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.
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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