turn to the detailed reconstruction of the 15-megaton 1908 impact near the Tunguska River in Siberia, which leveled a thousand square miles of forest. New calculations reveal that the impactor was a stony asteroid about 60 meters in diameter. Other contemporary impact events include the 1947 Sikote-Aline and 1965 Revelstoke meteorite showers, the 1972 fireball over the Teton Mountains that skipped out of the atmosphere, and recently declassified military surveillance data on upper atmospheric explosions, which are recorded several times each month. Yes, the Earth is still a cosmic target!

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CHAPTER 8: PEARLS ON A STRING

Shoemaker-Levy 9 was not the astronomers' comet; it was the people's comet. It was the comet that had the entire population of the world watching as it hit Jupiter and wondering, "what if it had hit here instead?" -- Comet discoverer David Levy, in Three Minutes to Impact (February 1997)

No astronomer had expected to witness an interplanetary collision; prior to 1993 such events were judged too rare. Interplanetary space is immense and very empty. Imagine compressing the inner solar system to fit inside Houston's Astrodome. A ping pong ball slowly floats over the stands. That's Jupiter. Beyond second base, at the edge of the outfield, is a brilliant basketball-sized sphere -- the Sun. A bluish B.B. is behind home plate. That is our Earth, to scale. The larger asteroids are but a few hundred motes of dust, hovering over the seats. Comets, like Hale-Bopp, which make beautiful apparitions in our skies, are little more than isolated molecules. In such a vast emptiness, it is difficult to believe that worlds can collide. Yet during the near eternity that our solar system has existed, the circulating comet-molecules occasionally try to occupy those same few cubic millimeters as our B.B. -- a microscopic pock-mark is then made on the surface of the B.B., and Earth's ecosystem is changed forever.

Since Jupiter is a bigger target than Earth, it is struck much more frequently by cosmic projectiles -- but still very rarely by human standards. The remarkable events of July 1994, when about twenty fragments of Comet Shoemaker-Levy 9 crashed into the giant planet, may have represented the largest jovian collision since Rome fell. How lucky we were to see it in our lifetimes! How fortunate that telescopic instruments had been devised and spacecraft launched, so that we could study this exceptional event. We have recounted how the repetition of such rare impacts over the aeons has gradually cratered the planets, shaped the surface of our own world, and affected the evolution of life. But to actually watch an impact...what a treat!

Gene Shoemaker was among the scientists who had only dreamed of witnessing a major impact. In the early 1970s, he had retrained himself as an astronomer in order to discover near-Earth asteroids and comets. For two decades, Gene and his wife Carolyn regularly drove south from their home in Flagstaff, Arizona, through the Prescott Mountains, across the barren Sonoran and Mohave deserts to Palomar Mountain, northeast of San Diego. There stands the dome of the 200-inch telescope, for years the world's largest. Gene first began to use Palomar's much smaller 18-inch wide-field telescope to survey asteroids in the early 1970's, as we told in Chapter 2. Until he Carolyn said good-bye to Palomar in 1995, they conducted a multi-faceted search program. While Carolyn racked up comet discoveries (she has found more comets than any other woman in history), Gene concentrated on finding Earth-approaching asteroids.

The nights are long and cold at Palomar, and the daytime work of developing and searching the films is tedious. The Shoemakers' funding has been minimal, so they engaged volunteer assistants, who donated their time to the cause. During the 1990's, their prime assistant was amateur astronomer David Levy. Levy, who grew up in Montreal and earned a degree in literature, was always interested in astronomy. As a young man, he moved to Arizona to pursue his hobby seriously under the clear skies of the desert. That is where he developed his "telescope farm," which at one time comprised about 50 telescopes, ranging from toys made from toilet paper cardboard tubes to substantial instruments he still uses nightly to sweep the skies for comets.

Years before Levy joined forces with the Shoemakers, he had discovered more than half a dozen comets with his own eyes and had made a name for himself throughout the world of amateur -- and professional -- astronomy. Levy was just the right person to become part of the Shoemaker-Levy media event in 1994, because he had already honed his skills as a public lecturer. Often supplemented by taped music, Levy translates the arcane world of astronomy with soft-spoken words that touch a layperson's sense of wonder.

In March 1993, a major spring rainstorm was approaching the California coast, as Gene, Carolyn, David, and a fourth assistant began yet another observing run on Palomar Mountain. A canopy of wispy cirrus clouds had spread over the whole southwest, and the forecast was dismal for the next several nights. Clouds are the usual bane of astronomers, forcing them to close up their dome, play a little pool, and finally head for bed if the skies aren't clear by 3 a.m.

Comet Shoemaker-Levy 9 might never have been discovered, certainly not at Palomar, were it not for a fluke accident. The team had found, to their chagrin, that someone had opened their box of specially prepared films, and much of their supply had been light-struck. They had another box of good film, but it is expensive (and the Shoemakers had no NASA funding at all in 1993), so they would not have wasted good film on a cirrusy night. David Levy suggested, however, that they might as well try some film from the middle of the light-struck supply, where only the edges had been affected. It was on one of these films, the next day, that sharp-eyed Carolyn found the unique "squashed comet" that was to bear their names.

Discovery reflects ineffable qualities of the human brain. It is a

wonder that she recognized the elongated photographic smudge as a comet at all, for it did not look like any of the other 30-or-so comets she had discovered, nor any other comet ever witnessed by human-kind. Indeed, other observers had captured the object on their films and failed to recognize it for what it was. But the three observers soon concluded that it was not a fixed object (like a galaxy) in the night-time sky. For confirmation they phoned a colleague observing with Tom Gehrels' Spacewatch Telescope on Kitt Peak several hundred miles to the east, where the clouds had not yet thickened.

Astronomers around the world learned of the discovery within days through a bulletin service run by the International Astronomical Union. Within a week, photographs taken by larger telescopes revealed an extraordinary "string of pearls": about 20 separate little comets, whose images practically touched each other in telescopic photos but which were actually spread across hundreds of thousands of kilometers of space. A handful of other comets had been seen to split into a few pieces, but S-L 9 was exceptional indeed. Its appearance alone would have made it one of the more famous comets of recent times, but there was much, much more to come.

Soon, enough measurements had been made to specify S-L 9's motion through the heavens. The comet was close to Jupiter's position in the sky, and astronomers saw that it shared the giant planet's motion against the background stars. In other words, S-L 9 was accompanying Jupiter in the giant planet's orbit around the Sun, in a long slow orbit around the planet. (This is not wholly unprecedented -- a few other comets have been seen in temporary orbits around Jupiter.) It wasn't long before S-L 9's path could be pinpointed accurately enough to project its motion into the future, and backwards in time, as well.

Such orbit predictions are the job of Brian Marsden, who runs the International Astronomical Union bulletin service. Marsden is a bouncy sandy-haired man who retains his strong British accent even after decades of living in Massachusetts. Using the position observations reported by many astronomers from around the world, Marsden used his computer programs to run the comet backwards in time, hoping to find some clue about its remarkable string-of-pearls configuration. He found that, about a year beforehand, S-L 9 had passed very near Jupiter, just a sixth of Jupiter's diameter above its cloud tops. At that distance, Jupiter's gravity pulling on the nearer side of the comet more strongly than on the far side must have exceeded the comet's self-gravity holding it together. A solid, cohesive body might well have zoomed by Jupiter unaffected, but S-L 9 was evidently more loosely held together, like a gravel pile. Unable to resist the mighty planet's forces, the comet was literally torn to shreds. With further analysis, the motions of each of the dozen brightest fragments of S-L 9 could be individually projected back to a single date -- July 7, 1992 -- where they converged to the same spot, the point of disruption next to Jupiter. Incontrovertibly, this once-whole comet, which had orbited Jupiter since 1929, had accidentally ventured too close and been torn asunder.

It was the comet's disruption that made it visible to the Palomar observers. Wide-field telescopes, like the one used by the Shoemakers, can search large patches of sky at once (as big as the bowl of the Big Dipper), but they have the drawback of being able to do so only for stars (and comets) that are relatively bright. The limit of the 18-inch Palomar telescope isn't so shabby, around ten-thousand times fainter than the faintest star a person can see in a dark, moonless sky. But as a tidy, modest sized object perhaps a mile across, far-away S-L 9 would have been simply too faint to see with such a search telescope. Indeed, subsequent perusal of photographic archives, even though we now know precisely which parts of which photographs to search, has turned up no trace of the comet in the years before break-up. However, the disruption of S-L 9 made it much brighter. Not only the twenty pieces of the comet train, but innumerable smaller pieces, ranging down to clouds of dust particles, all caught the sunlight and reflected it toward Earth, producing an apparent "smudge" that was bright enough for Carolyn Shoemaker to recognize on the light-fogged film.

The discovery of Comet S-L 9 also solved a mystery that had puzzled planetary geologists ever since the Voyager flybys of Jupiter in 1979. During those historic exploratory sweeps through the jovian system, the spacecraft cameras had photographed about a dozen long "crater chains" on Jupiter's outer big moon Callisto, and another two on the next inner moon, Ganymede. No one had ever figured out a satisfactory explanation for these strings of small craters precisely aligned, marching across the topography of Callisto and Ganymede. Then came the photographs of S-L 9, and a clever young planetary geologist, Paul Schenk, put the story together. He suggested that other comets had passed this way before and been pulled apart into a string of pearls, just like S-L 9. Of the numerous comets that had been disrupted over past aeons, a few would have accidently intersected one of the large jovian moons during the days following disruption; the fragments, colliding with machine-gun-like regularity across the moon's surface, would have replicated on the landscape the string-of-pearls arrangement of S-L 9 in the heavens. And indeed, the number of craters and their spacing and alignment perfectly match the calculations for S-L 9. This comet was not unique, but just the most recent example of something that had been happening throughout solar system history.

Well over 99.9% of such broken-up comets would not have hit one of Jupiter's moons immediately. Most of them must have missed, and subsequently some of them encountered Jupiter like S-L 9, while others were dispersed though the inner solar system -- eventually, perhaps, to collide with a planet like Earth. Thus if we can understand S-L 9's break-up and subsequent evolution, it would help us to understand not only what happens when a disrupted comet strikes Jupiter (S-L 9's fate) but also how comet fragments are formed that threaten us here on Earth.

Interesting as it was to project S-L 9's motion backwards in time to learn why it was now in twenty pieces, Brian Marsden's responsibility was to turn his attention to predicting where the comet would be in the future. It was about a month after the initial discovery of S-L 9 when he came to the startling realization that the comet train would actually crash directly into Jupiter a little more than a year later, in July 1994.

Following the 1992 disruption, the broken comet sailed almost directly away from Jupiter's southern hemisphere. Like a baseball thrown up overhead, S-L 9 was about to turn the corner in July 1993 and fall straight back down toward Jupiter. It would make a series of impacts as each fragment stuck in turn, one following another over a period of a week from July 16 to 21, 1994. Marsden issued his prediction, with characteristic British understatement, in one of his regular announcements: "[Our] initial estimate is that more than half the nuclear train could collide with Jupiter -- over an interval approaching three days . . . [although] it must be emphasized that a collision [of the entire train of fragments] with Jupiter is not assured . . ."

Astronomy is the quintessential science of prediction. Over millennia, its practitioners had gained respect by predicting the times of sunrise and sunset, eclipses, and spectacular appearances of comets. In 1910, they even predicted that Earth would pass through the near-vacuum of Comet Halley's tail. But never before had one solar system body been predicted to collide with another. One would no more expect to witness worlds colliding than one would expect to watch an Ice Age come. Astronomers around the globe began to make plans.

Not everyone initially recognized the importance of the prediction. But NASA scientists Kevin Zahnle and Mordacai Mark MacLow were almost too fast: they quickly modified their existing computer programs to predict the depth of penetration of the comet fragments and the dimensions of the expected explosion plumes, and they rushed a technical paper to the international scientific journal Nature within a few weeks of Marsden's announcement. Caught unprepared, the editors of Nature rejected the paper out of hand as too speculative and "not of interest to the readers of Nature." (Nature quickly realized it had missed the mark, so to speak, and they solicited an account of the impact prospects from one of us [Chapman] for a subsequent issue.) Although unpublished, the Zahnle-MacLow calculations quickly became the basis for planning observations to be made by the Galileo spacecraft (as we describe in detail below), and dog-eared copies of the Zahnle-MacLow manuscript circulated for months around the NASA Jet Propulsion Lab (JPL) and other research institutions.

The comet impact presented a unique chance for astronomers to plan ahead for a world-wide campaign. A year's notice was just what was needed. There was time for funding agencies, like the National Science Foundation, to solicit proposals, evaluate them, and award funds so that scientists could design and build new instruments and mount observing campaigns. There was time to organize teams of observers to apply for time on telescopes around the world -- telescopes that are usually scheduled at least half-a-year in advance. There was time for theoreticians to construct computer programs designed to simulate a Jupiter comet crash, and to make predictions about observable phenomena. When a unique event is about to occur, one wants to have thought of every possible outcome in advance, so that nothing is overlooked.

In order to keep track of S-L 9, astronomers named the individual mini-comets for the letters of the alphabet. Fragment A would be the first to hit, on the evening of Saturday, July 16th, as seen from Europe (mid-day in the U.S.); the show would conclude with W's demise the following Thursday evening, as seen from Pacific longitudes. Although individual fragments brightened and faded, there were always about 20 fragments to track, each with its own short tail, with the whole array immersed in a dusty cloud. The individual mini-comets were very small, however, and their sizes and their chemical composition remained matters of conjecture up to the time of impact.

The year's advance notice was critical for using the two most important observatories -- the Hubble Space Telescope (HST) and the Galileo spacecraft. Hubble, in orbit around the Earth, is not troubled by clouds or the blurring effect of the Earth's atmosphere. To be sure, its own optics were fuzzy, but astronauts fixed its vision seven months before the comet crash, enabling it to take the sharpest pictures of any telescope in the world. One year was actually short notice, but eventually xx [check] hours of observing time were scheduled on HST during comet-crash week, providing the most detailed views of what transpired on Jupiter.

The other observatory requiring the full advance notice -- Galileo -- is a modest one in size, but it had a unique advantage. A NASA spacecraft, launched by the space shuttle Discovery in October 1989, Galileo was en route to Jupiter on the last leg of a six-year journey. It was designed to drop a spinning probe into Jupiter's atmosphere, after which the rest of the spacecraft would go into orbit to study the giant planet's moons for two years. Although by July 1994 Galileo would be three times closer to Jupiter than all Earth-based observatories, that was not its chief advantage -- big Earth-based telescopes and HST could more than make up for their lack in proximity to Jupiter by their physical size. Galileo's advantage lay in the direction from which it could watch the crash: by great good fortune, it would see the pre-dawn longitudes where all the fragments would hit. Marsden's calculations showed that S-L 9 was going to crash into Jupiter's back side, over the planet's horizon as seen from Earth. One of us (Chapman) had the dubious distinction of being the first to tell Gene Shoemaker of that sad fact. Nature had beaten all the odds by getting a comet to crash into Jupiter for us -- and then we lost the final flip-of-the-coin about whether or not it would hit on the front side where we all could see it. Galileo alone was positioned to watch.

Like Hubble, Galileo is a complex facility, run by a team of hundreds of engineers. Although they were provided no more funds by NASA for the task, Galileo Project officials at JPL boldly decided to try observing the comet crash with only a year's advance warning. Normally, Galileo's commands were programmed two years in advance. Changing the interplanetary cruise operations for a comet watch campaign was an even more daunting task, however, for the spacecraft was seriously crippled by an earlier failure of its large, high-gain antenna. Without the high-gain antenna, Galileo was reduced to dribbling data back to Earth over a second, tiny antenna, designed for rudimentary communications only. After passing through the asteroid belt, Galileo would be so far from Earth that it could tell its story at the rate of only one character per second, slower than the transmission rate of a telegrapher sending Morse code. Since a picture is worth ten thousand words (actually more), Galileo could hardly send back a few dozen pictures at that slow rate. So JPL engineers were working on improving the effective data transmission rate, by designing data-compression software, by building and using larger antennas on Earth to receive the weak signals, and in every way they could other than shaking loose the main antenna. By 1996, when Galileo began looping among Jupiter's moons, engineers managed to have enough fixes in place so that the cream of the data could be returned to Earth.

Through heroic efforts, some of the Galileo software under development was readied, much earlier than planned, in time for the comet crash. The observing sequences were changed to enable Galileo to take multiple exposures on the same frame. That saved the day. Otherwise Galileo's camera would have filled the spacecraft's tape recorder with only xx [check] frames, which might have missed recording any impacts at all. In June 1994, when instructions for the final observing sequence had to be transmitted to the spacecraft, the impact times were known only to within the nearest half hour. With multiple exposure capability, however, as many as 64 images of Jupiter could be packed into a single frame. Time-lapse pictures could be taken for a whole hour around the predicted time of impact, and then later -- after Earth-based observers told Galileo engineers the precise time an impact actually occurred -- the recorded data bits could be dribbled back from just a single frame.

Astronomers planning to observe from mountain-tops around the world also wanted the best predictions of the impact times, so that they could develop minute-by-minute plans to observe most efficiently. For Galileo, however, with its constrained capabilities, good predictions were not just desirable but absolutely essential for getting the proper data recorded on its tape recorder. Good predictions involved a worldwide campaign to measure the positions of the evolving comet fragments as they headed for their climatic impacts. Two JPL scientists, Donald Yeomans and Paul Chodas, put all of the data into a sophisticated computer program; every week or so during late spring of 1994, they posted on the Internet their revised predictions for each fragment. The Galileo Project was the prime beneficiary, but the Yeomans/Chodas predictions also shaped travel plans as astronomers scattered to observatories around the Earth.

For most astronomers in the Northern Hemisphere, Jupiter would be low in the southwestern sky after sunset, providing a couple hours of good viewing each night of comet crash week. It was a bit better in southern latitudes, but only two or three impacts could be witnessed from any particular longitude on Earth. Given the vagaries of weather, no single observatory could be counted on to cover an impact. Furthermore, there were many types of observations to make -- for example pictures as well as spectra at visible, infrared, and radio wavelengths -- which can hardly be obtained at once from any particular observatory. So nearly every telescope on the planet was scheduled for comet crash observations, and portable telescopes were shipped to remote locations to cover longitudes not well served by in-the-ground observatories.

One observing team ventured to Renunion Island, in the western Indian Ocean. Another trucked delicate instruments for days, at slow speeds, across pot-holed roads into a mountain range in Baja California. Still others readied specialized equipment at every dome in the "telescope city" that has been built atop 14,000 foot Mauna Kea on the Big Island of Hawaii. A special team prepared to fly NASA's Kuiper Airborne Observatory to the Southern Pacific, where they would observe some of the comet impacts through a hatch in the side of the C-141 aircraft from an altitude of 40,000 feet -- far above most of the Earth's murky atmosphere. In the bitter cold and round-the-clock darkness of Antarctic winter, a University of Chicago team set up operations at the South Pole; if the snow stopped blowing, they hoped to see every one of the fragment impacts.

Theoreticians worked overtime trying to predict what would be seen. At Sandia National Laboratory, the world's most massively parallel computer had been put to work calculating a visual presentation of the physics of a typical impact. At a March scientific conference, scientists cautiously suggested that the debris plumes from some of the larger, later fragment impacts might rise high enough above Jupiter's back side to peek over its horizon, as seen from Earth. There was hope, after all, of seeing something of the explosion from observatories other than the well-placed Galileo spacecraft. Though hopes were raised, few astronomers counted on it.

More likely, they thought, the brilliant cometary meteors plunging through Jupiter's atmosphere at 60 kilometers per second would produce flashes so blinding -- though shielded from view from Earth's direction -- that Jupiter's moons would light up by reflection. Therefore plans were made to monitor the brightness of Jupiter's moons at the predicted times of impacts to see if their already sunlit surfaces would briefly brighten for the few seconds as a fragment scorched its way down into Jupiter's clouds. Then, according to theoretical modelers, each comet fragment would disintegrate, far below the clouds, and explode with the force of millions of megatons of TNT. A superheated bubble of jovian atmosphere would be expelled back up above the clouds, and erupt into space. This so-called fireball was expected to appear 10 or 20 seconds after the meteor struck, perhaps making an even brighter reflection off Jupiter's moons.

During the half hour after each impact, Jupiter's fast axial rotation would carry the impact point around into sunlight and direct view from Earth. Then, and only then, could the power of modern astronomical instrumentation be focused on analyzing what had happened. Nobody knew for sure what to expect. Even after a year of study, astronomers disputed how big the comet fragments might be. Some, analyzing the break-up mechanics, thought they might be only a few hundred meters across. Others, analyzing HST pictures, thought some fragments might exceed 4 kilometers across. If the HST team was correct, the series of impacts would add up to a yield equivalent of the K-T impact on Earth 65 million years ago.

Surely, the optimists said, there would be some chemical changes to be seen by the world's most sensitive astronomical spectrographs. Surely, at least to Hubble's sharp view, there would be some changes in the visible clouds -- some new "spots" on Jupiter's belted face. A few observers even hoped to see waves spreading away from the target points, like ripples from a stone dropped in a pond. If jovian seismic waves could be detected, after Jupiter was "rung" like a bell by the impact explosions, they would provide invaluable knowledge about the interior of the solar system's largest planet.

Surely, if millions -- perhaps hundreds of millions -- of megatons of energy were deposited in Jupiter's atmosphere, all near the same latitude of about 45 degrees south, astronomers would see some perceptible changes during the ensuing half hour. For example, stratospheric hazes might form and spread, possibly forming a bright patch that even amateur astronomers could see during subsequent weeks. That was the optimistic view. Astronomers' articles and public statements began to capture the interest of the news media and the broader public. The Nature Company signed on David Levy to promote its telescopes and other astronomical paraphernalia as well as a new book Levy had written. Amateur astronomers practiced looking at Jupiter through their backyard scopes, so they would know its pre-crash appearance in the off-chance the comet crash changed it somehow. CNN, PBS, and ABC's Nightline made advance preparations for televised specials in the middle of comet crash week.

