Return to IMPACT HAZARD
INTRODUCTION
This scientific and political history of the impact hazard was originally prepared as the first part of my "Prediction Science" Case Study, but had to be excised due to length. This is the first part of the obsolete October 1998 version of the Case Study. The second part of the October 1998 Case Study is a now-obsolete analysis of XF11. The updated Case Study omits this historical section and should be referred to for updated analysis of XF11, as well as subsequent developments.
Return to IMPACT HAZARD
INTRODUCTION
Clark R. Chapman
Revised: 7 October 1998
Abstract. An impact by an asteroid or comet larger than 1 km in diameter (30,000 Megaton energy) occurs about every 100,000 years, or 1 chance in a thousand per century. Objects of this size could cause serious regional disasters (e.g. tsunami) and objects only slightly larger would have global environmental consequences (e.g. severe ozone loss, injections of water and dust into the stratosphere, wildfires) that might threaten the future of civilization as we know it. (Smaller impacts can create damage similar to other major natural disasters, like earthquakes and floods, but they probably account for <0.1% of such disasters.) This "impact threat" was virtually unknown until the past two decades.
The first part of this Case Study reviews the history of how this "new" hazard came to be "discovered" by the scientific community and about how knowledge of the hazard has spread to the general public in the last few years. I also review the modest efforts by the Congress, other U.S. governmental agencies, and international groups (national, international, military, and private) to deal with the impact hazard. The unusual nature of the hazard (enormous consequence, extremely low probability of occurring in our lifetimes) has presented difficulties in getting it considered along with other natural hazards, especially by agencies with responsibility for mitigation.
It now appears likely that the discovery rate of Earth-approaching objects will increase dramatically in the next decade, raising a practical issue that has not been adequately addressed: how scientists should communicate with officials and the public concerning discoveries of objects that may impact the Earth in the near future. The second part of this Case Study reviews a recent (March 1998) event in which the world was alerted to an erroneous calculation by a supposedly reliable scientist implying a >0.1% chance of an impact by a mile-wide asteroid 30 years from now. The event dominated headlines for a day before a "correction" was issued, which undercut public confidence in astronomical calculations.
Public and political awareness of the impact hazard seems to be reaching a threshold so that it can no longer be ignored. Yet profound questions -- psychological, social, legal, political, scientific, and technological -- will not be easily dealt with. As an end-member in the spectrum of natural hazards, cosmic impacts are more likely to provide us with a mirror for assessing other rare but important hazards than to become real-life disasters like those depicted in the blockbuster movies of the summer of 1998.
This "impact hazard" by near-Earth objects (NEOs) is strikingly different from most natural hazards in two ways: (1) the potential consequences of a major impact exceed any other known natural or man-made hazard (including nuclear war), and (2) the probability of a major impact occurring in a politically relevant timescale (say, during our lifetimes) is extremely low. It is interesting, however, that multiplying the probability of a major impact by its consequences yields an annualized death rate similar to that of some of the traditional natural hazards being discussed in this Workshop (hundreds of deaths per year worldwide, see Morrison et al., 1994). The impact hazard is, therefore, the ultimate high-consequence, low-probability hazard.
Another interesting feature of this hazard, in the context of "prediction", is that this hazard is evolving from one that was almost unknown two decades ago (hence a serious impact would certainly not have been predicted) to one in which 75% of the risk could, within two more decades, be predicted so exactly that mitigation measures within our technological capability could be applied with a high degree of effectiveness. For the remaining 25%, the prediction would either be too late to mount more than partial countermeasures or there might (if a comet were suddenly to appear from the direction of the Sun) be a wholly unforecast "act of God".
In this Case Study, I outline the history of how this hazard was "discovered" by scientists. Then I delve more deeply into the recent history of how the world's political entities became aware of the impact hazard and what is, and is not, being done about the problem. (Conceivably, there are other important but as-yet-unrealized hazards awaiting discovery; perhaps this history will be instructive about recognizing them and understanding the difficulty of gaining full public awareness of them.) Then I analyze a recent "impact scare" [second, obsolete, part of this document or the updated Case Study] in which an erroneous prediction of a potentially civilization-ending impact within our lifetimes made headlines worldwide before being withdrawn the next day. Finally [also accessed by above links], I address some of the policy issues that are emerging and how this unusual hazard fits into the framework of the "Prediction in the Earth Sciences" project.
For an immature hazard, which has never happened in recorded history and whose potential for happening was not widely realized until the past decade, the policy goals are different from all others addressed in this report. Sheer recognition by policy-makers and federal and international governmental agencies that a problem exists has yet to occur in a serious way. I consider that the goal is to establish an effective national and international dialog -- including accompanying research -- to meaningfully assess the appropriate level of response to a threat that is very real and very serious, if also very unlikely to occur in our lifetimes. Should the outcome of such dialog be a decision that something should be done, it would then be necessary to develop from scratch ways of evaluating the hazard and preparing for mitigation.
