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.
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.
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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.