Other astronomers grew nervous, however. They had been burned by comets in the past. In the 1970's, a careless prediction was issued that a faint new comet would brighten into the "comet of the century." Comet Kohoutek, they said, would shine brightly enough to be seen in the daytime sky, yet it had proved to be a dud. Comet Halley itself had disappointed the public during its long-awaited return in 1986. JPL comet expert Paul Weissman made world-wide headlines a few days before fragment A was to hit. Fearing that Shoemaker-Levy 9's crash into Jupiter would prove to be another cometary dud, he predicted a "cosmic fizzle."

4600 words (5/6/97)

CHAPTER 8: PEARLS ON A STRING

SUMMARY

Before 1992, no scientist had expected to witness an interplanetary collision in our lifetime. Remarkably, however, our generation was lucky enough to be at the right place at the right time. We describe how Gene and Carolyn Shoemaker, with amateur astronomer David Levy, discovered the comet that bore their name (Shoemaker-Levy 9, or S-L 9), how it was torn apart by the gravity of Jupiter, and how astronomers calculated that the 20 fragments of this object would crash into Jupiter during one exciting week in July 1994. From first-hand experience we describe the reaction of the astronomers, and tell how major space systems such as the Hubble Space Telescope and the Jupiter-bound Galileo spacecraft were brought into the world-wide campaign to record these impact events. Other astronomers fanned out across the world, to mount the most concentrated observing effort in history. Yet there remained many uncertainties, and as late as one week before the impacts, some astronomers predicted a cosmic fizzle.

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CHAPTER 9: THE JUPITER COMET CRASH

The solar system no longer seems quite so far away as it did before July 1994. Here we are, close to the edge, protected from the true immensity of the universe by a thin blue line. A day will surely come when the sheltering sky is torn apart with a power that beggars the imagination. It has happened before. Ask any dinosaur, if you can find one. This is a dangerous place. -- NASA physicist Kevin Zahnle, quoted in Ferris: Is This the End? (New Yorker, Jan 27 1997)

It was lunch-hour in the dining hall at Kitt Peak National Observatory on Saturday, July 16, 1994. Normally, the southern Arizona observatory closes for the summer due to budgetary shortfalls, but an exception had been made for Comet Shoemaker-Levy 9. A group of scientists were comparing notes: MIT observers, several staff from the University of Arizona's Steward Observatory, plus other astronomers awarded time on the National Observatory's telescopes. They lingered after the meal to converse about the evening ahead, when the second comet fragment (named "B") was scheduled to hit Jupiter.

Despite mostly cloudy skies, the evening weather forecast provided a glimmer of hope. One of us (Chapman) had been assigned the 2.1-m (84-inch) telescope. During the previous evening, he and his assistants had ironed out several bugs in the observing procedures. Our hope was that when fragment B struck during the fading twilight, there might be a measurable enhancement in the brightness of Jupiter's inner moons, reflecting the brilliant meteor trail and subsequent explosion that would be hidden from our direct view behind Jupiter's horizon. Or, if we were very lucky, a plume from the impact might ascend high enough to be visible at the edge of Jupiter itself.

While some of the observers returned to their domes to give their instruments final check-outs, several remained in the lunch room shortly before 2 pm when someone returned to report a rumor: the impact of fragment A had actually been seen! We rushed back to our own dome, fired up our lap-top computer, and logged onto the Internet. An electronic-mail "exploder" had been set up in a University of Maryland computer, and several hundred would-be comet crash observers had signed up. Any one of us could send a message to the Maryland address and it would be instantly copied into the e-mail boxes of all the other participants.

The historic first report on the comet crash came from the Calar Alto Observatory in Spain. An international team of observers at this Spanish-German facility used a camera sensitive in the infrared (wavelength of 2.3 micrometers). Their electronic report was succinct: "Impact A was observed with the 3.5 m telescope at Calar Alto using the MAGIC camera. The plume appeared at about nominal position over the [edge] at around 20:18 UT. It was observed in 2.3 micron methane band filter brighter than Io."

What amazed the astronomers at Kitt Peak was the last sentence of the Calar Alto report, which said that the flash of impact A was brighter than Jupiter's moon Io. For astronomers, that is really bright! As the Sun dropped in the Arizona sky, additional reports of success were posted from observatories in Europe, the Canary Islands, South America, and even the South Pole -- all situated in the narrow strip of Earth where Jupiter had been above the horizon and observable in dark skies when fragment A struck Jupiter. As the Sun set in Arizona, half an hour before B's scheduled impact, we nervously inserted filters into our instrument and began tracking Jupiter, which -- we could see through the dome slit -- was fading in and out as clouds drifted past. Then we noticed that our telescope operator, who is responsible for moving our telescope, had posted a large, orange (colorized) image on his computer console. It was an infrared picture of Jupiter taken at the European Southern Observatory (ESO). Abutting the edge of Jupiter was an enormous bright "thing" -- the overexposed ejecta plume of fragment A. Soon he had retrieved from the World Wide Web a similar image posted by the South Pole observers. That was when it hit home to us that we were in for quite a show.

The Internet was the communications medium. Before comet crash week was over, literally millions of requests for images were made to accumulating archives of S-L 9 data, and the information superhighway slowed to a crawl. Yet this new system worked well enough to let hundreds of thousands of amateur astronomers, computer hobbyists, and media representatives look over the astronomers' shoulders as the drama unfolded.

We returned to monitoring our instruments but saw no hint of the impact of fragment B near its scheduled time, 7 hours after the excitement in Spain. Logging onto the Internet, we read similar negative reports from other observatories. Fragment B must have been a will-o'-the-wisp, a cloud of dust (making it visible from Earth) but lacking bigger chunks like those in Fragment A. We had missed our only chance to measure one of the S-L 9 impacts. But as we drove down the switchbacks on the mountain road, our vicarious exhilaration about the spectacular impact of fragment A crowded out any disappointment that the prime impact predicted for observers at our longitude had been a washout.

For most people, Comet crash week will be remembered for spectacular infrared pictures of glowing impact "flashes," for the televised cork-popping of champagne at the Space Telescope Science Institute when the first Hubble image of plume A was posted, and for the excitement of astronomers and laypeople alike about the unexpectedly dazzling show from Jupiter. Many people all over the world could share the elation of Gene Shoemaker, Carolyn Shoemaker, and David Levy as "their" comet made history. In retrospect, the circumstances of the come crash could hardly have been better, unless it had happened a year later when the Galileo spacecraft was in orbit around Jupiter.

Seeing each impact in silhouette on the edge of Jupiter turned out to be a lucky break. As demonstrated by Hubble's portrait of plume A ballooning into view, the eruptions were so spectacular that they quickly soared above Jupiter's edge anyway. As debris was propelled upward and then arched ballistically back down toward Jupiter's stratosphere, the plume was viewed against the blackness of space. Had we instead been looking down on a plume projected against Jupiter's sunlit clouds, we would hardly have seen its shape -- indeed it might have been totally lost in Jupiter's glare.

We were also lucky that the impacts struck Jupiter's pre-dawn longitudes, rather than Jupiter's post-twilight side. By the time the ejecta were cascading back down into Jupiter's stratosphere, the planet's rapid (10-hour period) rotation had carried the impact sites around into direct view, allowing astronomers to watch the awesome re-impact of the debris into Jupiter's stratosphere. As Jupiter's rotation carried the impact points further into sunlight and more directly across Jupiter's Earth-facing side, observers were able to study how the dramatic dark impact scars evolved during the first five hours after each impact. Waves spreading out from ground-zero looked like bulls-eyes; their velocities could be measured for hours until they faded from view. The wave velocities may provide clues about Jupiter's atmosphere far below the visible clouds, where the deepest effects of the explosions set the planet's atmosphere reverberating. And spectroscopists could study the rapidly changing chemistry of Jupiter's stratosphere in the immediate aftermath of its contamination by both cometary debris and by gases dredged up from far beneath Jupiter's ammonia cloud deck.

We were lucky that S-L 9 hit where it did, but the geometry was confusing, nevertheless. It took many months for observers to sort out what it was that they had seen. The stunning pictures of what appeared to be huge explosions on Jupiter (misleadingly called "flashes"), shown on television and printed in news magazines, were not really the impact explosions at all. These infrared flares, which lasted for many minutes and peaked more than a quarter hour after each impact, were actually the re-impacts of the erupted plumes back into Jupiter's atmosphere. The debris plummeted down, pulled by Jupiter's immense gravity, at about 10 km/s. Over regions larger than the entire planet Earth, stratospheric temperatures rose to more than 500oC. Such heat is unheard of in Jupiter's frigid part of the solar system, and the sensitive infrared detectors mounted on the Earth's largest telescopes literally went off scale as these regions came into direct view.

This phenomenon shouldn't have caught everyone by surprise, but it did. As we described in Chapter 6, several years earlier Jay Melosh had published this exact scenario for broiling the dinosaurs alive, by the heat of re-impacting K-T ejecta. But the physicists who used the world's most capable computers to predict what might happen on Jupiter were best equipped to study the first seconds and minutes of impact, so that's what they concentrated on, neglecting the development of the plume and its fall back into the jovian atmosphere. Those who did think about the later stages of the impacts failed to allow for the re-condensation of the hot plume gases into solid dust particles, which in turn were heated to incandescence during re-impact. We had the knowledge all along to have predicted late-stage infrared flares, but instead we were surprised and, at first, confused.

Another, unexpected shock were those big, black bruises on Jupiter. We are used to whitish cirrus clouds and volcanic hazes in our own planet's stratosphere; perhaps that's why astronomers imagined that if S-L 9 induced any changes at all in Jupiter's appearance, it might be a whitish haze. Instead, observers -- including kids using small backyard telescopes -- were treated to giant, gaping black spots in Jupiter's mid-south latitudes. Some of the spots were as large as Jupiter's famous Great Red Spot, larger than the planet Earth, and they were darker and more prominent than any atmospheric feature ever seen on Jupiter since the invention of the telescope. In hindsight, chemists realized that when you take a chemical stew of Jupiter's air and the carbon- and sulfur-rich debris from a comet and heat it with multi-million megaton explosions, you might expect a sooty residue.

The black spots, which were drawn out by stratospheric winds into a

globe-encircling black belt, were composed of very fine dust floating at the top of Jupiter's stratosphere, were they persisted for several months. Initially black to Earth-based observers used to Jupiter's bland colors, the dark palls weren't really opaque or dense enough to block sunlight entirely. But calculations show that warming at high altitudes and cooling at depth were great enough to have affected Jupiter's climate, over Earth-sized regions. A year after the impacts, the dark pall had faded, much of it spreading laterally around Jupiter's enormous girth and thinning out. On Earth, of course, there would be nowhere for a global pall to go, so it might well persist for more than a year. The persistence of S-L 9's atmospheric effects were noteworthy: two years after the impacts, the spectroscopic signatures of some rare chemical compounds formed in Jupiter's stratosphere were still strengthening.

With the impact of fragment W on July 22 -- its immense mushroom-shaped plume duly captured by Hubble's camera -- Comet Shoemaker-Levy 9 was history. Exhausted astronomers began to pack up their instruments and return from the far-flung outposts to which they had travelled. However, for Galileo Project scientists, the fun was just about to begin. NASA's spacecraft, out beyond the asteroid belt, had apparently performed most of its computerized instructions correctly, or so the sparse engineering information that was radioed back indicated. The primary challenge was to locate and transmit down to Earth the most critical few percent of all the data now recorded on the spacecraft's tape recorder. That's all that could be dribbled back, at just 1 character per second, before work had to begin six months later preparing Galileo for its 1995 encounter with Jupiter.

The Galileo team, under the leadership of one of us (Chapman), made the difficult decisions required to estimate exactly when each fragment had struck and commanded the spacecraft to play back just those specific images. By August 8th, engineers had processed the first samples from the K impact, one of the most spectacular as observed from Earth. Just a few lines were returned, widely spaced across a frame that covered about two minutes of imaging data. It was like looking at a scene through cracks between the slats of a fence. But there it was: a brilliant spot of light, projected against Jupiter's night side. Even the preliminary data revealed that fragment K's explosion was visible to Galileo for more than half a minute. The impact of the final W fragment was obtained more simply: Galileo's shutter snapped pictures as fast as it could, once every 2.3 seconds, and the time-lapse sequence was recorded on successive frames in arrays of 8-by-8 images, or 64 images per frame. One week after samples of K, Galileo radioed back the first sample data for W: just one row of 8 images, covering less than 20 seconds of time, but continuing the meteoric flash.

The first five images in the row showed nothing but a gibbous Jupiter, looking like the Moon between first-quarter and full. But there, in the sixth, seventh, and eighth images was a brilliant point-of-light (actually saturating the central picture element in the seventh image), suspended against Jupiter's night side. The sequence revealed what a human observer aboard Galileo would have seen with a powerful pair of binoculars. First there was nothing . . . then, suddenly, a several-second-long flash right next to Jupiter's illuminated face, about as bright as one of the brightest stars in the sky. The flash was already fading in the final image on that row. Not until January 1995 were the remaining pictures of W returned from the spacecraft, but already on August 15th it was clear what had been seen: it was the last remaining fragment of the broken comet making its fiery plunge into Jupiter's atmosphere -- the mightiest meteoric bolide ever witnessed.

It is from the entire suite of data -- taken by Galileo, by Earth-

orbiting satellites like Hubble, from airplanes high above the Pacific, and from mountain-top observatories around the world -- that scientists have pieced together some understanding of what transpired on Jupiter in July 1994. One of the earliest efforts to compare and reconcile the various observations took place on August 18 at The Hague, about 3 weeks after the final impact. Once every three years the International Astronomical Union holds a General Assembly, attended by leading astronomers from around the world, and by coincidence the 1994 meeting directly followed the comet crash.

Observers from all over the planet converged in The Netherlands, many still suffering from a combination of sleep deprivation and excess adrenaline. It was immediately obvious that the various visible and infrared observations of "flashes", "fireballs", and "plumes" could not all be referring to the same phenomena -- not unless observatory clocks all over the world had substantial errors. Torrence Johnson of JPL, the Galileo Project's Chief Scientist, wandered from one person to another with a time-line of the W event sketched on a notepad, confirming that the observed phenomena spanned nearly half-an-hour of time. At The Hague, the scientists were like the proverbial blind men and the elephant, each trying to make sense of a different, narrow aspect of the comet crash. Only after months of analyzing their voluminous data and comparing notes at a series of scientific conferences, did astronomers reach a consensus about the major lessons learned from this remarkable natural experiment.

Just what happened when a comet fragment screamed into Jupiter at 60 km/s? Galileo's instruments, with their unobstructed view of the impact sites, best characterized the first minute, the period when the entire kinetic energy of an impactor's motion was converted into a multi-million megaton explosion. What the spacecraft camera saw first was the meteor flash, before the disintegrating fragment plunged out of view beneath Jupiter's main cloud deck. Almost immediately the superheated tunnel of atmosphere just traversed by the meteor began to explode. Three Galileo instruments could observe simultaneously in three wavelength bands: ultraviolet, visible, and infrared. Together, they determined the temperature, altitude, and dimensions of the developing fireball. The G impact, perhaps the biggest of all, epitomized Galileo's results. A few seconds after the meteor, Galileo measurements reveal that there was a parcel of jovian atmosphere, located just above the main cloud deck, which was several kilometers across and radiating at a temperature hotter than the surface of the Sun. Within the next minute, this bubble of atmosphere was observed to rise, cool, and expand. Without a doubt, these were the early stages of the plume that Hubble a few minutes later watched soar into sunlight, more than 3,000 kilometers above the clouds.

Twelve minutes after G's impact, Peter McGregor, observing from Siding Springs Observatory in Australia, took perhaps the most memorable picture of the entire week, as the plume fountained back toward Jupiter, creating an infrared spectacle. Two hours later, Hubble took its most detailed picture of the bulls-eye debris apron, and then visual observers around the world began to gape through their eyepieces at the most prominent feature ever to be seen on Jupiter.

That is what was seen, but it's not all that happened. Unquestionably, the more energetic comet fragments gave up most of their energy in explosions far below Jupiter's visible cloud deck. What observers watched was just the top of the explosions. Observers initially claimed that their data showed that the fragments had disintegrated above the clouds, and that the computer models predicting fragment penetration to hundreds of kilometers below the clouds were wrong. However, it remains unknown just how far the explosions penetrated into Jupiter's depths, for the deeper effects were swallowed up. What's clear is that the tops of the explosions were mighty enough to produce all the phenomena observed.

All, that is, except for the bulls-eye rings. Andy Ingersoll, a lanky professor at Caltech, is one of the world's experts on the atmospheres of the outer planets -- Jupiter, Saturn, Uranus, and Neptune. During the Voyager mission press conferences, he became a familiar face as he made his outer-planet "weather predictions," most of which proved true. Unlike our own Weather Bureau, which has to issue the daily forecast whether they feel confident or not, Ingersoll was free to issue only the predictions he was sure would come true. Five years after Voyager's last outer-planet encounter, S-L 9 enticed Ingersoll to make his most famous prediction of all. And this time he even put his money on it.

Ingersoll measured the bulls-eye rings radiating away from the largest impact scars. But their spreading velocities made no sense if the waves were generated at Jupiter's visible cloud surface. The rings, he concluded, must be the uppermost crests of waves generated far down below Jupiter's erstwhile water clouds. But even then, he couldn't make the velocities fit the theory . . . unless, he concluded, Jupiter has ten times as much oxygen (for every hydrogen atom) as the Sun does. His conclusion flew in the face of the widespread idea that Jupiter is pretty much a chunk of solar material and should have the same percentage of oxygen (which would make up the H2O) as the Sun. Cocky from his Voyager success and with reporters listening in, Ingersoll bet all comers $10 that the Galileo spacecraft would prove him right when its probe penetrated through Jupiter's water clouds in December 1995 and directly measured the humidity. Several dozen of his colleagues bet against him, and he lost every dollar. In fact, the Galileo Probe didn't find any water clouds at all, Ingersoll immediately paid up. Jupiter experts now think that the Galileo probe struck an unusually dry spot on Jupiter, and that overall the planet might be as wet as Ingersoll had expected. But even three years after the S-L 9 impact, Ingersoll still does not understand the nature of the expanding dark rings. He does have a graduate student working o the problem, however, as a possible thesis topic.

A major dispute among S-L 9 observers concerned the sizes of the fragments. Some thought they must have been very large; how else to explain the plumes rising 3,000 km high? Yet there were other indications that the fragments were small. All observers who tried to detect the meteor flashes by reflected light from Jupiter's moons were disappointed. One moon was even eclipsed from sunlight in Jupiter's shadow when the K fragment struck, yet no flash reflection was detected. No matter how pretty Galileo's images of the meteor flashes are, they were far fainter than some early predictions for large impacting fragments.

Apparently the fragments of the comet were not especially massive. Probably the whole original comet measured less than 2 km across. The individual fragments may have been tight swarms of smaller pieces, loosely held together by their own gravity during their final two-year trajectories following break-up. As the swarms plunged toward their demise, they stretched out a bit and might have been a kilometer long when they rammed into Jupiter's stratosphere, generating the visible plumes. However, the mass of each swarm would have made an object only a few hundred meters across, were it all compressed together. Research also shows that this modest remnant from the solar system's birth had virtually no strength at all, given the ease with which Jupiter's gravity pulled it apart. S-L 9 was little more than an aggregate of dirt and ice, with a bulk density of about 0.7 grams per cubic centimeter (less dense than an ice cube). It essentially fell apart in July 1992 as it sailed close to Jupiter.

In spite of remaining uncertainties about the nature of the comet, and of the jovian atmosphere it struck, there are some dramatic lessons that S-L 9 has taught us about planetary impacts. Each of the major observed phenomena on Jupiter -- the direct blast, the atmospheric waves, the plume blasting high above the atmosphere, the heat pulse from backfalling debris, and the long-lived dark clouds in the jovian stratosphere -- has a counterpart in terrestrial impacts. Environmental chnges previously inferred indirectly from the K-T boundary on Earth were seen unfolding before our eyes on Jupiter.

Perhaps most important, after the great comet crash, it was difficult for anyone to dismiss the idea of cosmic impacts as a fantasy. Jupiter's stratosphere was clearly devastated by the impact of a very modest-sized comet, and there can hardly be any doubt that a similar event on Earth would have terrible consequences for human civilization. In the remaining chapters of this book, we will focus on the nature of this contemporary risk and on ways of developing a planetary defense against comets and asteroids.

3800 words (5/6/97)

CHAPTER 9: THE JUPITER COMET CRASH

SUMMARY

This chapter continues our first-hand discussion of the collision of Comet SL 9 with Jupiter in July 1994, and of the lessons it taught us about impacts on our own planet. After describing events at Kitt Peak National Observatory, at the Hubble Space Telescope Science Institute, and at the Galileo mission control center, we summarize the results of the observing campaign: the giant plumes that rose nearly 4000 km above the jovian atmosphere, the fallback of ejecta that heated the atmosphere to incandescence, the atmospheric waves that rushed out from each impact like ripples on a pond, and the black stratospheric

palls that persisted for months following the impacts. Each of these phenomena was large enough to engulf the entire planet Earth. In summary, we quote physicist Kevin Zahnle, who later said: The solar system no longer seems quite so far away as it did before July 1994. Here we are, close to the edge, protected from the true immensity of the universe by a thin blue line. A day will surely come when the sheltering sky is torn apart with a power that beggars the imagination. It has happened before. As any dinosaur, if you can find one.

Having described the evidence for continuing impacts on Earth and the kinds of environmental damage that can result, we turn in the final six chapters to questions associated with the current risk and the ways we are trying to deal with the threat, including proposals to construct defenses against asteroids and comets.

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CHAPTER 10: SHOULD WE WORRY?

Asteroids can hit planets! They are doing it today. And while the chances may be one in a million, that millionth chance may be tomorrow. We are dumb if we don't try to get a handle on this. -- U.S. Representative George Brown (appearing on NBC-TV, February 1997)

We have seen in previous chapters that major impacts are exceedingly rare events. It is human nature to avoid unpleasant subjects and to defer dealing with issues that seem remote in time. It is not surprising, therefore, that most discussion of the role of impacts in terrestrial history has made little reference to current impact dangers. However, a few prescient individuals realized from the beginning that impacts have another dimension, as potential agents of the apocalypse. Recently, of course, the press and a number of political interests have spoken out on the impact hazard, and today we cannot discuss past impacts without also considering the implications for the future of the Earth and of our own civilization.