I should note briefly my own background and point-of-view about this issue. I am an astronomer and geoscientist by training and have long had a professional interest in both small planetary bodies (asteroids and comets) and in impact cratering of planetary surfaces. I am not, however, among the small group of astronomers who are engaged in constructing telescopes to search the heavens for threatening objects and have received no funding to study this subject. Although, as a scientist, I have served on policy advisory committees, my own participation in the policy-making process has been rudimentary (a decade ago, I was a member of the Pima County, Arizona, Planning and Zoning Commission and chaired the committee that drafted the county grading ordinance.) My view of how society should deal with the impact hazard has been shaped by the literature of risk perception in which it is argued that there is no single objective way for society to assess the relative importance of the hazards we face.
However, as a scientist, I believe that an important input to political evaluation of this hazard is the concrete scientific information we have learned about the hazard (about the frequency of impacts, about the consequences of those impacts, about the technological issues of how we find potential impactors and what we might do about them) so that policy makers are made aware of and have access to factual knowledge (and associated error bars) about the hazard. As a citizen, my personal belief is that the impact hazard is important enough to merit further study and analysis and that it is worth the cost to augment the search for potential impactors (although certainly not worth advance preparation of on-the-launch-pad defensive measures). But I also believe that some other natural hazards -- and many other hazards, mostly medical, accidental, and related to war -- deserve more urgent attention than the impact hazard.
At the present time, there is minimal funding of work on this hazard, although there are fledgling attempts in several countries, and by several international organizations (including the U.N.) to take the impact hazard into account. Neither FEMA nor the "hazards community" (i.e. the subscribers to The Natural Hazards Observer) has taken serious interest in this particular hazard, despite widespread media attention given to it in recent years, especially since the spring of 1997. Indeed, the previous Associate Administrator of Space Science in NASA deliberately chose to downplay the importance of this topic. However, a forthcoming "near-miss" of Earth by a mile-wide asteroid (predicted for the year 2028, with initial reports suggesting a serious possibility of actual impact) generated front-page news media coverage in March 1998. And two major blockbuster films released in the summer of 1998 are beginning to force more serious consideration of the impact hazard.
In an interesting way, a theme of this project -- prediction -- would seem to be a relatively minor player insofar as the impact hazard is concerned. For asteroids and comets that have been discovered, current technology can usually provide exceptionally precise predictions about when and where an impact might occur. (Astronomers have been applauded for centuries for accurate predictions of sunrise, sunset, and eclipses.) Approaches to mitigation, while preliminary in development and expensive, are inherently simpler than for many hazards, at least from the technical standpoint. And for those potential impactors that haven't been discovered (so far the great majority, though we could change that in the next couple of decades), the probability of impact is a simple cosmic lottery: our inherent chances-of-losing are readily calculated and well known. There are, indeed, uncertainties in the environmental consequences of impact -- akin to those of other meteorologically-based hazards, like storms, global warming, the ozone hole, and El Ni¤o -- but the sudden drama and exceptional character of an impact event itself would minimize the perceived importance of such uncertainties. In this case, the major predictive uncertainties involve questions of how individuals, the society, and political institutions may prepare for and conceivably respond to predictions of such a horrific but unlikely disaster.
There are other issues of prediction, however, which are relevant. They are analogous to those affecting other Case Studies addressed in this project, although I will continue to argue that they are relatively less important. The erroneous prediction of a possible impact issued on March 11, 1998, which made banner headlines around the world, illustrates the problem. The error -- public assertions by supposedly credible astronomers that there was a "small" (interpreted by others as >0.1%) chance that the Earth would be hit in the year 2028 by a mile-wide asteroid (1997 XF11) -- was corrected a day later; revised predictions were reported by a press somewhat disenchanted with the astronomers' credibility. On the other hand, such temporary errors, occasioned by an urgent press looking over the shoulders of naive astronomers (who should not have alerted the press until they knew what they were doing), was hardly as serious as forecasts of earthquakes that never happened (after counties had spent millions in preparation) or failures to forecast hurricanes that did happen. The mis- forecast impact only involved changing the odds from "very unlikely" to "zero", and no public funds had begun to be spent toward unnecessary mitigation of an event 30 years in the future. Nevertheless, astronomers dare not appear to be Chicken Little and lose credibility in an arena in which they conceivably might someday have to forecast an event that would deserve to be taken seriously at the highest public and governmental levels.