Beginning with this chapter, we turn our attention to the danger posed by near-Earth object (NEO) impacts. There has been discussion of impact dangers in the press, including cover stories in Newsweek, Time, US News and World Report, and The Economist. TV documentaries on the subject have appeared on an NBC National Geographic Special and on the Discovery and Science Fiction cable channels, and fictional impacts have been depicted in films, including the high-profile Spielberg movie "Deep Impact". Often these discussions have focused on the "what if?" question. For example, in the aftermath of the 1994 collision of Comet Shoemaker-Levy 9 with Jupiter, it was common to ask "what if the comet had hit the Earth?" And when describing the Tunguska explosion, people often imagine "what if it had hit a city instead of happening in the wilderness of Siberia?" We have already seen that impacts can do terrible damage, including the global environmental effects that lead to mass extinctions. But this is not enough. We must also consider how often such events happen. What are the actual probabilities of death by impact, and how do these risks compare with others that are faced every day?

Our own contribution to the impact hazard debate has been largely associated with efforts to evaluate this risk. We have also played a role in publicizing the impact hazard and stimulating discussion of public policy issues. In this and the following chapters, we will include our own personal impressions of some of the key personalities and critical events that have characterized the recent (and continuing) debates on this issue.

* * * * * * *

Although interest in the impact hazard seems like a recent phenomenon, concerns about threats from the sky go back a long way. More than 3,000 years ago, Chinese astronomers had recorded comets of different aspects, each held to be responsible for a different kind of earthly disaster. Had they actually correlated comets with death from the skies by meteorite storms, which a number of ancient reports suggest may have happened? Or were the ancient Chinese simply frightened by unusual changes in the usually predictable heavens?

From the Homeric epics through the Middle Ages until just a few centuries ago, comets were often regarded as evil portents. Yet their true nature, as corporeal objects, did not begin to be appreciated until the last century. As late as the 1920s, a standard astronomy textbook described the physical nature of comets as "airy nothingness" or "dust swarms," while admitting the possibility of more substantial components. With their nuclei hidden inside their glowing atmospheres, the existence of solid objects within comets remained conjectural until spacecraft sent back pictures of Halley's nucleus in the mid 1980s.

Asteroids first appeared as a potential threat after the Earth-crossing object Apollo was discovered in 1932. Before then, all asteroids were thought to be safely confined to the asteroid belt beyond Mars. Soon other Earth-crossers or NEOs were found, and Harvard astronomer Fletcher Watson realized a down-to-Earth implication: "When these cosmic bullets swing past at a mere million kilometers we start worrying about the likelihood of collision. . . [and] sizable bodies do strike the earth every few thousand years. . ." [from Between the Planets, 1941]

A few years later, astrophysicist and industrialist Ralph Baldwin said it better in his seminal book on lunar science, The Face of the Moon (1949): ". . . since the Moon has always been the companion of the Earth, the history of the former is only a paraphrase of the history of the latter. . . [Its mirror on Earth] contains a disturbing factor. There is no assurance that these meteoritic impacts have all been restricted to the past. Indeed we have positive evidence that [sizeable] meteorites and asteroids still abound in space and occasionally come close to the Earth. The explosion that formed the crater Tycho [nearly 100 km in diameter] . . . would, anywhere on Earth, be a horrifying thing, almost inconceivable in its monstrosity."

There was no technical analysis of the asteroid hazard until the late 1960s, when a class at the Massachusetts Institute of Technology was assigned to invent a solution to the hypothetical prospect of a collision with the Earth-approaching asteroid Icarus. This MIT analysis, which outlines a plan to intercept and destroy the asteroid with nuclear bombs, was later published as a book. Their "Project Icarus" study was surprisingly prescient, and even today it serves as a good introduction to the subject of asteroid defenses.

Even during the 1970s, as a handful of astronomers embarked on their search for NEOs, motivated by scientific interests, it was left to science fiction writers to develop the horrors of a potential collision (e.g., Lucifer's Hammer by Jerry Pournelle and Larry Niven; Shiva Descending by Gregory Benford and William Rotsler). Novels were followed by a 1979 Hollywood film starring Sean Connery and Natalie Wood called Meteor, which depicted joint US-Russian efforts to use nuclear weapons to disrupt a small asteroid on a collision course with Earth. However, Meteor was panned by most critics and quickly sank into obscurity. Generally speaking, neither scientists nor the public gave the impact hazard any more credence than faster-than-light starships and time machines. It is not that anybody proved that impacts couldn't happen -- they just didn't think about them.

Several scientific discoveries during the 1970's helped set the stage for our current awareness of the impact threat, as we have described in previous chapters. The most important was the association of the K-T mass extinction with an impact. In addition, the field of risk assessment developed as engineers tried to cope with the developing public perception -- fueled by the environmental movement -- that we are subject to dangers, both natural and man-made. Finally, the lonely proselytizing of Gene Shoemaker and Eleanor Helin for NEOs had gradually interested a few other astronomers, as the list of newly discovered NEOs steadily grew.

In 1980, contemporary with the initial Alvarez work on the impact origin of the K-T extinction, the NASA Advisory Council (a high-level group of outside advisors to the space agency) suggested that NASA might consider the implications of impacts for our survival. NASA then sponsored a workshop on the subject in the summer of 1981, chaired by Gene Shoemaker. This meeting marked the first official recognition of the impact hazard.

Shoemaker convened the impact hazard meeting at the Timberline Lodge in Snowmass, Colorado in July 1981. A mountain-side outpost of Aspen, Snowmass is a skiers' paradise in the winter. To tide it through the summer, Snowmass has long promoted a varied program of activities, ranging from hot-air balloon festivals to scientific conferences. About thirty American scientists gathered to consider a topic that had never before been the subject of a scientific meeting: the hazard of impacts by comets and asteroids. The invitees included astronomers and geologists, Nobel-winning physicist Luiz Alvarez, Bernard Oliver (Vice-President for Research from Hewlett Packard), several members from the military services, and one of us (Chapman).

The astronomers made presentations on the nature of asteroids and comets, cratering of the Earth and Moon, and meteors and meteorites. Then attention turned to the physics of collisions -- airwaves, seawaves, cratering in the ground, effects on climate, mass extinctions of the past, and the risk to human populations. Tom Gehrels, a University of Arizona professor, described his proposed new asteroid search project, which he called "the Spacewatch Camera". The "camera" was envisioned to be a telescope, designed to use new technology in a dedicated search for faint NEOs. It would sample the population, catching only a tiny percent of the NEOs that sail near the Earth, but it would be a forerunner of a more thorough search strategy. Gehrels had access to an old, seldom-used 36-inch telescope and a dome on Kitt Peak in Arizona, and he had promises of a much more capable 72-inch mirror that could be put into operation after the 36-inch had proved the technique. In response to his presentation, the Snowmass group recommended to NASA that Gehrels' Spacewatch Project receive funding.

The final talk on the first day of the workshop addressed the question of whether modern technology could ameliorate an impending asteroid strike. If an asteroid were found headed toward Earth, it probably could either be destroyed or diverted, using rockets and bombs. The chief uncertainty concerned the physical diversity of asteroids and comets -- they come in all sizes and were known to be made of materials as different as snow and metal. So the workshop's final recommendation was to mount exploratory spacecraft missions to NEOs in order to obtain critical engineering data that would be needed to deal effectively and safely with an oncoming impactor, in the unlikely event it would be necessary some day.

After the busy first day, the Snowmass Workshop broke into smaller groups to discuss three different facets of the impact hazard. Meeting around tables in separate rooms, one panel considered the Earth-approaching bodies themselves and how we could learn more about them, another considered the physical and environmental effects of collisions, and the third considered how human society might be affected by such impacts and what might be done to protect against them. In 1981, there were no experts in asteroid defense; this was an entirely new topic for professional scientists and engineers. Some of the expertise offered to the workshop was of the homegrown sort. For example, Michael Gaffey, a research scientist from the University of Hawaii, had been invited to the workshop because of his expertise on meteorites and asteroids, but his more salient contribution came when he dropped in on the human hazards panel. Beginning with his boyhood experience growing up on a farm in northern Iowa, Gaffey had developed an interest in agriculture in cold climates. He told the group how sensitive crops were to rather modest changes in climate, due to decades of agricultural research that had fine-tuned crops for specific temperature conditions. He warned that cooling far less than that proposed by the Alvarezes as ending the Cretaceous period could dramatically reduce food production worldwide.

One important insight of the human hazards panel was the recognition that there might be a threshold for global catastrophe -- an asteroid size that would cross the line from an essentially regional disaster to a worldwide calamity. The panel estimated that for some size of impacting asteroid -- perhaps in the range of 250 m to 2 km in size -- the number of expected fatalities might jump from millions of people (in the country that was struck) to billions of people around the world. These are all topics we will return to later for more detailed discussion.

In the month following the workshop, Gene Shoemaker gathered together the assorted pages, tables, and graphs produced by the panels and individual participants. He distributed these in the form of a 90-page draft report. Meanwhile, letters circulated among Workshop participants on what remains to this day the biggest controversy surrounding the impact hazard: should we recommend a defense system to shoot asteroids down? George Wetherill, a senior planetary scientist and member of the National Academy of Science from the Carnegie Institution of Washington, argued against a draft recommendation to fund research on interception and active defense. He wanted the report's "trigger-happy tone" softened. Wetherill pointed out that neither the Spacewatch Camera nor any other asteroid detection system would be likely to find, in the next fifty years, an object we would need to intercept. And by then, Wetherill wrote, "the present intercept technology will be irrelevant." Wetherill made two further arguments. First, he noted that it was contrary to national and international policy to place nuclear weapons in space, and he feared that the topic was too "delicate" to raise in the midst of the Cold War. Second, he was concerned that the public would focus too much on the sensational "shoot-'em-down" part of the report, would ridicule it, and would disparage the whole topic, including the "sensible" recommendations for additional study of NEOs.

As it turned out, the Snowmass report was never published. Although officials deny it, some participants believe that NASA was gun-shy about issuing the report in the face of the nukes-in-space debate. The simple fact is that the over-committed Gene Shoemaker never got the report into publishable form, and no one at NASA pressured him to do so. Eventually, many of its conclusions dribbled out in papers and books published later in the 1980s. The impact of these results was minimal, however, and public awareness of the hazard was not sparked until 1989, when a skyscraper-sized asteroid was discovered by the Spacewatch Camera on a path that brought it closer than the Moon.

This 1989 discovery was an indirect result of the Snowmass meeting, which had recommended NASA funding for Tom Gehrels's Spacewatch project. Gehrels, a Dutch-born American astronomer, is one of the most interesting characters in the impact story. After adventures during World War 2 as an anti-Nazi partisan and commando in Europe and Asia, he turned to astronomy and devoted most of his professional interests to the asteroids at a time when few scientists took much note of these small bodies. Avoiding the conventional, he studied Buddhism and other eastern religions, became a vegetarian, and participated in the American peace movement during the years of nuclear confrontation between the U.S. and Soviet Union. Today he is wont to begin his observing sessions on Kitt Peak by sitting on the top of the telescope dome in a lotus position, meditating on the sunset.

Gehrels felt a genuine concern over the danger posed by NEOs, and he had a plan to deal with it. His Spacewatch system was to be the prototype for a new and more efficient way to discover faint asteroids. Ever since the introduction of photography to astronomy in the 1890s, asteroids had been found by photographing the sky with wide-field telescopes. This was the approach still used, with improvements, by Gene and Carolyn Shoemaker and Eleanor Helin at their 18-inch telescope on Palomar Mountain. However, a revolution was taking place in other fields of astronomy in the late 1970s, as photographic techniques were supplanted by the new technology of CCD detectors.

CCDs (Charge Coupled Devices) are the electronic devices used to record video images in today's ubiquitous camcorders. As applied to astronomy, they permit the light of distant galaxies (or nearby asteroids) to be detected electronically on a silicon chip and recorded and analyzed in digital form. Not only are the devices much more sensitive than photographic film; the images they produce are ideally suited to computer manipulation and analysis. The objective of Project Spacewatch was to automate the asteroid search process, using computers rather than humans to compare two images of the sky and detect any object that had moved against the background stars between the two exposures. Specially developed software would subtract one digital image from another and identify the asteroids. At least this was the plan.

In practice, however, developing Spacewatch proved to be difficult. The first hurdle Gehrels had to overcome was funding. Although the Snowmass meeting had shaken loose some NASA money, Gehrels needed $200,000 more to get started. He tried the idea of "selling" asteroids; in exchange for naming a Spacewatch-discovered asteroid "Liztaylor," for example, perhaps the actress would send a check to Arizona. But the idea never caught on in Hollywood. Finally Barney Oliver of Hewlet-Packard put up a matching grant of $100,000 from his personal funds, and Gehrels managed to collect the rest from a variety of smaller contributors.

Even with start-up funding in hand, perfecting a working Spacewatch system proved to be a formidable task. Gehrels first mounted his new instrument on the 36-inch telescope in 1983, but it required three more years of frustrating effort before he discovered his first NEO with the new technique. Ultimately, however, his perseverance paid off, and Project Spacewatch began to compete effectively with the photographic observers discovering new asteroids, and ultimately to surpass them in the friendly rivalry to discover new comets and asteroids. But finding funds has remained a difficult challenge, and not until 1997 was Gehrels finally able to move his Spacewatch system to a larger 72-inch telescope, as he had proposed 15 years earlier to the 1981 Snowmass meeting.

The Spacewatch system differed from the photographic searches in another way. Gehrels had a larger telescope and more sensitive detector, so he could see much fainter asteroids. However, he could not cover as much of the sky as the wide-field photographic patrols. Most NEOs passed him by undetected, but those objects he found tended to be fainter and, on average, smaller than the photographically discovered NEOs. Spacewatch proved to be particularly adept at finding small asteroids very close to the Earth.

On March 23, 1989, Spacewatch discovered an asteroid about 200 m in diameter that was closer than the Moon. This object (originally designated 1989FC and later named Asclepias) was very near the Earth at the time of its discovery. A few hours earlier it had reached its minimum distance of 275,000 km, closer than any previously observed asteroid. Moving at a speed of 12 km/s relative to the Earth, Asclepias traversed the distance from the Earth to its point of closest approach in only 6 hours. To illustrate just how close this was, the press release from NASA stated (not quite accurately) that, had it come along 6 hours earlier, the asteroid would have hit the Earth.

The "near-miss" by asteroid Asclepias was widely reported in the international press. Also noted with concern was the fact that it had not been discovered until it was already past the Earth and receding from us. This little asteroid served as a kind of wake-up call, alerting many people to the asteroid hazard, including some members of Congress.

At about this time we found ourselves part of the awakening concern about impacts. Just a few months before the Asclepias near-miss, we had published a popular-level astronomy book called Cosmic Catastrophes, discussing a wide range of disasters ranging from ice ages to nearby supernova explosions that might fry our planet in high-energy cosmic rays. We also devoted several chapters to the danger of impacts, with emphasis on the K-T mass extinction. In the final chapter of the book we turned to the question of current impact hazards. One of our motives in discussing the hazard was to publicize some of the conclusions from the 1981 Snowmass meeting, conclusions that had lain dormant for nearly a decade. In the draft report from that meeting, Gene Shoemaker had estimated the probability of impacts by NEOs of various sizes and had derived some estimates of risk. We re-evaluated and extended those conclusions.

While working on that book, we realized that the greatest danger is posed by the largest impacts, those that cause environmental damage on a global scale. Until then most discussion had focused on the risk from the Tunguska-class impacts. While the Tunguska-class (15 megaton) explosions could indeed wipe out a city, the actual chances of one of them doing so are very slight. The unique destructive power of the largest impacts is what makes the impact hazard truly catastrophic in nature. In science, a situation is considered catastrophic if the very rare events dominate over the cumulative effects of many smaller, more frequent processes. Impacts have this characteristic. Any number of Tunguskas can take place without the collapse of ecosystems, extinction of species, or threats to the survival of civilization, yet just one big impact can place the entire ecosystem in jeopardy. Thus we concluded that our focus should be on the rare, large events rather than the more common, smaller ones.

In writing the final chapter for Cosmic Catastrophes, we made a quantitative assessment of the impact risk. By impact risk we mean the probability that an individual will die as the result of an impact -- the chances that it will say on your tombstone that you died from an impact rather than from an auto accident or a heart attack or being hit by lightning or any other cause. We discovered, rather to our surprise, that this risk is comparable to a number of other risks that many people take quite seriously. For example, the risk of dying from an impact appeared to be about the same as the risk that an average American will die from an airline accident (we will discuss the quantitative risk assessment more in later chapters). We found in discussions with the press and the public that this particular comparison struck a responsive chord. For the first time the risk of death from impacts was related to a fear that is shared to some degree by everyone who has ever flown in an airplane.

A few months after Cosmic Catastrophes was published, one of us (Morrison) received an invitation to speak to the Congressional Space Caucus, a group of interested staffers in the U.S. House of Representatives. On June 26, 1989 (3 months after the near-miss by Asclepius), he was warmly welcomed by an audience of about 25 young men and women. This group included two individuals who would become strong supporters of asteroid defense, William Smith and Terry Dawson from the staff of the House Committee on Science, Technology, and Space. The talk was short, focused on the evidence of past impacts provided by Tunguska and other terrestrial craters and on the statistical risk of such impacts in the future, and it emphasized the poor state of our current knowledge of even the larger near-Earth asteroids, and how few astronomers were searching for new comets and asteroids.

Although Morrison's subject was new to most of the listeners, they seemed to react positively, and there was some informal discussion after the talk about possible Congressional action to urge additional asteroid studies. Staffers requested an autographed copy of Cosmic Catastrophes for Congressman George Brown, Democrat from southern California and influential Chairman of the House Committee on Science, Technology and Space.

Encouraged by this modest show of interest as well as the wide press coverage given to Asclepias, the American Institute of Aeronautics and Astronautics (AIAA) decided to follow up and encourage Congressional action. The AIAA is a major aerospace professional organization that maintains a lobbying office in Washington. Under the leadership of physicist Edward Tagliaferri, the AIAA Space Systems Technical Committee wrote a position paper called "Dealing with the threat of an asteroid striking the Earth." In this document the AIAA stated that "Earth-orbit crossing asteroids clearly present a danger to the Earth and its inhabitants ... the AIAA recommends that a systematic and open program be established to detect and define the orbits of Earth-crossing asteroids with a precision which will permit the prediction of impacts with some confidence."

Representatives of the AIAA discussed their proposal with members of Congress, receiving favorable receptions from Congressman George Brown and from Terry Dawson of the Science Committee staff. Brown asked Dawson to draft language adding the following to the 1991 NASA Authorization Bill (dated September 26, 1990): "The Committee believes that it is imperative that the detection rate of Earth-orbit-crossing asteroids must be increased substantially, and that the means to destroy or alter the orbits of asteroids when they threaten collision should be defined and agreed upon internationally. The chances of Earth being struck by a large asteroid are extremely small, but since the consequences of such a collision are extremely large, the Committee believes that it is only prudent to assess the nature of the threat and prepare to deal with it. We have the technology to detect such asteroids and to prevent their collision with the Earth. The Committee therefore directs that NASA undertake two workshop studies. The first would define a program for dramatically increasing the detection rate of Earth-orbit-crossing asteroids; this study should address the costs, schedule, technology, and equipment required for precise definition of the orbits of such bodies. The second study would define systems and technologies to alter the orbits of such asteroids or to destroy them if they should pose a danger to life on Earth. The Committee recommends international participation in these studies and suggests that they be conducted within a year of the passage of this legislation."

Thus, for the first time, there was official governmental recognition of an impact hazard and encouragement to develop programs to deal with this threat. If not exactly worried, Congress had at least expressed its concern. The next step was up to NASA.

4200 words

CHAPTER 10: SHOULD WE WORRY?

SUMMARY

Beginning with this chapter, we turn our attention to the present-day danger posed by near-Earth object (NEO) impacts. Our own contribution to the impact hazard debate has been largely associated with efforts to evaluate the risk in quantitative form. We have also played a role in publicizing the impact hazard and stimulating discussion of public policy issues. In this and the following chapters, we include many personal impressions of the key personalities and critical events that have characterized the recent (and continuing) debates on this issue. This chapter focuses on recognition and early evaluation of the hazard, beginning with a NASA workshop in 1981, the first time this subject was addressed by scientists and policy-makers. One result of this meeting was funding for Project Spacewatch in Arizona, the prototype of modern asteroid searches, pioneered by astronomer Tom Gehrels. It was Spacewatch that reported in 1989 the widely-publicized near-miss by asteroid Ascelpas. In the same year we published our book Cosmic Catastrophes, comparing the risk of death from impact to that of a round-trip commercial air flight. These two events, plus lobbying by the American Institute of Aeronautics and Astronautics, spurred the US Congress to request that NASA study the hazard, propose ways to accelerate the discovery of NEOs, and convene a workshop on the technologies that might be used to protect against a cosmic impact.

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CHAPTER 11: SPACEGUARD SURVEY

"It is an intolerable uncertainty that right now there are some 1700 or so Earth-crossing asteroids larger then one kilometer in diameter and we don't know whether any one of these will be hitting the Earth with the force of hundreds of thousands of Hiroshima bombs. That requires the greatest urgency." -- Spacewatch astronomer Tom Gehrels (on Three Minutes to Impact, 1997)

The United States Congress took a bold initiative in asking NASA to study the impact hazard. At that time, most NASA managers were skeptical of becoming involved in the issue. There was good reason to be cautious. In 1991, the space agency was suffering from an apparently endless series of technical and public-relations disasters. The Hubble Space Telescope, pride of the NASA space science program, was found to have been launched with an optical flaw that seriously degraded its resolution, giving it fuzzy vision. Erratic performance of the Space Shuttle main engines led to repeated launch delays. The nation's editorial writers and political cartoonists were savaging NASA for both technical and management failures. The request to study the dangers of "space rocks" and "killer asteroids" seemed to NASA officials like an opportunity best declined.

The example of Vice President Dan Quayle reinforced NASA officials' sense of caution. A fan of space exploration, Quayle had established the President's National Space Council as an independent voice in U.S. space policy. One of the Space Council staffers, on detail from the Air Force, had included a reference to the asteroid danger in one of the Vice President's speeches. Always happy to find fault with Quayle as a mental lightweight, the press ridiculed his "Chicken Little" concerns. The Vice President, stung by the criticism, banned all reference to asteroids from his future speeches, and NASA took note. The "giggle factor" had become an ingredient of public discussion of the impact threat, and officials at NASA Headquarters did not want to risk further ridicule in the press.

Although NASA managers may not have relished becoming entangled in the asteroid impact business, a study had been formally requested by the Congress, so they had to comply. In the spring of 1991 two separate committees (or "workshops") were set up to deal with the Congressional request. We participated in both. The first part of the Congressional charge, the search for potentially threatening asteroids, seemed to be a straightforward technical issue, and NASA assembled an international team of astronomers chaired by one of us (Morrison) to prepare a response. The second element of the problem, how to organize defense technologies to deal with an incoming object, lay outside NASA's usual mission, especially if the politically sensitive issue of nuclear weapons were raised. This is exactly the concern from George Wetherill that contributed to the failure of NASA to publish the results of the 1981 Snowmass conference. To chair the second workshop, NASA looked beyond the space science community and appointed John Rather, who had recently joined NASA after a career in defense-related projects. In this chapter we will tell the story of the Morrison team from our perspective as active participants, returning to the space defense workshop in the next chapter.

The members of the "Morrison Committee" were mostly astronomers, including asteroid hunters Gene Shoemaker, Eleanor Helin, Ted Bowell, and Tom Gehrels, and orbit experts Brian Marsden, Don Yeomans, and Steve Ostro. In addition to U.S. scientists, the team included members from Russia (then the USSR), Finland, France, India, and Australia.

The official name of Morrison's team was the NASA International Near Earth Object Detection Working Group. Clearly a better term was needed to characterize both the group and the comprehensive asteroid survey we were charged with designing. The inspiration for a new name came from the opening pages of the novel Rendezvous with Rama by the famous science fiction writer Arthur C. Clarke. In his prologue, Clarke had described a fictional 2077 asteroid impact of catastrophic dimensions that had wiped out Venice and adjacent parts of Northern Italy. According to the novel, humanity responded with an international effort to ensure that the Earth would never again be struck by such a cosmic missile, and this program was named "Project Spaceguard". Spaceguard seemed like an excellent name for what we were proposing. With Clarke's enthusiastic permission, we called our proposal the Spaceguard Survey and named our team the Spaceguard Working Group.

Arthur Clarke followed the progress of the Spaceguard Working Group closely from his home in Sri Lanka, and when asked to write an essay by Time magazine in the summer of 1992, Clarke chose asteroid impacts as his subject. The essay, printed in a special issue of Time, was later expanded into a novel, The Hammer of God, which refers directly to the NASA Spaceguard study. This novel, in turn, is the basis for the 1997 Spielberg film Deep Impact.

The Spaceguard group examined both the nature of the impact hazard and how best to search for potentially threatening asteroids. We needed to look at the hazard issue in order to answer the questions of what objects needed to be searched for: asteroids or comets; large ones or small ones? The first task, therefore, was to subject the risk from impact to a rigorous statistical analysis and place it in context with other hazards. How big is the risk, and what is its nature? In particular, what do impacts by comets and asteroids of various sizes actually do to the environment? In terms of what they do and how often such impacts happen, which are the most hazardous?

Analysis of a problem often begins with concrete examples. Most of our direct experience with the impact hazard was limited to just two events. At one extreme was the gigantic Chicxulub impact of 65 million years ago that had led to the extinction of most species on Earth, including the dinosaurs. Clearly such a global catastrophe would kill most (if not all) human beings, but it is equally clear that such enormous events are very rare, happening perhaps only once in a hundred million years. The other example was the 1908 Tunguska blast, which would have wiped out a city had it hit one, yet it didn't -- such events were calculated to take place on the land area of the Earth about once per millennium. The range of projectile mass from Tunguska to the Chicxulub impactor is enormous, and the effects are entirely different depending on the size of the projectile. To make any sense of the hazard, we had to analyze the effects and risks of different sized projectiles.

At smallest end are the meteorites -- rocks from space that strike the atmosphere yet fail to burn up entirely during their fiery plunge to the ground. Astronomers estimate that several tons of meteorites reach the surface every year, but this is only a minute fraction of the total mass of impacting cosmic debris. Clearly, the great majority of these fragments burn up in the atmosphere and do not reach the ground. Those that do are slowed by atmospheric braking and strike the ground like a freely falling rock, at less than 10 m/s. This means that you have to almost literally be hit on the head to risk injury or death. Since such direct hits are extremely rare, the hazard is negligible. We can walk freely in our neighborhoods with no need to fear being struck dead by a space rock.

An impact by a larger projectile is much more serious. That is not only because it is larger, but such objects can reach the ground with most of their original speed, and hence energy, intact. When such a hypervelocity projectile strikes the ground or disintegrates in the lower atmosphere, it explodes causing widespread destruction. The question is: at what size (energy) does an incoming projectile penetrate the atmosphere without being slowed by atmospheric friction? Only objects larger than this threshold size will cause substantial damage.

At the time the Spaceguard Working Group began its task, conventional wisdom, at least in some quarters, was that the threshold for atmospheric penetration was at a mass of a few tons, corresponding to a diameter of a meter or so. A recent report from Livermore National Laboratory had estimated this threshold at 4 m diameter and concluded that hypervelocity impacts releasing tens of kilotons of energy take place annually on the Earth, an amazing prediction to which we will return in the next chapter. But common sense tells us that natural blasts the size of the Hiroshima bomb are not annual events on Earth. Something must be wrong with such calculations. Fortunately, Chris Chyba and Kevin Zahnle of NASA Ames Research Center had an explanation, as we described in Chapter 7. In 1991 they were just developing computer simulations that realistically accounted for the Tunguska event.

The new analysis showed that the smallest diameter rocky object that can penetrate the lower atmosphere is about 50 m (or 100 m for an icy, cometary projectile), with a corresponding impact energy of about 10 megatons. Smaller impacts detonate harmlessly in the upper atmosphere, except for projectiles made of iron, which fortunately are rare. Thus we need not worry about natural Hiroshima-scale events. Only those much rarer impact explosions with energies greater than the largest thermonuclear bombs penetrate low enough to do any harm -- and they happen somewhere on Earth only about once per century.

An impact above the 10-megaton threshold would be destructive in a populated area, but it would do little harm if the projectile hit in the ocean or in a wilderness like Siberia. Most impacts in the 10 to 100 megaton range would cause brilliant, devastating airbursts. Projectiles with energies exceeding 100 megatons would crash into the ground and form explosion craters -- the more energetic the impact, the larger the resulting crater. At 10,000 megatons, for example -- equivalent to the world's total nuclear arsenals -- the crater would be several kilometers across and the zone of devastation would approach a hundred thousand square kilometers. Such events occur somewhere on Earth roughly once in 100,000 years.

It is easy to estimate the risk we each run of being killed by such an impact, which could, after all, happen anytime -- we do not necessarily have a grace period of 100 millennia! Since such an impact would be lethal only to those near ground-zero, we term such an event a "locally destructive impact" to distinguish it from the larger, rarer events that could harm the whole global ecosphere. We would be safe from such a 10,000 megaton event unless we were less than 200 miles away from ground-zero. The hazardous zone, within 200 miles radius of ground-zero, has an area of about 100,000 square miles. For comparison, the total surface area of the Earth is approximately 200,000,000 square miles, larger by a factor of 2000. Thus, by random chance, only one in 2000 of these locally destructive impacts will be close enough to us to do us harm.

Since the frequency of such impacts on our whole planet is only about one per 100,000 years, the average interval between impacts that endanger us is about 100,000 x 2,000, or 200 million years. Expressed as a probability, this means the chance each year of any one of us being killed by such an impact is only 1 in 200 million. Most people consider such a low probability -- much less than the chances of an average American being killed by a poisonous insect or by a lightning bolt -- to be negligible. (Note our phrase average American. A golfer on the greens during a thunderstorm, or a person with known allergies to bees trying to remove a swarm of killer bees, would run a much higher than average risk. Locally destructive impacts, however, strike at random, and we are all at roughly the same, minimal level of risk.)

There is a major, qualitative difference between locally destructive impacts and impacts large enough to produce a global environmental disaster. The Chicxulub impact was a global holocaust, of course, but even far short of events that cause mass extinctions, an impact can cause short-term changes in global weather with catastrophic consequences for human civilization.

In Chapter 6 we discussed recent research on the environmental effects of impacts by objects of various sizes. To make an estimate of the actual hazard, however, we must first decide exactly what we mean by a global catastrophe. There were many, roughly equivalent concepts: the collapse of modern civilization . . . a new Dark Ages . . . something like the effect of the plague on 14th century Europe . . . the sudden death of a large fraction of the world's population . . . something analogous to the aftermath of global nuclear war. In order to define the onset of global catastrophe, we also needed to understand just which globally destructive environmental consequence can be produced by a blast of any given size. While an impact of 10 million megatons or more was required to ignite global wildfires, a smaller impact might be sufficient to inject dust into the upper atmosphere worldwide, block the sunlight, and lead to crop failures in all nations. In 1991, as the Spaceguard team groped for a definition for the onset of global catastrophe, it seemed that mass starvation due to collapse of world agriculture was the critical factor.

We decided to define a global catastrophe as an event that leads to the death of 25% of the world's human population, primarily due to starvation and attendant disease and instability. As it happens, collapse of world agriculture had already been studied as an environmental consequence of a large-scale nuclear war. We had available to us the results on nuclear winter from the original TTAPS team, and one of the TTAPS scientists, Brian Toon, became interested in what might happen if the stratosphere were loaded with impact-produced dust instead of smoke. Applying the computer simulations that had been developed to study nuclear winter, he and his colleagues concluded that the injection of about a billion tons of fine dust into the stratosphere would lead to a global cold snap and crop destruction. An impact with an energy of a few hundred thousand megatons would produce that much dust. That is the impact energy of a mile-wide asteroid. Toon provided this information to the Spaceguard Working Group for our evaluation of the hazard associated with a global impact winter.

Even as we learn more about the impacts themselves and their potential environmental consequences, we may never understand how human civilization would react to such a global disaster, which would vastly exceed the severity of World War 2. Would such massive stress lead to international cooperation or to war? Would social, political, and economic structures collapse, too, along with agricultural production? Would this be a short-term crisis, or the end of civilization? Europe survived the death of a quarter of its population during the plague, and Germany was prospering less than two decades after losing World War 2. But since all nations would suffer simultaneously from impact winter, there would be no provider of a Marshall Plan. Would our social and religious systems and our individual wills to live muster the hidden strengths that would enable us to re-build? Or would destructive chaos imperil us, as depicted by novelists William Golding in The Lord of the Flies and Larry Niven and Jerry Pournelle in Lucifer's Hammer? No one really knows, and we surely don't want to do an experiment to find out.

Adopting Toon's estimate of about 1 mile for the threshold diameter of an asteroid capable of delivering a globally catastrophic blow to the environment, we can calculate (from the known frequencies of impact by asteroids and comets of various sizes) the relative hazard of locally destructive impacts and globally catastrophic events, and we can compare the probabilities of death. (These risks, for each range of size, are derived in the appendix.) An important conclusion is that the larger the impact, the greater is the total risk, confirming the work we did earlier for the book Cosmic Catastrophes. Of course, the damage increases with the size of the impact, but one might expect that the overall hazard would be more than compensated by the increasing rarity of such events. Not true. Despite the relative infrequency of larger impacts, we are more at risk from the very rare global catastrophe than from all of the smaller, more frequent impacts of the Tunguska class. Nearly 90 percent of the total hazard is associated with impacts at, or a little above, the global threshold of about 1 mile diameter. If we could protect against these impacts, we would not only ensure that there are no global, civilization-threatening disasters, but we would also remove most of the total risk.

Our hazard analysis led to another important conclusion: the probability of death from impact (chiefly the globally catastrophic ones) is actually similar to that from more familiar natural hazards such as earthquakes, volcanic eruptions, and severe storms. Worldwide, the lifetime chances of an individual dying from the aftermath of a large cosmic impact is about 1 in 20,000. We will compare this number with other hazards and discuss people's responses to such levels of risk in a later chapter.

For purposes of writing the Spaceguard Survey Report, settling on the level of the threshold was an important issue. The specific value for the threshold diameter supplied by Toon was 1.7 km, but he realized this value was uncertain by a factor of two either way. We wouldn't want to design and construct Spaceguard to find only those asteroids larger than 1.7 km, and then have later research show that 1.0 km asteroids, or even smaller ones, might be capable of triggering a global catastrophe. So some members of the Spaceguard Working Group felt that we should be conservative, and adopt 0.5 km diameter as the threshold size, to be sure of protecting against anything that might conceivably have such unparalleled consequences as the end of civilization. Other members took an opposite philosophical view of scientific "conservatism". They rejected adoption of the smaller threshold size as being alarmist. To them the "conservative" decision would be to adopt a larger value, perhaps 3 km, a size of projectile that we all could agree would have undoubted catastrophic effects.

Ultimately we compromised on a value of 1 km for the threshold -- at least for the practical purposes of the survey. Even though the real threshold is probably a bit larger, Spaceguard would provide Earth with a margin of safety by searching to somewhat smaller diameters. Although almost no one was satisfied, the Spaceguard Working Group nevertheless had taken its first step toward protecting the Earth from incoming asteroids: We had decided to design the search to locate all, or at least most, potentially threatening asteroids 1 km or larger.

Now we had to figure out a way to locate the objects themselves long before any of them struck the Earth. If we could determine the orbits for each of the Earth-crossing objects larger than 1 km and project them forward in time by computer calculations, we could establish once and for all whether any of them pose a near-term hazard of impact with our planet. Some members of the team calculated that it should be possible to predict most of these orbits accurately for at least a century ahead.

How many asteroids are there with diameters of 1 km or larger? As we discussed in Chapter 2, Gene Shoemaker and others have estimated this number as being approximately 2,000. Of these, only about 100 were known in 1991. Our objective was to find the other 1900 in a timely way.

Asteroid surveys, such as those done with the 18-inch telescope on Palomar Mountain and the 36-inch Spacewatch instrument on Kitt Peak, were finding new Earth-approaching asteroids larger than 1 km in diameter at a rate of about one a month. At this rate, it would require 1900 months, or about 160 years, to discover them all. The challenge was to increase this discovery rate to at least 10 per month and to carry out a comprehensive all-sky survey.

Although these are called near-Earth asteroids because they can come close to Earth, they do so only occasionally in the normal course of their loops around the Sun, when Earth and asteroid both happen to be near the crossing points of their orbits. If we are to complete the survey rapidly, we can't wait for these chance close encounters. The way to increase the discovery rate is to extend our reach to a greater distance (where the objects are fainter) in order to find NEOs even at the far points of their orbits.

Some people misinterpret the Spaceguard Survey as a search for asteroids that are heading straight for Earth -- part of a last-minute defense system analogous to military radars that detect incoming bombers or ballistic missiles. But this is not the case. Spaceguard has never been designed to detect asteroids on their final fatal approach, since long lead time is required for an effective defense. The point is to survey a large volume of space and pick up each asteroid on one of the thousands or millions of times it crosses the Earth's orbit before actually striking. Ideally, the warning of a future impact would come decades, perhaps centuries, before the actual collision with our planet.

Spaceguard team members used computers to model various survey approaches in order to zero in on an optimum search strategy. These simulations showed that for a complete survey to be accomplished in a practical length of time, it would be necessary to increase both the size of the telescopes and the area of sky to be scanned beyond that of Tom Gehrels' Spacewatch system. Basically, the telescope size is determined by the requirement to see dark, 1-km asteroids out to a distance of about 200 million km; anything much less lets too many objects slip past undetected. This requirement in turn demands telescopes of diameter 2 m or larger -- that is, with about 5 times the light-gathering power of the 36-inch Spacewatch telescope. To provide sufficient sky coverage, we concluded that six such telescopes were needed, three in the northern hemisphere and three in the south.

The survey would be impossible to conduct without modern detectors and computers to analyze the data. Visual or photographic techniques are woefully inadequate, and even the electronic approach pioneered by Tom Gehrels would need considerable refinement. We concluded that identification of faint, slowly moving asteroids against a heavenly background of millions of stars is a task ideally suited to a computer, but perhaps slightly beyond the capabilities of state-of-the-art computers in 1991. We were confident, however, that within a few years even relatively inexpensive work stations would be up to the task.

Congress had asked that we estimate the cost of this survey system. While the astronomers on the Spaceguard Working Group lacked expertise in such budgeting, we enquired into the purchase price of a 2-m telescope and concluded that each of the Spaceguard telescopes, complete with the best detectors and computers, could be bought for about $8 million -- a modest sum compared with the $80 million price tag of the largest ground-based telescopes under construction in Hawaii and elsewhere. The comparison with Earth-orbiting telescopes is even more dramatic; each Spaceguard facility would cost less than 1 percent as much as the Hubble Space Telescope. The whole Spaceguard system would cost about $50 million for construction and perhaps $10 million per year for operations. By NASA standards, this sounded like a real bargain!

The Spaceguard Survey Report was completed in January 1992, just in time for the second Congressionally mandated study, which was to deal with defense technologies. If Spaceguard discovered a comet or asteroid on a collision source, did the means exist to intercept it and deflect it? We were about to find out.

3900 words

CHAPTER 11: SPACEGUARD SURVEY

SUMMARY

The United States Congress took a bold initiative in asking NASA to study the impact hazard. At that time, most NASA managers were skeptical of becoming involved in the issue, warned by unfavorable press reactions to comments by Vice President Quayle on this topic, but they had no choice but to follow congressional instructions. One of us (Morrison) was appointed to head the international working group that defined the hazard and studied ways to increase the discovery rate of NEOs. This group met throughout 1991, as recounted in this chapter, determining for the first time how the danger is related to the size of the impacts. We found that the greatest risk is from impacts near the threshold for global ecological catastrophe (about 1 million megatons, corresponding to a mile-wide asteroid or comet), rather than from the much more frequent smaller blasts (like Tunguska). With these results in hand, we designed an international astronomical survey system to find these objects, which we called the Spaceguard Survey (borrowing the name from a science-fiction novel by Arthur C. Clarke). The cost of the Spaceguard Survey was estimated at $250 million, spread over the 20 years that would be required to discover all the near-Earth asteroids larger than a mile across. Our Spaceguard Working Group completed its report to NASA and the Congress in January 1992.

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CHAPTER 12: NUKES FOREVER!

Needs opening quote.

Congress had charged NASA, through its Spaceguard Working Group, to develop a specific plan for an asteroid survey. The second part of their request was less well defined, asking only for an analysis of the technology that might be used for asteroid protection, if a threatening object were actually discovered. In addressing this assignment, John Rather took a different approach from that adopted by the Spaceguard team. He decided to hold a single brainstorming meeting in January 1992 where ideas on defense technologies could be brought forward. Rather invited many of his former colleagues from the nuclear weapons laboratories (Los Alamos and Livermore), and he held his meeting at Los Alamos National Laboratory in New Mexico, the home of "The Bomb".

A few astronomers also attended the defense technology workshop. Except for Gene Shoemaker, who had been invited early on, the Spaceguard members had received their invitations at the last minute, reportedly at the insistence of NASA officials who wanted balance. Most of us had never visited Los Alamos, constructed in the remote mountains of New Mexico during World War 2 for the sole purpose of designing and constructing the first nuclear bombs. It was apparent from the beginning that the environment here would be different from that of NASA scientific meetings and workshops. A prominent sign set the tone for the meeting: "By Invitation Only." John Rather barred several reporters who showed up, even though the meeting was unclassified. We entered the meeting room past a long gallery of portraits of such great atomic physicists as Robert Oppenheimer, Niels Bohr, Enrico Fermi, Hans Bethe -- and Edward Teller, who was also present in person. As we describe in this chapter, a new constituency interested in asteroid defense was about to make itself heard -- the bomb builders.

Since most of the people who assembled in Los Alamos were new to the asteroid defense issue, John Rather had sent them each a package of background reading. This package included an unpublished 1990 report by Lowell Wood and his colleagues at Livermore National Laboratory, called "Cosmic Bombardment II: Intercepting the Bomblets Cost-Efficiently." In this report, Wood asserted that any asteroidal projectile larger than 4 m in diameter would penetrate though the atmosphere and impact explosively, with a potential for widespread damage and many fatalities. He estimated that the Earth was struck by projectiles of 4 m or larger diameter about annually, producing property damage of about $60 million and hundreds of deaths per year, on average. In his words, "it's the stuff between a truck and a house in scale which rains down on our fair planet at rates of dozens to hundreds of strikes per century." Since these smaller impacts are much more frequent than large ones, Wood recommended that any defensive system should concentrate on the 4-m projectiles. He went on to outline such a system, featuring nuclear-tipped interceptor rockets that could be launched on short notice, rather similar to some proposals for defense against enemy nuclear attacks. He concluded that "erecting such defenses should be commenced at the earliest practicable date."

We Spaceguarders were incredulous when we read Wood's paper. Not only had he made a basic technical error in his calculations of the threshold for atmospheric penetration, he had also apparently failed to test his hypothesis against simple common sense. How could he assert that cosmic bomblets were killing all these people and doing hundreds of millions of dollars in property damage, when there were no newspaper or historical reports of even one such destructive event having taken place? Obviously, there was some fallacy here. During the days before flying to New Mexico, we wondered: why had Rather included this erroneous report in his collection of background reading? Was it coincidental that the 4-m adversaries that dominated Wood's paper were about the same size as an ICBM?