There are important policy issues that must be addressed. One is how the scientific community should advise officials and the public about the chances of an impact occurring shortly after discovery of a potentially threatening body, but before it can be proved that the object cannot hit. (This problem, articulated in 1997 drafts of this Case Study, played out dramatically during the early days of the March 1998 media frenzy over 1997 XF11.) Another is to clarify how a plethora of national and international laws (e.g. the National Environmental Protection Act and the Outer Space Treaty) might be interpreted in terms of mounting a defensive infrastructure to deal with asteroids or, in the unlikely event that an object were found to be on an imminent collision course with Earth, to mount emergency countermeasures. Will national and international responses be driven primarily by legitimate public concerns? Or will they be torqued by the self-interested motives of those entities that would stand most to benefit from mounting large scientific or military responses to the threat? Finally, how does a society responsibly decide on the relative allocation of funds to address, on the one hand, mundane, everyday problems that kill millions (e.g. diseases and accidents) or instead to deal with rare but horrifying disasters (biological/chemical warfare or terrorism, exceptionally destructive earthquakes, or even asteroid impacts)?
Comets have been perceived as threatening objects since antiquity. Literature through the nineteenth century sometimes dealt with cosmic calamities and worlds in collision. But the scientific concept that comets are physically solid bodies (which would be potentially dangerous if they were to impact) was not proposed and widely accepted until the mid-20th century when Whipple (1951) proposed the "dirty snowball" model for comets. The greater danger (probably) is posed by asteroids. The first asteroid was not discovered until 1801, around the time that meteorites were first understood to be stones from interplanetary space. And the first asteroid found to be in an orbit that crosses the Earth's orbit around the Sun ("Earth-crossing asteroid") was not discovered until 1932. Soon thereafter, a Harvard astronomer, Fletcher Watson (1941), wrote cogent sentences about the dangerous prospects of such an asteroid (a few miles wide) crashing into the Earth. With hindsight, the technical literature reveals that some other important scientists also forecast both the enormity as well as the rarity of a potential impact. Such eminent pre-Space-Age researchers included Ralph Baldwin (1949) and Ernst Opik (1957).
From at least the 1950's through the 1970's there were also (we now recognize) prescient remarks (again by Opik, by USGS planetary geologist Harold Masursky [1967], and by one of the founders of modern planetary science, Nobel laureate Harold Urey [1973]) about the possibility that mass extinctions of species in the geological record might be explained by impacts of large asteroids or comets. Other astronomical ideas (e.g. nearby supernova explosions) competed with traditional speculations of paleontologists (e.g. that early mammals ate dinosaur eggs) as alternatives to the impact idea, and the astronomical hypotheses -- in particular -- were ignored and not further developed. (Powell [1998] argues that the numerous paleontological hypotheses weren't well developed or widely accepted, either, creating a vacuum that made it possible for new ideas to be proposed and considered seriously.)
In the late 1960's (Kleiman, 1968), an M.I.T. space systems engineering class was presented with the problem of what would they do if an Earth-approaching asteroid (they chose Icarus) were found to be on a collision course with Earth. Their conclusions, published as "Project Icarus" (loc cit.), represent the first serious consideration of what human society might do if confronted with this threat. Yet, the M.I.T. study was conducted in a political and cultural vacuum and led nowhere. During the 1970's, several science fiction novels were published with asteroid/comet impact as a central theme. Some were of limited circulation but at least two were by world-renowned science fiction writers: "Rendezvous with Rama" (Arthur C. Clarke) and "Lucifer's Hammer" (J. Pournelle and L. Niven). There was also the 1979 grade C movie "Meteor".
Perhaps (or perhaps not) these antecedents led to the sudden convergence around 1980, on several fronts, of the idea that asteroids and comets might pose a significant hazard to humanity. The fundamental scientific context was the "Golden Age" of space exploration, commencing with the Ranger and Mariner investigations of the Moon, Mercury, and Venus, and the Apollo explorations of the surface of the Moon. The early Mariners showed that Mars and Mercury are also cratered worlds, and the moon rocks returned by Apollo astronauts provided radiometric calibration of the ages of geological units on the surface of the Moon; lunar crater counts combined with the ages yields a cratering rate for the Earth-Moon system. The research of Eugene Shoemaker (cf. 1963) and others, substantially motivated by the dawning of the Space Age and the Apollo program, finally settled a long-standing debate about the origin of various "crypto-volcanic" craters on the Earth -- Meteor Crater in Arizona, and others, were shown to be impact craters.
By 1980, telescopic observational programs (chiefly by Helin and Shoemaker, 1979) to survey Earth-approaching or Earth-crossing objects, as well as continuing exploration of the planets (e.g. Voyager's imaging of the surfaces of the four large moons of Jupiter, three of which showed cratered surfaces while the innermost moon, Io, exhibits extraordi- narily active volcanism that erases any impact craters almost immediately after they are formed) provided an overwhelming planetary science context for understanding that the Earth, too, is living in a cosmic "shooting gallery".
I have not been able to definitively pin down all of the antecedents to the critical July 1981 Snowmass, Colorado, workshop on the impacts of asteroids and comets, chaired by Gene Shoemaker (I was a member). One important influence was that asteroid- researcher Tom Gehrels, of the University of Arizona, had been promoting the construction of a telescope (now called "Spacewatch") to search for Earth-crossing asteroids. Gehrels had been the leading scientist surveying the properties of asteroids since the late 1950's, and he became more interested in the near-Earth asteroid population by the late 1970's (he had organized a scientific conference concerning the Earth-approacher Eros in the mid- 70's, cf. special Eros issue of Icarus, Vol. 28, No. 1, May 1976).
In June 1980, the NASA Advisory Council (composed of high-level outside advisors to the space agency) held one of its annual retreats ("New Directions Symposium") on Martha's Vineyard, providing a forum for NASA's management to step back from the day- to-day programs and find a "vision" to define and support its future programs. These officials, thinkers, and technologists proposed several future themes for NASA. One subpanel, including the late Barney Oliver (vice-president of Hewlett-Packard), proposed a theme of investigating the potential hazard to the Earth posed by asteroids and comets.
An outgrowth of that summer meeting was the convening, by NASA, of the Snowmass "Spacewatch Workshop" the following summer, organized by NASA Planetary Astronomy program head Bill Brunk and chaired by Gene Shoemaker. I was asked to participate, in part because of my then-recent role, as a member of COMPLEX (Committee on Planetary and Lunar Exploration), helping to formulate the National Research Council's report on the next decade's strategy for investigating small bodies (comets and asteroids).
In the meantime, Luis and Walter Alvarez and their collaborators, had discovered the infamous iridium layer at Gubbio, Italy, and had developed the idea that a 6-mile-wide asteroid crashed to Earth 65 million years ago, causing the mass extinctions that define the Cretaceous-Tertiary (K/T) boundary, most famously exemplified by the demise of the dinosaurs. Their epochal paper in Science (Alvarez et al., 1980) had appeared by the time of the Snowmass meeting, and the Alvarez team was represented at the meeting by Frank Asaro. But the history of the Alvarez hypothesis (e.g. see Alvarez, 1997) suggests that they looked to the evidence from planetary science only after developing the core idea; I know of no evidence that the Alvarezes' work on the Gubbio mystery informed the NASA study that led to the Snowmass meeting.
So, following the confluence of various scientific threads, by summer of 1981 a representative group of scientists and technologists was assembled, including military experts (e.g. Gen. Theodore Taylor), astronomers (e.g. Tom Gehrels), geoscientists (e.g. David Roddy), aerospace engineers (e.g. Alan Friedlander), and others (e.g. Barney Oliver) to discuss and debate the proposition that human civilization was at risk from comets and asteroids and that there might be something that could be done about it. During the Spacewatch Workshop, I participated in a core sub-group that defined the magnitude of the hazard in terms that could be compared with other natural hazards. Nothing was fundamentally different from the original insights of Fletcher, Baldwin, and Opik nor from the more thoroughly developed later analysis by Chapman & Morrison (1994), which was based on the preliminary thinking at Snowmass: there is a small, but non-zero, risk of a civilization-threatening impact happening -- a catastrophe that would be much smaller than the K/T boundary impact 65 million years ago but far larger than the 1908 Tunguska impact (which could have killed millions had it unluckily struck in the center of a city, but in fact killed only a couple of people at most). Preliminary thinking about how to discover potential impactors (Tom Gehrels presented the Spacewatch concept) and about how to destroy a potential impactor, if found, (by nuclear explosives launched to rendezvous with the object) were all discussed at Snowmass.
NASA intended to release a report of the Snowmass findings, but that never happened. In fact, a 90-page draft report was written by the members of the Workshop during the months following the meeting. Important philosophical issues were raised and debated during the writing of the report. For example, George Wetherill, of the Carnegie Institution, expressed deep concern about the potential destabilizing influence on the Cold War political climate by advocacy of using nuclear weapons in space for destroying or deflecting threatening asteroids. This was one factor among many leading to some delay in issuing the report.
Several years ago, I interviewed Gene Shoemaker, some committee members, and the cognizant NASA official (Bill Brunk) to determine why the report was never completed and released. I concluded that the chief reason was Gene Shoemaker's overcommittment (a characteristic of this exceptionally talented scientist, who tragically died in a July 1997 automobile accident in the Australian Outback) -- he simply never finished the report -- and it was not because of any reluctance on NASA's part due to the controversial nature of the proposed mitigation measures. (The latter hypothesis had been suggested by some.) Shoemaker's scientific conclusions about the frequency of impacts of objects of various sizes, developed for the Snowmass meeting, were eventually published in an important review article (Shoemaker, 1983). The Snowmass meeting and its conclusions were summarized in the final chapter of a trade book in 1989 (Chapman & Morrison, 1989). Copies of the draft report were circulated to participants of at least one of the major meetings of the early 1990's, and thus it entered into discussion at that time.