We had heard of Lowell Wood before. A burley man with a red beard and booming voice, he was famous in the defense community as a favorite of Edward Teller's who had achieved a high position at Livermore and considerable prominence in Washington. He had also gained national notoriety as the chief advocate of the x-ray space laser that had provided the original justification for President Reagans' Star Wars policy, a technically flawed concept that was ultimately withdrawn as unworkable after the expenditure of hundreds of millions of dollars. More recently Wood had taken command of the defense programs at Livermore called "brilliant pebbles" and "brilliant eyes", and he was touting these as the answer to the challenge of an impenetrable missile defense.

Wood's interest in shooting down small asteroids was supported by his mentor, Edward Teller. Teller biographer and critic William Broad of The New York Times has characterized him as a major architect of the Cold War and probably the most influential scientist of the 20th century. Broad believes that isolation from collegial criticism has become a problem for Teller and has encouraged him into major mistakes such as his support for the x-ray laser. But in spite of the x-ray fiasco, Teller remains a formidable presence (following the Republican Congressional victory of 1994, House Speaker Newt Gingrich, himself a space supporter, invited Teller to present the opening keynote address to the newly-elected members of Congress).

Now in his waning years, Teller is stooped and peers through nearly-closed eyelids; his famous eyebrows are white and less prominent. Leaning on his staff -- a thick pole made of a light-colored wood, with a leather grip at shoulder height -- Teller immediately commands attention, even from those who are not his disciples. His gravelly voice is clear and authoritative, and his cadence is exceptionally slow, which makes his delivery all the more dramatic. Each sentence is grammatically perfect, each word is enunciated precisely in a Hungarian accent, still perceptible after five decades in the United States.

From the moment he entered the Los Alamos conference room, Teller's presence dominated the asteroid defense workshop. He was treated with obsequious deference and always given priority in speaking. Once, when Rather, as chair of a session, began to comment after a speaker had finished, Teller's rumble became audible. Rather instantly interrupted himself in mid-sentence to give his mentor the floor: "Oh, Dr. Teller, I am so sorry...I believe you want to speak?" He was always formally addressed as "Dr. Teller", and at Los Alamos he always had the final word on any issue.

Both Morrison and Shoemaker spoke in the first session at Los Alamos, describing the results from the Spaceguard Working Group. In particular, we noted that small asteroids did not, and could not, penetrate the Earth's atmosphere, in contradiction to Lowell Wood's paper. No questions were asked, and we assumed that we had made our points. When the weapons scientists began their presentations, however, we found that most of them dealt with ways to locate, track, intercept, and destroy 4-m asteroids shortly before they would plunge down to Earth. Following Wood's lead, each speaker assumed that the greatest danger was from these 4-m "bomblets," which could be located only at the last moment, hence requiring a virtually instantaneous response -- interceptors on the launch-pad -- to shoot them down. In the discussion period the astronomers again explained that Wood's analysis was in error, that small projectiles were not the problem, and that if the Spaceguard Survey were carried out we would have a warning time of many years, not just a few hours, before an impact. The audience stared back blankly, refusing to accept the heresy of a challenge to Teller or his protoge, Wood.

Those of us from the Spaceguard team were not sure what was happening. We were already struggling with the unfamiliar vocabulary of the weapons builders, where a nuclear bomb is a "device", a rocket is a "vehicle", and a spy satellite is an "asset". More fundamentally, however, we saw that the weapons scientists did not conduct their business through the open questioning and discussions that astronomers were used to. Outwardly, everyone seemed excessively polite and deferential toward each other. Statements made by speakers went unchallenged even when they seemed contradictory or technically ill-founded. Perhaps this was because so much of the research done by this community is classified and thus not appropriate for public discussion and debate. Or maybe they were just more polite, and less critical in public, than astronomers. Or possibly it was Teller's inhibiting presence that stifled open debate.

John Rather had invited a wide range of participants, and there was no shortage of ideas to lay on the table. Many of the Los Alamos and Livermore technologists seemed like kids with their hands in the cookie jar. The bombs, lasers, and miniature spacecraft had been their playthings through the era of Star Wars, and they welcomed an opportunity to keep playing. The concepts they proposed often seemed outrageous to the NASA scientists. One idea, for example, was to focus giant laser beams on an incoming asteroid, using lenses in space that were several miles in diameter and laser systems that consumed more power than an entire city. Another scheme was to deploy a phalanx of thousands of high-speed "darts" against an asteroid, striking it so fast that they would slice it apart like a potato in a vegetable dicer. Most of the plans, however, depended on nuclear weapons to disrupt or destroy cosmic projectiles.

Several schemes were suggested for exploding asteroids into billions of small fragments. In one approach, it was necessary to get a large nuclear bomb into the center of the asteroid. This would be accomplished by launching a series of closely spaced nuclear warheads, like firecrackers on a string. The first one would be detonated at the surface, excavating a hole. The second warhead, following a fraction of a second behind, would explode within the hole, deepening it. After a dozen or more such explosions, the hole would be deep enough for a large bomb to penetrate and provide the coup de grace.

An alternative was suggested by Teller himself, who noted that a sufficiently large bomb detonated at the surface could disintegrate any comet or asteroid. The challenge was to make the bomb big enough. He noted that for the larger asteroids, a bomb would be required that was a million times more powerful than any that had yet been developed. Such a multi-million-megaton weapon would have no use in terrestrial warfare, of course, but Teller suggested it might be appropriate to develop such a bomb as part of a cosmic defense system.

All this talk of potential uses for nuclear weapons was well received by most of the participants, many of whom had been involved in the design and testing of nuclear bombs. This spirit was best captured by Lowell Wood, who noted in a comment made from the back of the room that there was considerable convergence of ideas from Los Alamos and Livermore scientists on the need to develop nuclear devices to deal with the impact hazard -- "but that is not surprising, since we share the same motivation: Nukes Forever!". The phrase "Nukes Forever!", later quoted in The New York Times and other newspapers, came to symbolize the Los Alamos meeting.

Although overshadowed by the seemingly outrageous proposals, a number of good ideas were presented at Los Alamos, and the meeting served to introduce the astronomers and weapons lab scientists to each other. Each group had previously been pursuing their interests in asteroid defense in isolation. A great deal of time was wasted, however, in consequence of Wood's cosmic bombardment paper and its focus on small bomblets. Wood's analysis had been accepted by Teller, Rather, and almost all of the other bomb-builders, who received the counter-arguments from the astronomers with open skepticism and resistance. Only late in the meeting, when Wood's colleagues from Livermore gave a talk in which they admitted that the atmosphere would block most small asteroids, did the majority of the audience consider backing off from their preoccupation with a short-range defense founded on a "shoot-first, look-later" philosophy. But by then the meeting was nearly over.

Although much of the talk at Los Alamos concerned ways to blow up an asteroid, the defense strategy of choice is deflection, not destruction. Any effort to blow apart an incoming projectile might well make things worse, transforming (in effect) a cannon ball into a cluster bomb. If, however, an offending asteroid could be gently nudged while still a long way from the Earth, it would be possible to change the orbit so that it would miss the Earth entirely. Two academic scientists, Tom Ahrens of Caltech and Alan Harris of NASA's Jet Propulsion Laboratory, presented a report on deflection requirements at Los Alamos and subsequently published their paper in the journal Nature, making theirs one of the first published papers to deal with asteroid defense issues. It is characteristic of the two cultures that Ahrens and Harris, the "outsiders", would publish their research findings in a widely available technical journal, while Wood and his colleagues produced only internal reports (such as the "Cosmic Bombardment" series, which had grown to "Cosmic Bombardment IV" by 1994) with circulation limited to a few colleagues in the weapons labs.

One of the unique aspects of the impact threat is that, as a natural hazard, it alone can be eliminated totally -- at least in principle. We cannot stop an earthquake or hurricane, but we can avoid an impact, given sufficient warning. The purpose of the Spaceguard Survey is to provide that warning. If a complete survey of the larger asteroids is carried out, any threatening object could -- most likely -- be identified decades before it actually struck the Earth. We would not need to have rockets and bombs ready on the launch pad. We would have many years during which to plan a defense strategy, study the object in detail, develop and build the necessary hardware, and perhaps try several ways to nudge it into a different orbit.

How can one nudge an asteroid? If it is small enough, then simply impacting its surface with a high-speed rocket might provide sufficient energy to change its orbit slightly. If a lot of energy must be provided, however, either because the threatening asteroid is large or the time available is short, then a nuclear device is the only choice. A one-ton conventional (non-nuclear) warhead has the chemical energy of one ton of TNT, but a one-ton nuclear warhead can pack a million tons -- a megaton -- of energy. Since any mitigation device must be launched into space from the surface of the Earth, the enormous advantage of nuclear defense schemes in the amount of wallop they carry per pound is clear.

How does one use a megaton of energy to change an asteroid's path? Simply detonating a megaton bomb on an asteroid's surface won't do the trick. It risks breaking the asteroid apart, especially if the asteroid is small (or fragile) and the bomb is large. We can provide a gentler nudge if the bomb is exploded in space near the asteroid, perhaps at a distance from the surface equal to the radius of the asteroid.

When a bomb explodes in the Earth's atmosphere, it releases much of its energy in the form of shock waves and supersonic winds composed of superheated air rushing outward from the fireball. If the bomb is exploded in the vacuum of space, there is no air to form such a blast wave. Instead, neutrons -- sub-atomic particles that are copiously produced in nuclear reactions -- slam into the surface of the asteroid, heating it and boiling off the rock. As the rock vapor flows away into space, it exerts a reaction force against the asteroid to move it in the opposite direction. This is an example of Newton's Third Law of an equal and opposite reaction, which accounts for the operation of jet engines and rockets.

Rough calculations presented at Los Alamos indicated that a megaton-size device would do the trick, assuming it was used several years in advance of the predicted impact. Even gentler accelerations could be provided by a series of smaller explosions. The most efficient approach is to use neutron-rich bombs, devices that had been discussed for several years by cold warriors as a way to kill people without damaging structures. The NASA astronomers wondered if such neutron bombs actually existed; the physicists from the weapons labs simply smiled.

Whatever its technical merits, the Los Alamos meeting was a public relations disaster. Having been barred from attending, the press heard indirectly about the discussions of nuclear weapons and Wood's "Nukes Forever!" outburst, and they were quick to attack. When NASA managers read the press reports and saw the weird ideas that Rather was promoting under the banner of new technologies, they moved quickly to disassociate themselves from the meeting. NASA released a sanitized summary that omitted most mention of nuclear weapons and Star-Wars technology, and the space agency refused to sanction publication of the more detailed reports that had been presented at Los Alamos. This task was delegated to the Department of Energy, which published a report a year later.

* * * * * *

Congress had asked for written reports from the two asteroid studies, and eventually it received them. The next step was to hold a hearing on the results. In March of 1993, a meeting of the Space Subcommittee of the House Committee on Science, Space and Technology was duly announced on the subject of "the threat of large Earth-orbit crossing asteroids". Letters of invitation were sent to David Morrison and John Rather as chairs of the two NASA studies, as well as to the headquarters offices of NASA and the Secretary of the Air Force. The Air Force demurred, pleading that the Clinton administration team had not had time to consider this matter, but on March 25 the three NASA witnesses presented themselves in the plush, high-ceilinged hearing room of the Rayburn House Office Building and sat down under the bright television lights to talk about asteroid impacts.

Presiding at the hearings was tall, courtly Congressman Ralph Hall of Texas, the chair of the Space Subcommittee and a strong supporter of the Space Station. Next to him sat Congressman George Brown of California, chair of the parent committee and one of the most knowledgeable members of Congress on matters of science and technology. Congressman Brown was the most vocal advocate of the Spaceguard Survey in the Congress, and he was especially anxious that the Air Force should join with NASA in pursuing asteroid searches. Other members of the subcommittee wandered in and out, apparently curious to see whether this strange topic was something that should actually be taken seriously by the official representatives of the American people.

Brown began the proceedings by noting that "for Members who are hearing about this subject for the first time, I know that the tendency is to be somewhat skeptical . . . None of our friends, relatives, or constituents have ever been killed by an asteroid." After enumerating some of the evidence that the Earth is struck occasionally by cosmic projectiles, he continued: "But why should Congress be involved? I think that it is the duty of the Congress to provide periodic oversight of all matters that relate to the health and welfare of the citizens of this country. This is particularly true for those issues where Congressional oversight might spur the Administration forward to take some appropriate action. I believe that the topic of Earth-threatening asteroids is just such an issue . . . I believe that the initiatives that are now getting underway to deal with this issue in a thorough and scientific manner have the potential for being one of the most important things that mankind has ever done."

Several other committee members also entered positive statements into the record, and then Morrison and Rather summarized the results of their two studies, followed by NASA Associate Administrator Wesley Huntress who spoke officially for the space agency. Huntress took a cautious tone, praising the two reports but declining to endorse their specific recommendations. Noting that NASA was doubling its support for astronomical studies of asteroids to $1 million per year, he mentioned the upgrade of Gehrel's Spacewatch Program and the funding of a new search telescope at Lowell Observatory to be used by Gene Shoemaker and Ted Bowell. The NASA objective was "to systematically and prudently increase the detection rate of NEOs and to enhance our understanding of these objects."

In the discussion that followed these statements, many of the questions were more concerned about scientific facts than programmatic issues -- questions about the nature of comets and asteroids, and about the evidence for historical impacts and their effects. The scientific exchange, which extended long beyond the scheduled time for the hearings, was a major departure from the usual business conducted in Congressional meetings.

At the end, Chairman Hall seemed somewhat bemused by the whole experience. Looking quizzically at the three witnesses sitting before him in their identical grey suits, he mused: "I think it's been a very interesting bit of testimony. I think it's obvious that you men and those with whom you work know what you're doing, and I guess it must be stressful to you to know some of the things that ought to be done . . . because you have no funds and no infrastructure [to discover and track potentially threatening objects]. You seem like pleasant men, but you must lead a frustrating life knowing that you're not likely to get any funds out of Congress at this time."

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CHAPTER 12: NUKES FOREVER!

SUMMARY

Congress had charged NASA, through its Spaceguard Working Group (described in Chapter 11), to develop a specific plan for an asteroid survey. The second part of their request was less well defined, asking only for an analysis of the technology that might be used for asteroid protection, if a threatening object were actually discovered. This charge formed the topic of a workshop, held in January 1992 at Los Alamos. This technology workshop was attended primarily by defense department scientists, many of them involved in the design and testing of nuclear bombs. A few astronomers, ourselves included, were invited, apparently as an afterthought. This chapter tells of our Los Alamos confrontation with the "star warriors" and describes the technologies proposed for a defense against asteroids and comets. It also introduces Edward Teller, the "father of the H Bomb" and a strong proponent of asteroid defense, together with his protege, Lowell Wood. We found the style of the star warriors very different from our own experience -- more secretive and apparently less critical of technical issues. The were also much more hierarchical, especially in the obsequious deference given to Teller. On defense issues, we describe how explosives could be used to deflect or destroy incoming NEOs, given adequate warning time. Nearly all of the suggested approaches involved nuclear technology, and Wood's outburst "We all agree on one thing -- Nukes forever!" has come to symbolize the Los Alamos workshop. The chapter concludes with a description of the 1993 testimony before Congress by Morrison and two other NASA witnesses.

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CHAPTER 13: ASTEROID DEFENSES

Regarding nuclear explosives, we have enough knowledge. But regarding the interaction of these explosives with comets and asteroids, we have much too little knowledge. I believe that calculations are important, but I believe that experiments are much more important. We must carry out experiments to determine how nuclear explosions of various kinds will react with heavenly bodies of various kinds. We cannot find out about these in any way other than by carrying out experiments. -- Physicist Edward Teller in a speech at the Space Protection of the Earth conference, Snezhinsk, Russia, September 1994.

In the months following the Los Alamos meeting, the astronomers and the weapons builders continued their dialogue. In time, both the United States Air Force and the Russian nuclear establishment became interested, as we describe in this chapter. An influential participant in all these discussions was Edward Teller, who had adopted the asteroid defense issue as a major theme for international cooperation in the post-cold-war world.

At Los Alamos, Teller had followed Lowell Wood's lead and focused on the problem of defending against small asteroids, which could not be detected more than a few days in advance of their collision with the Earth. This concept led to a "shoot first" defense strategy centered on a fleet of nuclear-tipped anti-asteroid missiles held at the ready. However, Teller's attitudes soon began to evolve. Later in the spring of 1992, when he and Morrison gave back-to-back keynote lectures at a meeting of the National Space Society in Colorado Springs, Teller had accepted the argument that small projectiles were not a problem and that some sort of comprehensive asteroid survey should be carried out. At that time, however, he advocated conducting a space-based search for NEOs using Star Wars technology. A series of small satellites would be placed in Earth orbit to scan the sky for asteroids, transmitting their observations to the ground to analysis.

Teller's primary argument for a space-based NEO search was that serendipitous astronomical discoveries might result from wide-field space telescopes. At Colorado Springs, Morrison countered that a ground-based search was much cheaper and more cost-effective: the job could be done for a hundred million dollars from the ground rather but would cost more than a billion dollars from space. At that meeting, the two could only agree to disagree. But by the time of the next hazard meeting, in January 1993, Teller was apparently reconciled to the cost-effectiveness of a ground-based asteroid survey like Spaceguard.

Looking ahead to the problems of deflection, however, Teller expressed concern about our lack of knowledge of the physical properties of comets and asteroids. Such knowledge would be necessary if we were to plan a comprehensive defense. He pushed for a major program of spacecraft visits to explore the comets and asteroids, using small and relatively inexpensive probes based on the "brilliant eyes" surveillance system being proposed by Wood at Livermore. He also suggested that initial spacecraft visits should be followed by active testing, including the experimental deflection of asteroids and comets to develop the technology that might someday be needed for defense against a real impact threat. Teller's proposal to start nuking asteroids stimulated a strong response from Carl Sagan and other astronomers, which we will discuss in a later chapter.

Although he remained in the background during the early interactions between astronomers and weapons scientists, one of the most influential figures in the impact hazard debates was Col. Pete Worden of the U.S. Air Force. A handsome career military officer with a Ph.D. in astrophysics, Worden was a long-time "space cadet". A severe critic of NASA, which he viewed as technically timid and excessively bureaucratic, Worden had embraced Reagan's Star Wars program as a means to develop new space technology and perhaps, if NASA faltered, to create the basis for an alternative space exploration program. Having served on the staff of the National Space Council under Vice President Quayle, in 1992 he was in the Pentagon managing advanced technology for the Star Wars program. There his small cadre of Air Force enthusiasts sometimes called themselves the "space Nazis".

With Teller's support, Pete Worden decided to organize another conference on asteroid defenses for the spring of 1993. This meeting would be everything that Rather's Los Alamos conference should have been. Focusing on near-term issues, Worden's meeting would exclude the fringe elements and their impractical "new technology" schemes. His meeting would be international, with the participation of leaders from the Russian defense establishment. And far from banning the press, Worden invited not only science reporters but also leading critics of the Star Wars program from a number of Washington space and peace advocacy groups. Bringing together 50 of the leading astronomers, weapons scientists, and their critics in an environment conducive to open discussion and compromise, Worden sought to forge a consensus in support of asteroid defense. For a location, he chose the picturesque town of Erice, perched atop a mountain on the west coast of Sicily. We both attended.

Erice, an ancient walled hill-town with narrow cobbled streets and glorious views of the Mediterranean, was developed in the 1970s as a site for international science conferences and summer courses, complete with modern meeting facilities, classrooms, and dormitories, all constructed in the renovated interiors of ancient monasteries. Erice was well known to Teller, Worden, and other insiders of the U.S. defense establishment. A series of annual "Seminars on Nuclear War" held there had provided opportunities for high-level military and civilian defense strategists from the U.S. and the Soviet Union to exchange views on a regular basis. With the end of the Cold War, these seminars had been broadened and retitled as "Seminars on Planetary Emergencies". The subject of Worden's Erice seminar in May 1993 was "The Impact on Earth of an Asteroid or Comet".

Worden set the tone for the meeting in his introductory talk, expressing hope for a consensus based on recognition of the reality of the impact hazard and a commitment for an international effort to deal with it, initially through support for the Spaceguard Survey. When we reported our work on the nature of the risk, emphasizing asteroids 1 km or more in diameter, we found none of the hostility that had characterized Los Alamos. The discussions were constructive, and gradually agreement emerged among the participants, in spite of their different backgrounds. The Russians seemed especially anxious to share their decades of experience with nuclear weapons in the service of a new, international cause.

On the final day of the Erice conference, we all gathered in the main monastery building to agree upon a statement that summarized the conclusions we had reached. The first four points of the statement were relatively straightforward. The group agreed that "cosmic impact is an environmentally significant phenomenon which has played a major role in the evolution of life on Earth", and that "the threat is real and requires further internationally coordinated public education efforts." They also agreed that "gathering of additional physical knowledge of NEOs and their effect on the Earth is a scientifically and socially important endeavor," and that "dedicated international astronomical facilities similar to the proposed Spaceguard System should be developed."

On a final question, however, consensus broke down. Worden did not want to include any discussion of defense systems, but Teller insisted that this issue be addressed. Teller was convinced that nuclear experiments on deflecting real asteroids and comets were necessary. He had two reasons. One was technical: the need to learn how to accomplish such a task in advance of any actual emergency. His second reason was political. He argued that it would be extremely difficult to obtain international agreement for any use of nuclear devices in space, and that we had better start now to work out ways to deal with this problem. Teller considered the social experiment of forging international consensus in support of experimental deflections to be more challenging than the technical problems of accomplishing the deflection once it was approved.

In the Erice environment, Teller did not automatically get his way. Opposed by Worden, Shoemaker, and all of the Spaceguard astronomers attending the conference, Teller found himself engaged in a real debate. Most of us wanted to say explicitly that we did not favor experimentation at this time, but Teller stood his ground. He would not agree to any statement that called for deferring experiments. The majority would not sign any statement that endorsed such experiments. After heated arguments, the stalemate was resolved in the waning minutes of the conference and compromise wording was reluctantly approved: "The study of potential mitigation systems should be continued. Many of us believe that unless a specific and imminent threat becomes obvious, actual construction and testing of systems that might have the potential to deflect or mitigate a threat may be deferred because technology systems will improve." The "many" referred to in the statement was nearly everyone at the meeting except Teller.

In spite of this last-minute dispute, Pete Worden had accomplished his goal. Everyone at last agreed on the general nature of the impact threat. The Russian attendees were enthusiastic about joint efforts that might make use of their capabilities, including unused tracking telescopes that they had available. And, perhaps most remarkable, all of the critics who attended were now converts and supporters of continued efforts to deal with the impact threat.

Science is founded on the principle that observations, experiments, and calculations can be reproduced and verified. The conclusions of an individual scientist or a team should be checked through repetition by others. In practice, however, one cannot check or reproduce the vast amount of data and interpretation that are presented at meetings and published in technical journals. Individual scientists therefore develop a sense of who and what can be trusted. We learn, through a combination of experience and intuition, who are the most reliable observers, the most precise experimentalists, and the cleverest theorists. When the NASA astronomers and the physicists from the nuclear weapons labs first came together in January 1992, they had no basis for trust, no way to know who in the other camp was reliable or brilliant and who might not be. Between Los Alamos and Erice, a sort of Darwinian process eliminated the fringe elements and allowed some members from each community to gauge the worth of the others. One of the products of this interaction was a sense of trust and shared values.

* * * * * *

The emergence in Russia of concern about the impact hazard happened at the same time as in the United States, with Russian astronomers and weapons builders playing roles that paralleled those of their American counterparts. Many Russian scientists were interested in defense against asteroids for the same reasons as Americans: a mixture of scientific curiosity, desire to perform a socially useful purpose by promoting protection of the Earth, and self-interest associated with retaining their jobs and research funding. They sponsored the first international meeting specifically devoted to the NEO impact hazard, held in October 1991.

At just the moment the Soviet Union was disintegrating, astronomers from the Institute for Theoretical Astronomy of the USSR Academy of Sciences invited their colleagues from Russia and abroad (ourselves included) to meet in St. Petersburg (then newly renamed from Leningrad). Few of the Russian attendees were previously involved with the impact hazard, but after three days of open and vigorous discussion, they endorsed an asteroid survey. The organizers were particularly interested in making a case for Russian government funding for a new search telescope and for follow-up studies of NEO orbits, a scientific specialty of the Institute for Theoretical Astronomy. Following the meeting they formed an international organization on the impact hazard to support the legitimacy of their funding requests.

While the Russian astronomers continued to exchange information with their counterparts from the Spaceguard team in the United States, a quite different group of Russians began discussions with Teller and his colleagues from the U.S. weapons laboratories. For many years the defense strategists and weapons builders from the U.S. and Russia had worked together, first as cold-war rivals and more recently in a spirit of international cooperation. These scientists were especially interested in applying their experience with nuclear testing to problems associated with the effects of impact explosions. Since the USSR had set off much larger atmospheric tests than had the Americans, they were able to make unique contributions to this research. Within Russia the astronomers and the weapons scientists initially pursued their interests independently, but by 1994 they had begun to talk to each other, largely at the initiative of small, dapper nuclear physicist Vadim Simonenko, an energetic, outgoing leader of the new post-Soviet generation of Russian defense experts.

Simonenko worked at the Institute of Technical Physics of the Russian Federal Nuclear Center -- one of two major centers for research on nuclear weapons and the Russian equivalent of Livermore National Laboratory in the United States. For four decades the existence of this Federal Nuclear Center had been a military secret, and the city in the southern Urals where it was located did not even have a name -- just a postal code, Chelyabinsk-70. It was a major milestone in the emergence of this secret city when Simonenko obtained permission to host an international conference on asteroid defense there. The conference was named "Space Protection of the Earth 1994", and both Teller and Wood accepted invitations to attend, as did five other Americans, including Tom Gehrels and one of us (Morrison).

The American party arrived at the Ekaterinburg Airport in the small hours of the morning of September 25, having crossed 12 time zones in the flight from California. Soon we were in a bus, following the blinking blue light of a police escort through endless miles of white birch trees to our destination: a closed city that appears on no map of Russia. Only during the preceding year had it obtained a name: Snezhinsk, which means "snowy" in Russian. When we finally arrived, we entered Snezhinsk through high barbed wire fences guarded by soldiers with automatic weapons and unsheathed bayonets. Here, isolated from the rest of the world, lived the 15,000 workers at the nuclear institute and their families. A huge bronze statue of Lenin still dominated the central square, and several of us spoke with students in the schools who had never before seen or met an American. These talented people, who have spent two generations building nuclear bombs in this closed society, were looking for alternatives. Shooting down asteroids seemed like a possibility.

The Space Protection meeting was attended by about 150 Russians, the majority of whom had not previously worked with foreigners. These scientists and engineers were experts in nuclear weapons and missile systems, including space tracking and surveillance. For a week they met to discuss asteroids, with particular emphasis on schemes for interception and nuclear deflection or destruction of NEOs. Teller, as "father of the H-bomb", was idolized by the Russian nuclear community, and he and Wood had come to Russia with a message that this audience was happy to hear. The Teller-Wood thesis was simple: we must build an international defense system against cosmic impacts, and an urgent part of that effort is to conduct nuclear tests to learn how to deflect or destroy NEOs. Teller emphasized that such an experimental program was the only way to obtain the required data, and that nuking asteroids represented the most cost-effective kind of experimentation. In case anyone missed the message, Wood also told the audience that there were no international prohibitions against nuclear explosions in space, since the existing treaties dealt only with "weapons of mass destruction", not peaceful uses of nuclear explosives intended to develop a capability to protect the Earth. Teller added that only fear-mongers opposed the peaceful use of nuclear explosives.

When Tom Gehrels told the conference that the majority of American scientists opposed nuclear testing in space, he was fiercely attacked by both Teller and Wood for his efforts to "politicize" the meeting. These attacks seemed particularly inappropriate directed toward Gehrels, who had broad international experience and personal cultural ties to Europe and Asia as well as America. For good measure, Wood also publicly accused Morrison and the Spaceguard scientists of intentionally minimizing the impact risk and lying to the American Congress. Here, on Russian soil, the apparent consensus among the Americans evaporated, and Teller and Wood seemed less inhibited about their advocacy of nuclear testing in space. This was certainly a message that the Russians welcomed, desperate as they were to find a future for their nuclear profession in a world increasingly committed to disarmament and reduction in international tensions.

The Russian and American weapons builders have held two additional defense meetings after the Space Protection of the Earth 94 conference, at Livermore in 1995 and again at Snezhinsk in 1996, but these meetings were anticlimactic. Without funding, there was little technical progress on developing defense systems, only a recycling of the same ideas and proposals. In fact, not one scientist from either the Livermore or Los Alamos National Laboratories bothered to attend Simonenko's Space Protection of the Earth 96 meeting in the Urals. Meanwhile, the major Russian aerospace industries have taken up the NEO defense issue in their efforts to find new customers. The Lovochkin, Khrunichev, and Makeyav companies all made presentations at the 1996 Space Protection meeting favoring immediate deployment of a "Space Shield" defense system, based on the concepts of a short-range defense that had been discredited in the United States ever since the Los Alamos workshop in 1992. A.V. Zaitzev of Lavochkin went so far as to say that he could reveal for the first time to this audience the previously-secret development in Russia of all the elements of such a Space Shield. He said that their proposal was based on the Zenit rocket which can be readied for launch in just 90 minutes, the Mars-96 spacecraft which has the capacity to carry multi-megaton nuclear explosives, and the ASTRON space telescope which can be modified to serve as a NEO surveillance system. All this sounds pretty scary, but even the Russian scientists did not take this Space Shield proposal very seriously.

In the same week in September 1996 that the defense scientists met at Snezhinsk, world leaders at the UN signed the Comprehensive Nuclear Test Ban Treaty, adding to the Russians' desperate concern about their future in what had once been a model city, with its lovely parks and excellent schools. No one had been paid for three months, and a few days following the conference, the Director of the Federal Nuclear Center in Snezhinsk committed suicide. As we drank vodka toasts to international cooperation at the final conference banquet, flurries of snow were already falling through the birch forests, and the winter promised to be a cruel one. Whatever the future might hold for NEO defenses, these celestial threats could not solve the immediate problems of the Russian defense labs and their staffs.

3000 words (3/27/97)

CHAPTER 13: ASTEROID DEFENSES

SUMMARY

In the months following the Los Alamos meeting, the astronomers and the weapons builders continued their dialogue. In time, both the United States Air Force and the Russian nuclear establishment became interested, as we describe in this chapter. A continuing influential participant in all these discussions was Edward Teller, who adopted the asteroid defense issue as a major theme for international cooperation in the post-cold-war world. We discuss how US Air Force Col. Pete Worden brought the US and Russian participants together for the first time in 1993 in an isolated monastery in Sicily, and the return invitation from the Russian nuclear scientists to meet in 1994 in the Siberian city of Chelyabinsk-70. At Chelyabinsk-70, a formerly secret city still surrounded by double barbed-wire fences patrolled by soldiers and dogs, little has changed since the Soviet era. There is even a huge bronze statue of Lenin still standing in the central square. This was the occasion of Teller's first visit to the Russian nuclear center, and we provide first-hand descriptions of how he used the opportunity to urge the Russians to join an international program to test nuclear weapons against Earth-approaching asteroids. In telling the history of these events, we also further describe the technology that could be used for such space defense systems.

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CHAPTER 14: THE YEAR OF THE COMETS

Needs opening quote on comets

Although the category of Near Earth Objects, or NEOs, includes both comets and asteroids, we have focussed in the previous chapters on discovering, tracking, and defending against asteroids. This makes sense, because most impacting objects are asteroids, probably more than 90%. Nevertheless, comet impacts play an important role as well, and the recent appearance of several widely-publicized comets has called attention to the threat they pose. First came the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994, then in 1996 Comet Hyakutake came closer to Earth than any other comet this century, and finally Comet Hale-Bopp in 1997 was the most widely seen spectacular comet since 1910. Truly, the 1990s have been the years of the comets.

Actually, there are three classes of NEOs that we must consider, not just two (asteroids and comets). The long-period comets are the simplest and least-ambiguous category: loosely compacted icy and rocky objects (dirty snowballs) that enter the inner solar system on highly elongated orbits. By definition, a long-period comet is one that takes more than 20 years to orbit the sun. (Some astronomers insert another category, intermediate-period comets, for those with periods between 20 and 200 years). As a matter of fact, almost all long-period comets have periods of thousands of years or more, and many of them are probably entering the inner solar system for the first time in its 4.5-billion year history.

The second category is the short-period comets, those with orbital periods of less than 20 years. These are all comets that first approached the Sun on long-period orbits, but whose orbits have been made smaller, usually by the gravitational attraction of Jupiter. Most short-period comets are said to be part of the Jupiter family of comets, since their approximately 6-year orbits take them out only as far as the orbit of Jupiter. Finally, there are the near-Earth asteroids themselves, solid objects with orbits that bring them repeatedly close to the Earth. Typically, their orbital periods are less than 3 years. Although mostly rocky and metallic in composition, the near-Earth asteroids probably include a substantial number of dead or dormant comets, which have lost their near-surface ices as a result of repeated heating by the Sun.

Thought of as a cosmic bomb, the composition of a colliding object makes little difference. The energy of an impact, not the physical or chemical make-up of the projectile, primarily determines the damage done. This energy, however, is proportional to the square of the impact speed. Therefore it matters a great deal that long-period comets, swinging into the inner solar system on highly elongated orbits, approach the Earth at higher speeds than either the asteroids or the short-period comets. Typically, an asteroid strikes the Earth at a speed of 20 km/s, while a long-period comet strikes at two times this speed or 4 times the energy (since 2-squared is 4). In the case of Comet Halley, which circles the Sun in a backward or retrograde orbit, the encounter velocity is more than 60 km/s. So Halley would generate 9 times the explosive energy of a typical asteroid of the same mass. Comet impacts are few in number, but they pack a powerful punch.

Another distinction between long-period comets and objects in smaller orbits is that current search strategies are poorly suited to find the former. For example, the Spaceguard search strategy of discovering all potential impactors years before they pose a direct threat applies only to objects whose orbits thread repeatedly through the inner solar system. To find and catalogue these objects, we depend on the fact that they will pass near the Earth thousands of times before they actually hit. If we miss one of them this year, we will pick it up on its next orbit, or the one after that. This is an excellent strategy for near-Earth asteroids and short-period comets, but it fails completely for long-period comets. If one of these interlopers is destined to hit the Earth, it will not give us decades or centuries of warning. Instead, we will have to pick it up on the way in, and even with the best telescopes the warning time is not likely to be more than two years. Protection against long-period comets requires eternal vigilance, and if there is any prospect of defending ourselves, we must act on relatively short notice. We will return to the "nightmare scenario" of a predicted comet impact in the next chapter.

For now, let us agree on the terminology we will use. When we say asteroid, we are including the short period comets, which have similar orbits and impact speeds and can be catalogued in a Spaceguard survey just like the asteroids. When we speak of comets, we are referring to the long-period comets, those with orbital periods of more than 20 years. These are the ones that hit us fast and don't provide much warning. Comet, in this chapter, always means long-period comet. NEO, as before, includes both asteroids and comets with orbits that cross the orbit of the Earth.

While almost all astronomers agree that comets pose less of a hazard than asteroids, they don't agree how much less. One reason we are uncertain is that it is very difficult to measure the size of a comet's nucleus, which we must do in order to estimate its mass and hence the energy it would release if it hit the Earth. Radar and infrared techniques have both been applied to this problem, but to date few comets have revealed their size. The best measurement was made for Comet Halley in 1986, when the European Giotto spacecraft flew within 600 km of Halley's nucleus and photographed it directly. Irregular and elongated, the dark nucleus turned out to be 11 km long and about 7 km wide, roughly the size of the island of Manhattan south of 116th Street. The largest measured nucleus is that of Comet Hale-Bopp, revealed from both infrared measurements and direct photography with the Hubble Space Telescope to have a diameter of between 20 and 40 km. Halley is somewhat smaller than the K-T impactor that blasted out Chicxulub crater, and Hale-Bopp is substantially larger. Several other measured comets (including Hyakutake) have diameters of between 2 and 7 km.

Based or our current rather meager knowledge, it appears that comets make up about 5% of the impactors in any given size range. Since on average each comet hits with about four times the energy of an asteroid of its same size, comets make up about 20% of the hazard. These estimates are most secure for relatively small impacting objects, such as the 2-km objects that constitute the primary threat to Earth. At larger sizes, there is some indication that asteroids become under-represented relative to comets. As one example, note that there is no present Earth-crossing asteroid as large as Comet Hale-Bopp. Gene Shoemaker thinks that at the magnitude of the K-T impact (hundreds of millions of megatons), comets may be at least as important as asteroids. But fortunately, such major mass-extinction events take place only once in 100 million years or more. Thus we estimate that asteroids make up 80% of the contemporary NEO threat, comets 20%.

There is another proposal made by a few astronomers that would elevate cometary impacts to a much more important role. We mentioned in Chapter 7 that a handful of British neo-catastrophists, including Victor Clube and Duncan Steel, believe that most terrestrial impacts result from the breakup of giant comets entering the inner solar system for the first time. When one of these pristine comets, perhaps more than 100 km in diameter, passes close to the Sun, it can spawn thousands of Halley-size or smaller fragments, called a comet shower. The result might be several large terrestrial impacts within a period of only a few thousand years, a hypothesis Clube and Steel call coherent catastrophism. These astronomers believe that such a giant-comet break-up took place in the solar system between 10 and 20 thousand years ago. They suggest that we are currently in a lull, but that in about a thousand years the heavy cometary bombardment will begin again. The ideas of coherent catastrophism are explained in detail in two recent popular-level books by Duncan Steel (Rogue Asteroids and Doomsday Comets) and Geritt Vershuur (Impact: The Threat of Comets and Asteroids), and they have received considerable press coverage, especially in Britain and Australia. We don't agree with these arguments, and we see no evidence of such a comet shower taking place now or in the historical past. But if they are correct, then comets dominate in impact risk, and indeed the risk itself is (or soon will be) hundreds or thousands of times greater than we have portrayed in this book.

Comet orbits present more of a challenge than those of asteroids. Through a telescope, an asteroid is a tiny star-like point that can be measured with precision relative to the background of stars. And asteroids move inexorably according to Kepler's Laws. In contrast, the solid nucleus of a comet is generally invisible within the extended glow of its atmosphere and tail, so position measurements have large errors. In addition, comets have the disconcerting habit of shifting their orbits slightly. The problem is not a breakdown of classical gravitational theory, but rather the rocket effect produced on comets when they are near the Sun. The evaporation of material from the surface of a comet is concentrated in jets or fountains. These jets of gas and dust exert a tiny reaction force on the nucleus, gradually changing its orbit just as if a rocket engine had been fastened to it. The problem is that the location and force of these jets change from time to time, so this rocket effect is virtually unpredictable. Thus many comets diverge gradually from their predicted paths, and while these effects are small, they can be important if we are trying to figure out whether a comet might hit the Earth. As we saw in discussing ways to deflect an asteroid or comet headed for the Earth, very small changes in speed can make large positional differences after several years. The inherent unpredictability of comet orbits means that effective defense strategies would require rapid response. It could be difficult, even with very precise astronomical observations, to predict whether or not a comet will hit the Earth. It might not be until the last couple of months before impact that we could be sure of our fate.

Yet another problem with comet orbits is their vulnerability to perturbations from Jupiter. None of the near-Earth asteroids has a Jupiter-crossing orbit, so they generally follow stable paths. But both the long-period comets and the Jupiter-family comets can experience dramatic orbit changes when they approach the giant planet. We have seen two recent examples of such perturbations. Prior to its 1997 appearance, Comet Hale-Bopp had an orbital period of 4400 years, but now the comet has been pulled down into a 2200 year orbit. The fate of Shoemaker-Levy 9 was even more dramatic. This comet was probably first captured in 1929 into a chaotic orbit around Jupiter, then disrupted in a close Jupiter encounter in 1992, and finally consumed by the giant planet in 1994.

Given the difficulty of calculating comet orbits, it is no surprise that the first false-alarm in predicting an impact on Earth dealt with a comet. On October xx, 1992, astronomer Brian Marsden issued what appeared to be an official warning from the Minor Planet Center of the International Astronomical Union that Comet Swift-Tuttle might crash into the Earth on its next return, on August 14, 2126. Although we doubt any astronomers who received the warning expected to live so long, 2126 is practically tomorrow by astronomical standards. Indeed your grandchildren's grandchildren might still be alive, so the warning had a certain news value -- indeed it garnered immediate prominent attention by The New York Times.

Marsden is an expert in the ancient but esoteric field of dynamical astronomy, the precise computation of orbits. As director of the International Astronomical Union Minor Planet Center, Marsden is the "cop" of the astronomical community, charged with establishing orbits for newly discovered objects (such as comets, asteroids, and supernovae) and assigning them appropriate names and numbers. Amateur astronomers know him as the person who resolves issues of priority in comet discoveries, and thus decides what name each new comet will bear. Marsden had participated in the Spaceguard Working Group and had attended the Los Alamos meeting, where he played a generally conservative role as the advocate for careful observations and strong international cooperation. He was not likely to shoot from the hip, so a warning from him of a possible future impact was certain to be taken seriously.

While the name of Comet Swift-Tuttle was not itself familiar, many people knew indirectly of the comet's existence. Several times each year, as the Earth circles the Sun, its orbit intersects the orbit of one or another comet. The comet, of course, is usually nowhere near, being elsewhere on its orbital path. But since comets shed dust as they evaporate, their orbits tend to be dirty places, filled with tiny specks of cometary debris. As the Earth crosses these dusty comet trails (not to be confused with the more visible but ephemeral comet tails), we see the bits of dust burn up in our atmosphere as meteors or shooting stars. If the comet trail is very dirty, we see a meteor shower. The most well-known and dependable of the annual meteor showers is called the Perseids, peaking between August 11 and 14 of each year. The Perseid meteors, which are seen by both amateur astronomers and casual sky-watchers each August, are debris from Comet Swift-Tuttle.

Swift-Tuttle is a long-period comet, with a period of 133 years. It was discovered by American observers Swift and Tuttle during the Civil War, and it did not make its second pass by the Sun until 1992. Until it was first seen by a Japanese amateur astronomer in late September, 1992, astronomers were not sure of the orbit or period of the comet, but with new observations flowing in, Marsden and his colleagues were able for the first time to compute an orbit and predict the circumstances of its next return, in 2126.

The fact that the Earth annually encounters the dust trail of Swift-Tuttle indicates that the comet's orbit intersects that of the Earth. In 1992, however, the closest the comet came was 150 million km. A collision is possible only if the comet and the Earth occupy the same volume of space at the same time. The speed of the comet relative to the Earth is about 25 km/s, so it travels the width of the Earth in about 600 seconds, or 10 minutes. Thus to predict an impact, Marsden would have to calculate the position of the comet in its orbit with a precision of 10 minutes at its next close approach, in 2126.

What Marsden actually told the press in October 1992 was that he thought the uncertainty in the Swift-Tuttle orbital prediction for 2126 was at least two weeks. For the best calculated orbit, no collision would take place, but Marsden felt that the uncertainties permitted the possibility of a collision. Speaking offhand to reporters, he guessed that the chances of a collision might be about 1 in a thousand, which is a substantial exaggeration. Even in a state of ignorance, the likelihood that the comet would arrive nearly two weeks late, and do so in the exact 10-minute interval that would lead to a collision, were far less than 1 in a thousand. In any case, many press stories reported this as an actual prediction of a collision in 2126. Since we suspect that the nucleus of Swift-Tuttle is at least as large as that of Halley, such a prediction implied a catastrophe of unprecedented magnitude.

As it happened, there was a way to resolve the issue by calculating a better orbit. Marsden had used only observations of Swift-Tuttle made in 1862 and 1992, and some of these were ambiguous. Don Yeomans, an expert on orbits from JPL, knew that more observations extending over a longer time base would lead to a much refined orbit. Projecting backward in time, Yeomans concluded that the comet must also have come past in 1737, and a search of the historical records indicated observations were made of a comet that year with an orbit consistent with Swift-Tuttle. Combining these observations with the other data, Yeomans calculated that the 2126 return of the comet would bring it to the Earth's orbit on August 5, while the Earth itself was still 20 million km away. Thus with improved orbital calculations, the impact threat disappeared.