The increasing rate of discovery of NEA's by the search teams led by Shoemaker and by Helin (whose joint collaboration ended around 1982), soon to be joined by Gehrels' Spacewatch telescope, created a general awareness within the planetary science community by the late 1980's that the Earth is subject to potentially dangerous impacts from asteroids and comets. Two major interdisciplinary meetings about the Alvarez hypothesis concerning the K/T boundary (the Snowbird Conferences of 1981 [Silver & Schultz, 1982] and 1988 [Sharpton & Ward, 1990]) helped generate a consensus among most non- paleontologist scientists that an impact was responsible for the K/T mass extinctions. Shortly thereafter, the "smoking gun" was discovered: the immense Chicxulub crater in the northern Yucatan (first recognized as a geophysical anomaly in the 1950's by Mexican oil- exploration geologists, proposed as an impact crater in the early 1980's [see Sky & Telescope, March 1982, pp. 249-250], and finally recognized for its significance by the geological community following publication of the paper by Hildebrand et al. [1991]).
Despite continuing hold-outs -- generally paleontologists unwilling to admit to such a haphazard explanation for the evolution of species on our planet -- there is widespread acceptance of the hypothesis that the impact of a 6 to 10 mile wide asteroid in the Yucatan 65 million years ago rendered dinosaurs extinct and led to the evolutionary burst of mammals and the eventual emergence of homo sapiens. It is a logical consequence of the K/T impact that a hazard might still exist today; after all 65 million years ago is almost yesterday in geological history, long after the early epochs of higher impact rates in the solar system. Indeed, the statistical assessment of the near-Earth population of asteroids and comets was robust by 1990, and not very different from preliminary assessments decades earlier.
Recognition of a modern-day hazard, as distinct from the role of impacts in Earth history, took a little longer to develop. In spring 1988, David Morrison and I submitted an abstract on the impact hazard to the 2nd Snowbird Conference, but it was initially rejected. Reasons for rejection included concerns that discussion of the modern impact hazard would distort the hoped-for rational discussions of the K/T boundary and other revolutions in the evolution of life. We appealed, on the grounds that the possibility of a modern-day catastrophe needed to be appreciated by just the interdisciplinary scientists who would be present at Snowbird, and that the topic needed to be brought into the scientific mainstream. We were finally given poster space, but not a slot on the oral program.
In 1989, a small asteroid (later named 4581 Asclepius) was discovered to have made a "close" (by astronomical standards) pass by Earth. Due to an exaggerated and somewhat incorrect NASA press release[1],
this event led to headlines around the world as well as to a presentation by myself at the Meteoritical Society meeting that summer (Chapman et al., 1989). Later the same year, the trade book Cosmic Catastrophes by myself and Morrison was published (Chapman and Morrison, 1989), leading to further public discussion of the issue. We were invited to contribute to a Hazards session at the December 1989 American Geophysical Union meeting in San Francisco. Thus, by the end of 1989, the "impact hazard" had entered the scientific literature, if only as abstracts for several meetings. It was two more years before a technical article on the hazard appeared in the peer-reviewed literature (Ahrens & Harris, 1992).
It took a while for the idea of a contemporary hazard to reach the public and political leaders. A chapter written primarily by myself in Cosmic Catastrophes (Chapman & Morrison, 1989) was a critical influence. Our book dealt with a wide range of catastrophes, ranging from the ozone hole to the death of the Sun. In the final chapter of our book, "Chapter 19: Threat from the Skies: Will a Comet Strike?", we told the story of the 1981 Snowmass meeting and outlined the principal conclusions of the unpublished Shoemaker report. (Joint work on this chapter with my co-author David Morrison eventually led to a technical article on the impact hazard. Originally conceived as a piece for Scientific American, we decided that the topic needed to be addressed in the peer-reviewed literature, so we redirected the article toward Science. Science chose, for reasons that were never satisfactorily explained to us, not even to send the paper out for review, so we sent it instead to Nature, which published it in early 1994 [Chapman & Morrison, 1994].)
Congressional interest. A few months after publication of our book (and attendant modest, but nonetheless national, media coverage), my co-author, David Morrison, was invited to discuss the topic by the Space Caucus of the U.S. House of Representatives. On June 26, 1989, he met with about 25 congressional staffers, including Terry Dawson, who -- as staffer for the House Committee on Science, Technology, and Space working for the Committee's Chairman, Rep. George Brown -- played a pivotal role in keeping the topic before the Congress until the Republican take-over of the House in 1995.
Perhaps inspired by the low-key national discussion of this issue, the American Institute of Aeronautics and Astronautics (AIAA, the chief professional society of aerospace engineers) drafted a white paper recommending enhanced efforts to detect and calculate the orbits of potentially threatening bodies. Their lobbying efforts in the Congress were favorably received, resulting in draft language added by Dawson, at Brown's request, to NASA's 1991 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."