Marsden was at first reluctant to abandon his doomsday prediction. His primary motive in releasing the prediction had been to goad astronomers into making more accurate observations of Swift-Tuttle as it sped away from the Sun and Earth in the waning months of 1992, so that its orbit could be determined with higher precision. But he was also enjoying the publicity his comments had brought. He confided to friends that he thought the Swift-Tuttle warning had served as a timely wake-up call for the impact threat -- distant enough in time to avoid panic, but real enough to command serious attention from the media and the public. Not until December 10 did he issue an official "all clear", in which he told the press that "there is very little possibility of the comet's collision with the Earth during the next millennium."

If Marsden's motive was to stimulate a public discussion of the impact hazard, he certainly succeeded. One result was a cover story in the November 23 issue of Newsweek, entitled "Doomsday Science: New theories about comets, asteroids, and how the world might end." The story faithfully summarized the conclusions of the Spaceguard Working Group and downplayed the more divisive political issues. The story concluded: "Killer asteroids and comets are out there. And someday, one will be on a collision course with Earth. Of all the species that ever crawled, walked, flew or swam on Earth, an estimated two thirds became extinct because of an impact from space. Mankind may yet meet that fate too. But we're the only species that can even contemplate it and, just maybe, do something to prevent it."

Two years later, the actual collision of Comet Shoemaker-Levy 9 with Jupiter attracted wide attention and did much to legitimize the concept of collisional hazards, as we noted in previous chapters. Because of its large size, Jupiter is a much more frequent target for impacts than is the Earth. Indeed, George Wetherill has calculated that the presence of Jupiter protects the Earth from most comet impacts. In effect, it draws the fire away from our own planet in the cosmic shooting gallery. But the fact that Jupiter was hit during our lifetimes called public attention to the potential vulnerability of the Earth to similar catastrophes.

One of the groups that was impressed by the lesson of Shoemaker-Levy 9 was the Science and Technology Committee of the U.S. House of Representatives. Committee Chairman George Brown considered this an ideal time to renew his earlier call for a Spaceguard asteroid survey, an idea that had lain largely dormant since the completion in 1992 of the two reports called for in the 1990 NASA authorization bill. With his encouragement, staffers Terry Dawson and Bill Smith drafted a new request for inclusion in the NASA bill for Fiscal Year 1995. On July xx, in the very week of the great comet crash, Brown's committee adopted the following language:

Committee quote

This time NASA responded quickly to the Congressional initiative, and within two weeks they appointed Gene Shoemaker to chair a new NEO detection study. We were both among the alumnae of the Spaceguard Working Group who provided continuity with previous work. The weapons labs were represented by Teller protege Greg Canavan, a senior physicist at Los Alamos National Laboratory and a former Air Force officer. The Air Force also responded positively, and they were represented on the Shoemaker committee by both Col. Pete Worden, recently appointed Commander of Falcon Air Force Base, and civilian John Darrah, Chief Scientist of the USAF Space Command.

The Congressional charge to Shoemaker's team was technically challenging but straightforward in principle. The House Committee had accepted all the essentials of the Spaceguard Survey Report, including its focus on asteroids 1 km or larger in diameter as constituting the most serious hazard. Now they wanted NASA to prepare a specific plan to carry out the Spaceguard Survey, and to complete it in just 10 years, rather than 25 years as proposed in the 1992 report. Shoemaker's team had to try to find a way to achieve results quickly. In addition, NASA wanted the price of the survey cut to a fraction of the $250 million estimated in the original report. In other words, Spaceguard should be implemented according to the "better, faster, cheaper" philosophy of NASA Administrator Dan Goldin.

The Shoemaker Committee was remarkably successful in meeting its objectives. We determined that improvements in CCD detectors and computer systems had substantially simplified the task, and that the survey could be carried out in one decade with half of the number of telescopes originally proposed. When the cost of the new Spaceguard program was added up, it came to just $5 million per year for a decade total of $50 million, only one fifth the original pricetag. However, work on the report went slowly as Gene Shoemaker, now a global celebrity, found himself traveling far and wide receiving awards and giving lectures about his comet. Deadline after deadline passed, and the report bearing the good news about a faster, cheaper Spaceguard was not submitted to NASA until May 1995. When the space agency forwarded the Shoemaker report to Congress, it stated in a cover letter that NASA did not intend to implement to proposed survey, and that no funds would be requested from Congress for this purpose.

In the year since all eyes had been turned on Shoemaker-Levy 9, the American political landscape had changed dramatically. With Republicans in control of the 104th Congress, George Brown lost his chairmanship of the House Science Committee, and Terry Dawson, the primary Spaceguard supporter on the committee staff, lost his job. As part of a bipartisan effort to downsize government and reduce the national budget deficit, NASA faced a declining budget that threatened the survival of the space program. Had the science community anticipated the call from Congress and been ready with a Spaceguard implementation plan in the fall of 1994, we might have succeeded. But by the summer of 1995, no one was listening.

How quickly the situation had deteriorated. In 1995 Gene and Carolyn Shoemaker terminated their long-standing sky survey at Palomar, a project that had consumed one quarter of their time for more than a decade. Gene retired from his position at the US Geological Survey and moved to Lowell Observatory, where he began a collaboration with Ted Bowell to build a new survey telescope closer to home. A few months later the Australian government cut off funding for the photographic asteroid survey at the Anglo-Australian Observatory, and by the end of 1996 Duncan Steel was out of a job. Where four teams had been searching for asteroids at the time Shoemaker-Levy 9 was discovered, the worldwide effort was now down to just two underfunded groups, led by Tom Gehrels in Arizona and Eleanor Helin at Palomar. After rising steadily for the past two decades, the discovery rate of NEOs actually declined in 1995.

Yet there was good news too. The Shoemaker Committee really had figured out ways to simplify and accelerate the Spaceguard Survey, and NASA had increased its annual support to $1 million per year, primarily to encourage development of new instrumentation. Equally important, the US Air Force was now seriously considering NEO searches. Pete Worden was interested in all aspects of NEO studies, from searches to space missions to the development of defense systems, and he gradually converted some of his colleagues -- primarily the technically-minded Majors and Colonels of the Air Force Space Command -- to the asteroid issue.

The USAF Space Command has a substantial capability in celestial tracking and observation, dating back to satellite tracking stations built at the time of Sputnik. More recently, nearly 20 one-meter-aperture telescopes have been constructed to track Air Force satellites, particularly those secret or "black" satellites that maintain radio silence as they wait in high orbits for possible activation in time of emergency. This GEODSS (Global Electro-Optical Defense Space Surveillance) system has operated observatories in New Mexico, Hawaii, Korea, Spain, and Diego Garcia Island in the Indian Ocean. In principle, these telescopes could be instrumented to search for asteroids as well. Indeed, some Space Command staff thought an asteroid survey might provide the rationale to upgrade the GEODSS detector systems. Plans were made to carry out the upgrade of one of the New Mexico telescopes in 1995, leading to an experimental asteroid search program in 1996. But an independent NASA-USAF team beat them to the punch.

Without consulting the USAF high brass, Eleanor Helin and her JPL search group established a collaboration with Air Force scientists to carry out asteroid observations with the GEODSS telescope in Hawaii. They called themselves the Near Earth Asteroid Team (NEAT). With NASA instrument development funds and assistance from Livermore National Lab, NEAT built a new CCD-based camera and supporting software, which began sky scans early in 1996.

[***more here on NEAT accomplishments]

With two modern electronic search systems (Spacewatch and NEAT) on-line, the discovery rate of NEOs began to climb again. Impressed with the NEAT results, NASA began negotiations in 1997 with the National Science Foundation to install a second NEAT camera on an old 2-meter telescope at Cerro Tololo Interamerican Observatory in Chile, thus extending the searches to southern skies. Congress again expressed its interest, with the House Science Committee voting in April 1997 to encourage NASA to increase its spending on NEO searches from $1 million to $3 million per year.

Tom Gehrels was not idle either, and by 1997 he was well on his way to expanding the Spacewatch search to a larger 2-meter telescope. This had been his plan from the time he first proposed Spacewatch to the 1981 NASA Snowmass meeting, and 17 years later his dream was about to become a reality. Yet another modern search camera was under construction at Lowell Observatory in northern Arizona, in a NASA-funded collaboration between Gene Shoemaker and Ted Bowell. Called the Lowell Observatory Near Earth Object Search (LONEOS), this telescope differed from the Spacewatch and NEAT systems in using a wide-angle telescope, optimized for covering large sky areas and therefore for discovering the relatively large (1-5 km) asteroids. Meanwhile, the old 0.4-meter photographic telescope at Palomar, which had discovered most of the known NEOs over the previous two decades, was finally closed down for good.

NASA and the USAF also discussed expanding the NEAT survey to other GEODSS telescopes beyond the one Helin was using in Hawaii. Such a collaboration could provide the fastest way to accelerate the search, as proposed by the Shoemaker Committee in 1995. However, the Air Force was still not convinced that NEO surveys were consistent with the primary GEODSS role of tracking military satellites. Others, for their part, doubted whether if was wise for discovery of asteroids to pass from civilian to military hands. They worried about security issues that excluded most astronomers from the GEODSS sites and might encourage the classification of survey results. NASA has taken the position that if it supplies the NEAT cameras, the data from Air Force surveys must remain in civilian hands.

As a result of all these new systems, the core of a Spaceguard Survey was emerging in 1997, based on two Spacewatch telescopes (of 1 and 2 meter aperture), the 1-meter NEAT/GEODSS telescope in Hawaii, a proposed 2-meter NEAT system in Chile, and the LONEOS wide-angle camera. This extraordinary development was primarily the product of entrepreneurial efforts by individual astronomers rather than a coherent plan executed from above. Such a system should be able to catalogue all the 1-km NEOs within two decades, if not one. All that remains is to ensure cooperation among these individual searches. If each telescope scans the same part of the sky instead of working in a complementary mode, the full capability of the system will not be realized. But if the teams work together instead of competing to see who can discover the most NEOs, Spaceguard can be a reality before the end of this millennium.

4900 words (5/7/97)

CHAPTER 14: THE YEAR OF THE COMETS

SUMMARY

After the widely publicized 1994 impact of Comet Shoemaker-Levy 9 with Jupiter, Congress asked NASA to work with the US Air Force Space Command to develop a program plan to accelerate the timetable for NEO search and discovery, and we participated in the joint NASA/USAF study team led by Gene Shoemaker. In the same year the Air Force launched the first of its Clementine series of military spacecraft to near-Earth asteroids, while NASA moved toward a 1996 launch of its own Near Earth Asteroid Rendezvous mission. In this chapter we discuss the outcome of the Shoemaker team report and the continuing relationship between NASA and the Air Force, as both potential partners sought to define their role in asteroid searches and planetary defense. We describe the various asteroid spacecraft missions and the new proposals for less expensive ways to carry out the Spaceguard Survey. Meanwhile the US military dropped all reference to nuclear solutions, presumably in deference to public concerns about nuclear weapons in space, and proposed a formal planetary defense mission for the US Air Force. This chapter brings us up to date on where each of these programs stands, as we prepare for a new round of congressional testimony in 1997.

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CHAPTER 15: THE PUBLIC POLICY DEBATE

If a big impact happens, it would be an unprecedented human catastrophe. There's something like one chance in two thousand that such a collision will happen in the lifetime of a newborn baby. Most of us would not fly in an airplane if the chance of crashing were one in two thousand. (In fact, for commercial flights the chance is one in two million. Even so, many people consider this large enough to worry about, or even to take our insurance for). When our lives are at stake, we often change our behavior to arrange more favorable odds. Those who don't tend to no longer be with us. -- Carl Sagan, in Pale Blue Dot (1994)

Recognition of the NEO hazard has influenced far more than academic scientists, as we have seen in the previous chapters of this book. Compared with other natural hazards, impacts are quite remarkable. They are the only known natural hazard that can threaten the lives of billions of people and the survival of civilization. They are also the only major hazard that can, at last in principle, be avoided by the judicious application of technology. Many issues of public policy are associated with our new awareness of impacts. Is there a government responsibility to protect us against impacts? How much should be spent on this effort? Should NEO defenses be a national or an international responsibility, and should they be in the hands of civilian or military agencies? To address these questions, we must also inquire into public reactions to this newly-identified danger, and to how well we are communicating these issues to the citizens of the United States and the world.

Most of the scientific debates we have been discussing took place behind closed doors, involving only a few dozen technical experts. However, as the impact hazard attracted press interest, rumors of the conflicts between the astronomers and the weapons scientists inevitably leaked. In some cases, it was these conflicts, more than the impact hazard itself, that became the object of press coverage. The bitter 1992 Los Alamos confrontation between NASA astronomers and star warriors attracted a good share of press interest, especially after reporters were banned from the meeting itself. A week after Los Alamos, The New York Times printed an op-ed piece by physicist Robert Parks called "Star Warriors on Sky Patrol", which supported the astronomers but criticized the motives of Edward Teller and Lowell Wood, asking "who will protect us from the 'Nukes Forever!' mentality?". Then a front-page story in The Wall Street Journal titled "Never mind the peace dividend, the killer asteroids are coming!" falsely claimed that NASA "is about to recommend that the Earth start planning to assemble an arsenal of nuclear missiles to head off an attack by asteroids". These and similar press stories contributed to NASA's decision to dissociate itself from the Los Alamos meeting and focus on non-defense aspects of NEOs.

Some commentators saw a different issue involving turf conflicts between government agencies. In a long article published in March 1992, reporter Fran Smith of the San Jose Mercury News (serving the high-tech community of California's Silicon Valley) suggested that the asteroid astronomers had "inadvertently walked into a battle over NASA's fate" when they "seized on the public fascination with dinosaurs to stir interest in the lowly field of asteroid astronomy". She speculated that the debate between the astronomers and the star warriors reflected a broader struggle in Washington, over the survival of NASA's civil space program in the face of challenges from the Department of Defense to militarize space. She wrote that some asteroid astronomers were already backpeddaling, expressing their concern that proliferation of space weapons was more dangerous than the asteroid danger these weapons were supposed to counter. Clark Chapman told her that the projects proposed at Los Alamos were "Outrageous. Not outrageous because they're technically wrong. Outrageous because they're so radically more expensive and dangerous than the modest threat they would address."

On the whole, the American news media in the 1990s have treated the NEO hazard issue fairly and given it serious if inconsistent attention. The few really negative comments came primarily from the political right. Both the Washington Times and the Wall Street Journal editorialized that the impact "threat" was a ploy invented by astronomers to obtain more research funds from the government. In Europe, however, press coverage of impact issues tended toward wild exaggeration, alienating public support and frustrating astronomers who wanted to have their message heard.

After Shoemaker-Levy, the danger from NEO impacts was no longer treated as an unproven hypothesis. In fact, public interest in the comet crash stimulated production on several major television shows, both factual and fictional, that were broadcast in 1996 and 1997. The first documentary (name) was produced by the Public Television series NOVA, followed by a production (On a Collision Course with Earth) broadcast on the new Science Fiction cable channel. Much more widely viewed were a 2-hour production (Three Minutes to Midnight) broadcast on the Discovery Channel and a 1-hour National Geographic Special on NBC, both appearing in February 1997. Each of these shows presented balanced and technically accurate pictures of the role of impacts in Earth history, past and present, much of it described by scientists who were directly involved in researching these issues.

Probably even more widely seen, however, was the 4-hour NBC miniseries Asteroid, which depicted the destruction of Dallas by the impact of an asteroid several hundred meters in diameter. In spite of advertizing hype, this show turned out to be technically weak and nearly devoid of drama or character. In apparent response, the TNT network resurrected the 20-year-old film Fire from the Sky, in which a comet wipes out Phoenix. Both plots deal with the local effects of Tunguska-like impacts, not with a global environmental disaster. Even if these dramatizations were widely criticized, it is unlikely that many people remain in the US in 1997 who have never heard of the possibility of an impact catastrophe. Whether they consider such a risk credible is, of course, a different issue. The boundaries between fact and fiction are frequently blurred on American television.

Even among people who were aware that science recognized a threat from the sky, however, the actual risk seemed extremely small. Indeed, of all the hazards we face, an impact is the least likely to happen in any given year. We are concerned, not because a big impact is likely, but because each large impact can produce extremely severe consequences, in which hundreds of millions of people could die. It is the combination of low probability with high mortality that sets this risk apart from others.

There is a discipline of social science that deals with risk perception. In the late 20th century, American society has become preoccupied with risk evaluation, and many political controversies are based on perceived risk, in issues as diverse as the health effects of breathing second-hand tobacco smoke, the value of requiring motorcyclists to wear crash helmets, or the hazard of constructing nuclear power plants. Most issues involving product testing, drug screening, and environmental impact statements deal with risk and risk-perception. We have become a risk-averse society, and the study of how we deal with hazards is a field with tremendous practical as well as academic implications.

One of the pioneers in academic risk perception is Paul Slovic, a professor at the University of Oregon. We first met him in 1992, when he was speaking at the annual meeting of the American Association for the Advancement of Science. This organization, which publishes the widely quoted journal Science, is one of the few that brings together practitioners of the vast panoply of disciplines and subdisciplines that make up the contemporary scientific enterprise. Its annual meetings span the social, biological, and physical sciences, with talks presented so that the average educated layperson or scientists from another discipline can understand.

Following the planetary science presentations (organized to honor the explorations of the Americas begun 500 years earlier by Columbus), we wandered into a symposium on public perception of risk in the nuclear power industry. One soft-spoken man in the front row seemed to be the authority to whom other participants frequently turned. This was Paul Slovic, whose name was vaguely familiar to us from an article of his we had seen in Science. In that paper, Slovic had asserted that how people react to various hazards depends as much on their perception of the immediacy or dread of the danger as on its actual calculated risk. At that time Slovic had not heard of the cosmic impact risk, but we struck up an acquaintance that led to our collaboration in jointly studying this aspect of the impact issue.

For Slovic and other social scientists, the impact risk provided a fascinating new area for research. Discovering a new risk was analogous, for them, to the discovery by astronomers of a new celestial object or physical phenomenon. Starting to work now, before many people were aware of this risk, they had a unique opportunity to track this issue and gauge what influenced public opinion and political action as the impact hazard became better known and more widely discussed.

In a paper he wrote with us in 1993, Slovic asked how the public might respond to the statistical threat of impact, absent a specific prediction. He noted reasons from his previous studies to predict lack of concern about this issue, on the one hand, and -- for different reasons -- the possibility of a high degree of concern. There were several reasons Slovic expected lack of concern and possible opposition to large expenditures. First, natural hazards (perhaps including impacts) tend to be less frightening than mysterious Dr. Strangelovian hazards of modern technology; people perceive nature as fairly benign, and they react rather apathetically to the threat from natural hazards. For example, despite the very real and rather obvious hazards from earthquakes and floods, people continue to build homes along the San Andreas fault and in the floodplains of streams and rivers.

Moreover, the probabilities of an impact may be too low and the risk apparently too remote in time to trigger concern, in spite of their high consequences. Chances of "one in a million" (and that's roughly the annual danger from major impacts) mean that the chances are essentially zero, in the popular meaning of the phrase. Since such chances of impact are often phrased in terms like "every million years" or "every hundred thousand years," people mistakenly think that it means that it won't happen until a hundred thousand (or a million) years have elapsed, not realizing that an impact could as well happen tomorrow as on any particular day in the distant future.

Also, people are often insensitive to very large losses of life. Maybe it is because we are overwhelmed and numbed by stories of thousands (or millions) dying in a war or famine. Or maybe we simply can't relate to such large numbers. But we will anguish over the death of a single child or expend great effort to find an organ donor to save an individual life, but in a context of impersonal numbers or statistics, the lives of individuals lose meaning. Thus many people may respond to our words about the prospects of a billion people dying pretty much the same as if we had said that "only" a million might die.

Finally, Slovic considered that people might be unimpressed by the prospects that we could reduce the chances of an impact. We have noted that the Spaceguard Survey, and even all that the Defense Department might muster, could do nothing to save us from the 20% chance of a big impactor being a comet rushing at us from the edge of the solar system. While experts might value the protection from the 80% of doomsday rocks that we could find long ahead of time, most people tend to expect -- however unrealistically -- 100% insurance against a threat: if impact defense systems cannot provide complete protection, they may be undervalued.

On the other hand, there are also reasons Slovic expected the public to be concerned enough about impact hazards to support action. First, the risk is demonstrable (it happened to the dinosaurs), the news media showed comparable events happening on Jupiter a few years ago, and the reality of the hazard is endorsed by credible scientists. Moreover, the potential consequences of large impacts are uniquely catastrophic and are qualitatively different from other natural hazards. Catastrophic potential, whether from a nuclear melt-down or an Act of God, is an element of hazard that increases public concern. Indeed, the hazard is new in the public consciousness, it strikes (unless we do more) without warning from the skies, and it is (at present, at least) uncontrollable. Lack of control, dread, and catastrophic potential are all qualities of other risks studied by Slovic that are associated with public perception of high risk and a resulting strong desire for action to reduce risk.

Finally, there is the simple, rational fact that the probabilities of catastrophic impacts, while small, are not trivial. Considerable public funds are already being spent to deal with hazards with even lower annualized rates of deaths, such as death or injury from tornadoes or terrorist attacks.

At the end of 1992 (before the discovery of Comet Shoemaker-Levy 9), Slovic carried out a preliminary survey of attitudes and perceptions using a sample of 200 college students, 75% of whom had not heard about the impact hazard prior to participating in the study. He found that after reading about the impact risk, his group ranked impacts near the middle of a range of possible dangers faced by the American public. The impact risk was judged higher than risks from prescription drugs, medical x rays, bacteria in food, floods, and air travel, but lower than risks from earthquakes and hurricanes. Impact risks were rated as extreme with regard to being unknown to scientists and the public, distant in time (non-immediate), uncontrollable, and catastrophic. There was modest support for detection efforts but considerable opposition to use of weapons in space, even to deflect a threatening asteroid. Slovic also concluded that the scientists who are expressing concern about this threat were considered credible by his survey group, suggesting that the media have treated these activities in a positive light and have not interpreted these public statements as particularly ill-founded or self-serving. This feeling applied to the astronomers who had dominated media coverage to that point, but it might not be true of the nuclear scientists who want to build asteroid defense systems.