NASA Workshops and Studies. NASA's response was, and has remained (at least until very recently), decidedly low-key, responding to the Congressional interest in a minimally adequate way. Three actions taken in 1990, by the NASA official appointed to handle the matter (the late Jurgen Rahe) were (a) to ask me to chair an international scientific conference on Earth-approaching bodies (held in San Juan Capistrano, CA, in July 1991, co-sponsored by The Planetary Society), (b) to appoint David Morrison to chair what became known as the Spaceguard Survey Committee to study the possibilities for detecting dangerous asteroids and comets, and (c) to appoint John Rather to chair what became known as the Interception Workshop to investigate what could be done, technologically, should an object be found to be on a collision course with Earth.
The international conference was covered by the national network news, beginning a level of interest by the media in the impact hazard that continued to rise through March/April 1997, when most networks (Fox, NBC, ABC, PBS) aired special documentaries and, in the case of NBC, an expensive, heavily-promoted fictional miniseries on the topic (called "Asteroid", it was a critical flop; media coverage accelerated in 1998 due to the 1997 XF11 asteroid impact scare and the two box-office-winning movies [Deep Impact and Armageddon] with cosmic impact themes). The Spaceguard Committee largely repeated the analysis of the Snowmass meeting, adding technical details about how to efficiently search for near-Earth objects, and proposed a global network of six 2.5-m telescopes, costing $50 million for capital construction, that would find about 3/4ths of the dangerous objects (90% of the Earth-crossing asteroids larger than 1 km diameter) during the course of 25 years, at an annual operations cost of about $10 million. Many of the remaining 1/4 of the threatening objects (long period comets entering the inner solar system for the first time) were found to be much more difficult to detect. The Committee made the argument that it would be significant to identify most of the potentially hazardous objects; if, as expected, none of them were found to be on a near-term collision course, then the hazard would be reduced by a factor of 4. The report was published, in limited numbers, in early 1992 (Morrison, 1992).
The follow-on Interception Workshop was held in early 1992, following completion of the work of the Spaceguard Survey group, at Los Alamos National Laboratory. This meeting had a powerful impact on the subsequent political debate, so I will briefly outline its salient features. Strategic Defense Initiative (SDI) proponent Edward Teller and his disciple, Lowell Wood, were prominent participants of the meeting. By keeping the press away from the meeting, organizers only fueled the media's skepticism of what happened behind the closed doors. As it was, many of the Department of Energy researchers who had been recently introduced to the question of how to deflect asteroids, were provided with a technically flawed analysis by Wood et al. (1990) arguing that there is great danger from very small asteroids that impact frequently; in fact, they burn up harmlessly in the Earth's atmosphere, never posing a danger to anyone on the ground. Nevertheless, because of the Wood analysis, many of the analyses readied for presentation at Los Alamos concerned application of Strategic Defense Initiative ("Star Wars") technology originally intended to shoot down enemy missiles, which aren't very different from Wood's cosmic bullets .
Astronomers in the audience, including Morrison and myself, tried to explain that the small objects posed no significant hazard, and that it was the large (>1.5 km diameter) objects that were of concern. Indeed, schemes were discussed at the Detection Workshop that would mitigate impact of the larger ones, including stand-off detonation of a neutron bomb, but also including outlandish "blue sky" ideas involving anti-matter and other notions more appropriate for science fiction novels. I, in particular, asked that my name be omitted as a co-author of the Interception Workshop report (Rather, Rahe & Canavan, 1992), although I consented to having my doubts and critique of the report appear in an appendix of a more complete "Proceedings".
Having been deliberately excluded, the media were interested in finding out what really happened at the Interception Workshop. Subsequent investigative reporting by the San Jose Mercury-News, an op-ed piece in the New York Times by physicist Robert Park (Park, 1992), and pieces in such trade magazines as Aviation Week -- amplified by wider discussion in forums like Rush Limbaugh's radio talk show -- enhanced public skepticism about whether the ex-Cold Warriors were looking for anything under the sun to justify continuation of SDI. There was also concern about the self-interested motives of some astronomers interested in getting funding for their telescopes. Simultaneously, wider mass media commentaries played into the "giggle factor" that inevitably accompanied this difficult-to-relate-to "Chicken Little" hazard.
At least partly because of the giggle factor and the controversies surrounding Edward Teller's program -- which included "practicing" defensive technologies by actually deflecting an NEO with a bomb (Teller's word was "experimentation") -- NASA would have little or nothing to do with the topic. NASA met its responsibility by providing brief, watered-down reports on the work of the two committees to the Congress in spring 1993. But it printed only a minimal number of copies of the reports and issued no press releases. Wesley Huntress, who was in charge of NASA's Solar System Exploration Division and had become Associate Administrator for Space Science by the time he testified to the House Committee on Space on March 24, 1993 (he retired from NASA in autumn 1998), was uninterested in doing more than he had to about the hazard issue (he viewed it as competing with the basic scientific research he favored). NASA had issued a proposal opportunity in 1992, with a reported $1.5 million of available research funds diverted from elsewhere, to enhance the detection programs already in place; but the funds had shrunk to less than $500K by the time funding was allocated in 1993. Despite the tone of Huntress' testimony (he described the modest efforts that had just received enhanced funding), NASA effectively gave a negative response to the Congressional call for a two-decade-long program involving the construction of several large telescopes.
Civilian vs. military debates. Another important outgrowth of public discussion of the Los Alamos meeting was that Carl Sagan developed an interest in the topic. His passionate, politically oriented perspective, was articulated in his inimitable way in articles he wrote for scholarly publications (c.f. Sagan 1992) as well as for the large-circulation Sunday supplement, Parade. It was one of several major topics that Sagan then pursued until his untimely death in late 1996. Sagan was interested in, and acknowledged, the serious danger posed by impacts in the scheme of human and planetary evolution. But he was deeply disturbed by the prospects that discussion of asteroid mitigation and diversion might undermine international treaties that, in his view, prohibited the use of nuclear weapons in space under any circumstance. He even mused that scientists shouldn't even try to find NEA's, because of the potentially dangerous response of the military should one be found. Most disturbing of all, to Sagan, was the possibility that development of a technology to divert asteroids away from Earth could be used by a "madman" to divert asteroids to crash into the Earth (Sagan & Ostro, 1994). Sagan contributed to a technical analysis of this "deflection dilemma" (Harris et al., 1994), based on his debate with Edward Teller at the January 1993 Tucson meeting on "Hazards due to Comets and Asteroids".
The Los Alamos meeting was a public relations debacle for technologists from Los Alamos, Livermore, and the general military/DOE communities. (For instance, the Wall Street Journal headlined a post-Los Alamos story: "Never mind the peace dividend, the killer asteroids are coming!") During the subsequent two-and-a-half years, there were several attempts to improve communication between the "deflection community" and the generally academic astronomers and geophysicists investigating asteroids and environmental consequences. Col. Pete Worden, of SDI, then the Air Force Space Command, and more recently Deputy for Battlespace Dominance at the Pentagon, together with Stuart Nozette, organized a retreat at Erice, Sicily, in May 1993, which also -- for the first time -- had significant representation from the space policy community (chiefly American, but also international). Among the participants were John Pike (Federation of American Scientists), Bob Park (American Institute of Physics), and George Washington University space policy analyst John Logsdon. Morrison and Teller (1994) even collaborated on a joint chapter for the "Hazards" volume. (As of this writing, the proceedings of the Erice meeting have still not been published, although Stuart Nozette says that he has received a final draft; a portion of a 1998 Erice workshop on "planetary emergencies" was proposed to deal afresh with the impact hazard.)
Worden and Nozette have been continuing behind-the-scenes advocates within the Defense Department for programs oriented to deal with the impact hazard. An SDI technology demonstration project, the Clementine spacecraft mission, was expanded in scope to travel to a Near-Earth Asteroid, although it failed after a successful period orbiting the Moon. Worden and Nozette have facilitated funding of some of the telescopic search efforts.
Another meeting of the academic and military NEO communities was held in May 1994 at the Air Force Space Command in Colorado Springs, spearheaded by since-retired AFSC Chief Scientist John Darrah (SAIC, 1994). While those discussions led to some cooperative arrangements involving telescopes and instrumentation, the two groups began moving apart shortly afterwards. International interest. There has been a strong international component to early public discussion of the impact hazard, which is appropriate for a hazard that is indiscriminate when it comes to national boundaries. Even in the 1980's, there were modest telescopic search programs for NEO's underway in Australia and France. Jurgen Rahe, appointed by NASA to lead its response to the 1991 mandate from Congress, insisted that the Spaceguard Survey Committee have international representation. Indeed, probably the greatest interest in the impact threat has been expressed by Russians, from both its astronomical and military communities, even though in the post-Soviet era, they have reduced capability to address the problem. Russian interest is undoubtedly enhanced by the fact that the largest terrestrial impact in recorded history took place early this century in Siberia, and one of the largest subsequent impacts also occurred in Russia (Sikhote-Alin in 1947). An international conference was organized in late 1991 by astronomers in St. Petersburg just weeks after the city's name-change from Leningrad. There has also been considerable interest in Italy, which has an active group of asteroid researchers; it is only coincidental that in 1973 Arthur C. Clarke wrote a fictionalized account of a major impact occurring in northern Italy, which led -- in his story -- to the creation of a "Spaceguard Survey," whence the name of NASA's survey committee. The International Astronomical Union established a Working Group on Near Earth Objects in 1991, with Andrea Carusi (Instituto Astrofisica Spaziale, Rome) as Chair; he later became a major player in developing European awareness of the impact hazard. More recently, David Morrison has assumed chairmanship of the IAU Working Group.