Perhaps unfortunately, asteroid defense issues inevitably become entangled with public concerns about nuclear energy. The simple fact is that if we were faced with the prediction of an impact, nuclear energy provides us with the most effective, widely applicable tools for self-protection. There are no other options to deflect or destroy a typical incoming NEO, at least with present or easily projected technology. (Small objects, especially with long lead times, could be dealt with using more benign approaches. But it is difficult to get around the fact that nuclear weapons possess about a million times the power of equivalent chemical devices, per pound, and we are limited in how much sheer weight we can launch into space to meet an intruder.) And, through all of the other hazard discussions, Edward Teller has held fast to his conviction that we need to pursue a program of nuclear testing in space to learn how to deflect or destroy asteroids and comets. Repeatedly, the Father of the H Bomb argued that science depends on experimentation (meaning that we ought to blow up an asteroid to see what happens), and that we must not trust our future survival to an untried technology. Teller, with his great prestige, would not let the nuclear issue be swept under the rug.

One astronomer who was deeply disturbed by these proposals was Carl Sagan, who had written about the hazard of comet impacts in his 1985 book Comet. Sagan, probably the best-known scientist in the U.S., began to participate in the hazard debate in 1993 and remained active until his untimely death in 1996. Unlike Teller, Sagan had never known political power, but he compensated with a commanding public presence. Teller tells a story about having breakfast with Sagan in an airport, where three people approached to ask for Sagan's autograph, but no one asked for one from Teller. Teller remarked "Sagan won." In addition to his own distinguished career as a planetary researcher, Sagan had taught a generation of planetary scientists, and both of us had been his students in the 1960s, when he was a young Assistant Professor at Harvard.

Many participants in the impact hazard discussions have expressed concern about the potential danger of maintaining fleets of nuclear-tipped missiles as part of the space defense system. It seems obvious that the risks inherent in such a defense system -- risks from either accident or misuse -- might be greater than the danger of an asteroid impact, unless effective international controls are developed. But Sagan went a step farther. He questioned whether knowledge of the deflection technology itself might raise the possibility of someone, sometime, using it against the Earth. After all, he argued, the same technology that could deflect an asteroid from a collision course and save the Earth could also deflect an asteroid that would have missed the Earth to hit the planet. And there are many more "near misses" that could be deflected toward us than there are asteroids that must be deflected away to save the planet.

Sagan was not the first person to suggest that asteroids might be used as weapons. About a decade earlier an anonymous paper had circulated within the U.S. defense establishment called "Ivan's Hammer", suggesting that the Soviet military might develop a technology for aiming asteroids at the United States, raining far more megatonage upon us than could be achieved by a simultaneous attack with all of their missiles and warheads. This paper had also speculated that the Soviets might engineer such an attack secretly, blaming the resulting catastrophe on natural causes. At the time, no one took this "Ivan's hammer" threat seriously. Sagan, however, was quite serious about possible misuse of asteroid defection technology.

Sagan presented his arguments against developing deflection technology to a variety of audiences, including a mid-1994 article in Parade Magazine and as a chapter in his book Pale Blue Dot (published late in 1994). In collaboration with radar astronomer Steven Ostro of JPL, he also published these arguments as technical papers in Nature and the journal Issues in Science and Technology. Through these efforts, Sagan's arguments against asteroid defenses reached many people who had not previously been aware of the impact hazard issue in any form.

To make his point, Sagan turned to a story from ancient history. In about 560 BC the Sicilian city-state of Camarina was suffering from pestilence, which was attributed to the presence of marshlands that surrounded most of the city. At great expense, the rulers of Camarina undertook a public works project to drain the marsh and thereby improve the health of the citizens, surely a worthy and laudable project. The water was drained, and everyone was happy -- until the neighboring city of Syracusa recognized that the swamp, which had protected Camarina from invasion, was no longer present. When the attack came, the citizens of Camarina found themselves unable to defend the enlarged perimeter of the city; they were defeated, the city destroyed, and its citizens all killed. Thus for Sagan, the marsh of Camarina became a metaphor for the well-meaning misuse of technology, in which an act that seems to serve a worthy purpose backfires to produce disaster.

Sagan's basic issue is that the techniques for altering asteroid or comet orbits can work both ways: to deflect them away from the Earth or, in the wrong hands, to direct an otherwise benign projectile toward us. His focus in this concern was on objects large enough to cause a global environmental catastrophe. As we have noted, such impacts take place naturally only once every few hundred thousand years. But suppose, Sagan asked, that we completed the proposed Spaceguard Survey and identified all of these objects. For every one that can hit the Earth, there will be about 4000 others that come as close as the Moon, or about one of 1.7-km diameter every century. About once per decade an asteroid larger than 500 m in diameter comes as close as the Moon.

Sagan then asked us to consider a world in which the technology has been developed to modify asteroid orbits, perhaps by "herding" them through a series of small nuclear explosions. What are the chances that there will someday be a madman somewhere with access to such technology and a desire to destroy civilization? We have already had Stalin, Hitler, Pol Pot, and Idi Amin this century. Given the potency of ethnic and religious conflicts, it is surely possible that someone, sometime, should conclude that it was time for the Day of Judgement and be willing to help God achieve His apocalyptic objectives. If such a madman existed, he could find an inoffensive asteroid and turn it into a weapon with thousands of times the destructive power of all the world's nuclear arsenals. Presumably there will be few such opportunities and, we may hope, no such despots with the capability of taking such action. But can we be sure that this could never happen? Like the ancient Camarinans, we might create a greater hazard than the one we are protecting against. The cure might be worse than the disease. Sagan called this the "deflection dilemma".

Sagan's dreadful scenario was widely regarded as unlikely. Is the technology to deflect an incoming asteroid really sufficient to redirect an innocent asteroid into a collision course? The methods suggested so far to deflect asteroids by stand-off nuclear explosions are quite crude. We do not know with any confidence how large the explosion should be or how close to the asteroid; that is why we have considered the possible need for a series of explosions to accomplish the orbit change. Furthermore, the asteroids themselves are irregular in shape and often rapidly rotating, and it is impossible to predict with assurance just how one would respond to a nearby nuclear blast. We would be lucky if it went even approximately in the direction we wanted. Fortunately, it is not necessary to change the orbit in any particular way to protect the Earth. We really don't much care how much the orbit is changed; the body can pass to the east or the west, the north or the south, just so it doesn't hit us. We all know that it is easier to get out of a parking place than to get in. In this case the difference is much greater, and the contrast between missing the Earth and being directed toward any particular direction or target is huge.

Furthermore, the orbital change required to bring an asteroid from lunar distance into an impact trajectory is about a hundred times greater than that required to miss the Earth. If 5 bombs would be enough to accomplish the defensive deflection, it would require 500 to change an orbit from lunar distance to intersection. In this case defending the Earth from impact is a great deal easier than launching an offensive strike against the Earth, and the technology of deflection is probably not capable of steering a benign asteroid into a threatening orbit. Similarly, if a madman did manage by multiple small nudges to get an asteroid into a collision course, it would be relatively easy for a defender to give it a bigger, less precise impulse and get it out of the way.

So what was Sagan imagining? First, he suggested that someone will develop the technology to fine-tune an asteroidal orbit. Assuming that such a thing is possible using nuclear explosives, it would require much more sophisticated technology than the sort of defense system contemplated today. Second, he imagined that a madman gains control of this technology and convinces a large cadre of technical experts to join in this suicidal scheme. Third, he required that this group of conspirators launch dozens (or more) of warheads and carefully redirect an asteroid over a time scale of several years in order to shift it into an impact trajectory. Finally, he imagined that no other nation or organization exists on Earth to detect this activity and counter with a defensive countermeasure.

While it is difficult for us to take such a scenario seriously, it certainly cannot be proved to be impossible, or that another analogous scenario, which nobody has thought of yet but the madman might, could lead to the awful consequences that Sagan worried about. And that was Sagan's larger point. Unless we can be better than 99.9% sure that there isn't a viable madman scenario that could be used to turn asteroid deflection technology against us, then the dangers from the madman exceed the very remote possibilities of an asteroid striking an unprotected planet Earth. And how often are we 99.9% sure of anything, let alone about an esoteric possibility built of cutting-edge technology and the psychology of madmen?

Teller had a much shorter response when asked in the summer of 1994 about the possible misuse of deflection technology. Bristling, his voice rose as he responded: "Who could suggest such an idea? Only what's-his-name -- Sagan! I do not understand him. I think he must know many evil people to have such ideas!"

Sagan had his own opinions on Teller. Writing in his 1995 book The Demon-Haunted World, Sagan concluded that Teller has done a great deal of harm to society, and he interprets Teller's life-long advocacy of nuclear explosives as an appeal that "somehow, somewhere, he wants to believe [that] thermonuclear weapons, and he, will be acknowledged by the human species as its savior and not its destroyer".

In spite of his concern with the deflection dilemma, Sagan recognized the reality of the danger posed by impacts, and he struggled in his articles and lectures to find a suitable compromise. He wrote in Pale Blue Dot: "If we develop and deploy this technology, it may do us in. If we don't, some asteroid or comet may do us in. . . If we are too quick in developing the technology to move worlds around, we may destroy ourselves; if we are too cautious, we will surely destroy ourselves." He suggested, therefore, that we proceed at once with the Spaceguard Survey, to obtain a complete census of the larger near-Earth objects, while deferring the development of deflection technology to a time when either an immediate threat is identified or else maturing international safeguards allow us to invest in such technology safely. Optimistically, Sagan assumed that political institutions will improve and the risk of powerful madmen will be reduced to negligible levels within the next few centuries.

If the NEO issue is primarily seen as one of defending the Earth from catastrophic impacts, an obvious case can be made for the military to play a leading role. In 1994 a this idea received official recognition in a futuristic report from the US Air Force Air University called Spacecast 2000, prepared with the advice of industry and academic figures (including Carl Sagan). In their analysis of global power in the 21st century, the Air Force planners wrote that the military mission is to conduct "counterforce operations ... aimed at opposing or defending against threatening force anywhere on the planet or in space". The Spacecast 2000 document asserts that "although not a traditional enemy, the asteroids are nonetheless a threat that the DoD should evaluate and defend against. The role of the military has traditionally been to operate in and expand the frontier of space. ... Provisions for defense of the planet, as far away from the planet as possible, need to begin."

The Spacecast 2000 report was written at the same time the USAF was participating in the Shoemaker committee to study a new, streamlined Spaceguard Survey. Pete Worden and others wanted the Air Force to assume responsibility for NEO searches. This is an appealing idea, since the military have both the telescopes and the manpower to operate them in their existing GEODSS surveillance system. But there are concerns as well, primarily associated with issues of secrecy. Astronomers are by nature open in their communications with each other and the public, but the same cannot be said about the military, whose mission requires discipline and (on occasion) secrecy. It is interesting to speculate what the military would do if they found an NEO headed for the Earth.

In 1996, when Hollywood film-maker Paul Almond was researching a script on defense against asteroids, he asked a number of public officials, including former cabinet officers, how the government would react to the discovery of an impact threat. Several told him that they would favor keeping the discovery secret from the public while establishing a top secret, military-type organization to defend against the NEO. Authors of several earlier novels and filmscripts have made such a government cover-up a central part of their dramatization of an impact scenario. Yet we, and most of our fellow scientists, consider the idea of keeping the risk secret to be absurd, both morally and practically. It is impossible to imagine scientists such as Tom Gehrels, Gene Shoemaker, Steve Ostro, or Brian Marsden -- the people today most likely to discover a new NEO and calculate its orbit -- keeping this information to themselves. The skies are an open book, there for anyone to read. Nor can we imagine a civilian government agency such as NASA keeping such a secret for more than a few days, even if they wanted to. Dramatizations that depend on such a conspiracy for their plot elements are pure friction.

On the other hand, it is possible to imagine that a tightly-disciplined military organization like the USAF Space Command might be able to keep such a secret for quite a long time. We have no way of knowing whether they would want to, but some highly placed officials did tell Paul Almond that they would favor such a course. Perhaps this is a good reason to keep the Spaceguard Survey in civilian hands.

CONCLUSION STILL TO COME

5200 words (5/7/97)

CHAPTER 15: WHAT WE SHOULD DO: THE PUBLIC POLICY DEBATE

SUMMARY

Most of the scientific discussions we have been describing took place out of the public eye. However, rumors of the conflicts between the astronomers and the weapons scientists leaked out, stimulating widespread press interest, which has grown steadily since 1992. Additional events have accelerated the public debate: the widely publicized impact of SL 9 with Jupiter, publication of nearly a dozen popular books, and several TV accounts (both fictional and factual) of impacts that were broadcast early in 1997. In this chapter we analyze the public and press reaction, including studies of the psychology of risk perception. We then turn to current policy issues. Carl Sagan originally raised the question of the unexpected consequences of a planetary defense system, in which the risks of the defenses themselves might be greater than the impact risk they are intended to mitigate. Astronomers and weapons scientists have argued over the relative importance of the very large impacts, compared with more common but smaller impacts like Tunguska. We also ask whether the defense of the planet should be in the hands of military or civilian agencies, and to what extent the effort should be international. When we look closely, we see that comets pose a much more difficult problem than asteroids, one that current technology may not be able to handle. There are difficult questions, but ultimately these issues will be faced. On the long run, we must learn to protect ourselves against comets and asteroids. Our very survival as a species depends on it.

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CHAPTER 16: LOOKING TOWARD THE FUTURE

The Epilogue is still in preparation. It includes a summary of the most important conclusions from earlier chapters, exploration of several possible scenarios for the future, and recommendations on policies to deal with the impact hazard.

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APPENDIX: NEO HAZARD CHRONOLOGY

1758 Recovery of Comet Halley confirms that comets have periodic orbits

1801 Piazzi discovers first asteroid (Ceres)

1803 French Academy concludes that meteorites have extraterrestrial origin

1819 Non-gravitational forces are identified altering orbit of Comet Encke

1866 Perseid Meteor Shower is identified with Comet Swift-Tuttle

1891 Wolff introduces photographic search technique for asteroids

1892 Gilbert concludes lunar craters have impact origin but Meteor Crater (Arizona) is volcanic

1898 Witt discovers asteroid Eros (Earth approaching but not Earth crossing)

1906 Barringer drills unsuccessfully for meteorite at Meteor Crater

1908 Tunguska event: 15 megaton airburst in Siberia flattens 2000 sq km

1927 Kulik leads first expedition to Tunguska site

1931 Moulten calculates impact energies, explains why craters are circular

1932 Reinmuth discovers Apollo, first Earth-crossing asteroid

1947 Sikhote-Aline iron meteorite falls in Siberia

1950 Whipple proposes that comets have solid nuclei (dirty snowball model)

1951 Opik calculates impact probability for Earth-crossing asteroids

1953 Coesite (heavily shocked silica) is discovered as impact fingerprint

1960 Shoemaker completes detailed geological study of Meteor Crater

1961 Stishovite (heavily shocked silica) is discovered as impact fingerprint

1965 Stony meteorite explodes over Revelstoke, Canada

Mariner 4 discovers that martian surface is heavily cratered

1969 MIT class studies asteroid defense (Project Icarus)

Dating of Apollo lunar samples establishes terrestrial impact rate

1971 Gehrels organizes first major international meeting on asteroids

1972 Teton event: potential impactor skips off atmosphere

USAF surveillance satellites begin to observe atmospheric impacts

1973 Urey suggests possible impact contribution to extinctions

Shoemaker and Helin begum Palomar photographic survey (PCAS)

Clarke coins term "Spaceguard" in novel Rendezvous with Rama

1975 Chapman/Morrison/Zellner define physical classification for asteroids

1976 Helin discovers Aten, first asteroid with period less than 1 year

1977 Niven and Pournelle portray comet impacts in novel Lucifer's Hammer

1978 TV drama Fire in the Sky shows comet impact destroying Phoenix

1979 Alvarez team identifies extraterrestrial iridium in K-T boundary layer

Voyager discovers that heavy cratering extends to outer solar system

Film Meteor depicts joint US/USSR defense against asteroid impact

1980 Alvarez team publishes proposal of impact cause for K-T mass extinction

NASA Advisory Council recommends study of impact hazard

1981 NASA sponsors Snowmass Workshop on the impact hazard

First Snowbird conference: Interdisciplinary meeting on the K-T boundary

Penfield and Camargo propose large impact structure in Yucatan

1982 Shoemakers begin separate Palomar sky survey (PACS)

1983 Sepkoski and Raup suggest that mass extinctions have 26 Myr periodicity

Shoemaker publishes standard Earth-impact average flux curve

TTAPS authors propose concept of nuclear winter

1984 Gehrels discovers first NEO with new automated Spacewatch system

1985 Chicago University team identifies global wildfires at K-T boundary

Time publishes cover story on impact extinction of dinosaurs

1986 Flotilla of international spacecraft investigate Comet Halley

Davies publishes book Cosmic Impact

Raup publishes book The Nemesis Affair

1987 Series of international conferences discuss properties of Comet Halley

1988 Second Snowbird conference: Global Catastrophes and Earth History

1989 Chapman and Morrison publish book Cosmic Catastrophes

Spacewatch discovers asteroid Ascelpius passing much closer than Moon

Ostro radar images show asteroid Castalia to be a contact binary

1990 Hildebrand proposes Chicxulub in Yucatan as K-T impact crater site

Melosh suggests heat from backfalling ejecta caused K-T firestorm

AIAA issues position paper urging program to detect NEOs

US Congress requests NASA to study NEO hazard, surveys, and defense

Clube and Napier publish book The Cosmic Winter

1991 Chapman organizes first international NEO scientific meeting

NASA establishes NEO working groups chaired by Morrison and Rather

International Astronomical Union establishes NEO Working Group

First International meeting on the NEO hazard is held in St Petersburg

Galileo spacecraft obtains first close images of an asteroid (Gaspra)

DoD approves Clementine mission to orbit Moon and fly by Geographos

1992 Tunguska impactor is identified as asteroidal, not cometary

NASA releases report from Spaceguard Survey Working Group

DoE and NASA sponsor NEO defense workshop at Los Alamos

Ahrens and Harris publish technical paper on asteroid deflection

Close approach of asteroid Toutatis yields excellent radar images

Public concern is aroused over possible Swift-Tuttle impact in 2126

Sagan discusses impact hazard in article in Parade Magazine

Newsweek publishes cover story on impact hazard

Fortune publishes cost analysis of impact risk

Slovic conducts first public opinion poll on impact hazard

1993 Gehrels hosts major international impact hazard meeting in Tucson

Shoemakers and Levy discover Comet SL-9 at Palomar

Impact of SL-9 with Jupiter is predicted for July 1994

Los Alamos releases NEO Interception Workshop Report

US Congress holds hearings on NEO hazard and defenses

Worden organizes Erice international conference on impacts

The Economist publishes cover story on the impact hazard

Spielberg's film Jurassic Park stimulates public interest in dinosaurs

Clarke's novel Hammer of God deals with defense against NEO impact

NASA approves Near Earth Asteroid Rendezvous mission to Eros

1994 Chapman and Morrison publish technical analysis of impact hazard

Hills and Goda publish analysis of impact tsunami hazard

Clementine spacecraft is launched but fails before reaching Geographos

Galileo spacecraft discovers first asteroidal satellite (Dactyl/Ida)

Largest Earth atmospheric impact (50-100 KT) observed from space

First material remnants of Tunguska explosion recovered

Time publishes cover story on SL-9 and impact hazard issues

Astronomers mount unprecedented effort as Comet SL-9 impacts Jupiter

US Congress asks NASA for plan to discover all large NEOs in 10 years

NASA and US Air Force begin consultation on joint NEO surveys

NASA appoints NEO Science Working Group chaired by Shoemaker

Russians host impact defense conference (SPE-94) at Chelyabinsk-70

Spacewatch discovers closest NEO (1994XM, 105,000 km from Earth)

Shoemaker terminates his Palomar NEO survey (PCAS) after 22 years

USAF proposes NEO defense role for military in Spacecast 2000 Report

Sagan's book Pale Blue Dot includes extensive impact hazard discussions

1995 Gehrels publishes 1300-pg multi-author technical book on impact hazard

AIAA issues second position paper calling for action on NEO defense

Internet Homepage on Asteroid and Comet Impact Hazard established

The Planetary Society begins publication of quarterly NEO News

Shoemaker Spaceguard report submitted to NASA & US Congress

Livermore Lab hosts conference on asteroid defense issues

UN sponsors asteroid workshop in New York

Internat'l Astronomical Union sponsors Spaceguard Workshop in Vulcano

Steel publishes book Rogue Asteroids and Doomsday Comets

NOVA broadcasts TV documentary Doomsday Asteroid

1996 Australian government terminates NEO photographic search program

Helin terminates Palomar photographic survey (PCAS)

Spaceguard Foundation formed in Europe to promote NEO surveys

Parliamentary Council of Europe endorses NEO Spaceguard concept

NASA and USAF begin Near Earth Asteroid Tracking program in Maui

NASA launches NEAR space mission to asteroid Eros

DoD obtains funding for Clementine 2 mission to intercept two NEOs

Science conference on Tunguska held in Bologna

Russians host impact defense conference (SPE-96) at Chelyabinsk-70

Comet Hyakutake passes within 15 million km of Earth

Lewis publishes book Rain of Iron and Ice

Verschuur publishes book Impact: The Threat of Comets and Asteroids

US cable TV broadcasts documentary On a Collision Course with Earth

1997 NASA NEAR spacecraft flys past asteroid Mathilda

US Congress proposes that NASA triple its NEO research program

NASA and USAF agree to collaborate on Clementine 2 mission

Toon et al. publish detailed analysis of environmemtal effects of impacts

Comet Hale-Bopp brightest in decades, widely observed

Newsweek cover story on Hale-Bopp includes NEO hazard discussion

US cable TV broadcasts documentary Three Minutes to Impact

NBC broadcasts National Geogrphic Special xxx

NBC broadcasts miniseries Asteroid in which impact wipes out Dallas

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