In spring 1994, in cooperation with the Explorer's Club of New York, an international workshop was organized by the United Nations and held in the U.N. building in New York (the proceedings were published in mid-1997 by the New York Academy of Sciences [Remo, 1997]). Although it was a minor activity, by U.N. standards, it led to modest actions within the U.N., including reconsideration of some elements of U.N. space policy.
Media and public opinion. A final arena of increasing awareness is the general public. Virtually no one, outside of the few scientists involved, was aware of the impact hazard until 1989. Publicity that attended the publication of our book (Chapman & Morrison, 1989) and the "near-miss" by Asclepius began an ever-increasing interest on the part of the media. One of the early media mishaps occurred following our AGU talk and associated AGU press conference in December 1989, when a mistranslation of an American wire service story by the New China News Agency led to the false announcement of an imminent asteroidal impact into China; although the story was withdrawn some hours later, it already had led the national evening news telecasts in China, causing panic in some quarters, according to American press accounts. The "giggle factor" was already active when Vice President Quayle -- who had been the butt of derision by the media -- publicly expressed concern about the impact hazard during a speech at the national meeting of the AIAA. After columnists and humorists poked fun at him, Quayle is said to have become angry at Pete Worden, then serving on the staff of the National Space Council headed by Quayle, who arranged his AIAA speech.
The first general treatment of the impact hazard by the American news media accompanied the international scientific conference held in San Juan Capistrano in July 1991. Stories were broadcast on the evening newscasts by most networks, and accounts appeared in national news magazines. Press coverage reached a new level in autumn 1992 (including a cover story in Newsweek ["Doomsday Science", issue of Nov. 23, 1992]) connected with the mistaken announcement by Harvard scientist Brian Marsden of a century-in-the-future impact by Comet Swift-Tuttle. It was about this time that Paul Slovic (Univ. of Oregon), a leader in the field of risk perception, conducted his first poll concerning the impact hazard. As he reported, in early 1993 (see Morrison, Chapman & Slovic, 1994), only 25% of the public had heard of the impact hazard, at that point; respondents were fairly respectful of the scientific community's statement of "facts" about the impact hazard, but decidedly cautious about plans (particularly military ideas) to do anything about it.
By July 1994, the impact hazard was a topic of curiosity and was finally being debated in the technical literature, considered by policy analysts and governmental bodies, and being used as a justification for modest funding by both astronomers and by technologists in governmental labs. But the topic might have languished there were it not for the remarkable arrival of Comet Shoemaker-Levy 9.
The entirely serendipitous early-1993 discovery of Comet Shoemaker-Levy 9 (S-L 9) orbiting Jupiter, by Gene and Carolyn Shoemaker and David Levy (cf. Shoemaker & Shoemaker, 1995), and the comet's subsequent spectacular crash into Jupiter in July 1994 - - capturing banner headlines around the world for a week -- finally changed the impact hazard from a theoretical curiosity into a very distinct possibility in the public mind. Far from being an historical or paleontological event, the impacts were witnessed in real time by amateur astronomers and by the public, tuned in to the newly popular World Wide Web that was used to distribute telescopic images from such remote sites as the South Pole and the Hubble Space Telescope. And the resulting black spots in Jupiter's atmosphere, as large as the entire planet Earth and readily seen with small back-yard telescopes, provided dramatic evidence of the potential harm that could be done if such an impact were to strike the Earth (cf. Chapman, 1995). While Jupiter is hit at least a thousand times more often than the Earth is, the implications nevertheless became palpable.
Scientific developments. The modest incremental funding of telescopic search programs by NASA has, so far, yielded minimal scientific advances directly related to the impact hazard. (The search programs continue to find objects of theoretical interest to asteroid researchers, dynamical astronomers, and so on, such as the discovery of a small asteroid in a "horseshoe orbit" connected with the Earth.) Eleanor "Glo" Helin has moved her observing program from Palomar Observatory to Hawaii, where joint cooperation with the Air Force has put into operation the NEAT (Near Earth Asteroid Tracking) telescope, which is finding many Earth-approaching asteroids. The LONEOS (Lowell Observatory Near Earth Object Survey) telescope, being developed at Lowell Observatory by E. Bowell (and the late E. Shoemaker), is becoming operational just this year.
Perhaps the major scientific contribution directly related to the impact hazard in the last couple of years is a comparatively definitive evaluation of potential environmental consequences of impacts (Toon et al., 1997).
Policy and legal analysis. In some ways, the current GSA/NCAR workshop represents the first serious consideration of the impact hazard in the broader arena that interacts with the natural hazard community and public policy analysts. With emergency management practitioners involved in the latest disasters -- upper mid-west flooding last year, El Ni¤o-driven storms this year, and who knows what next year -- it is not clear how a hazard that has never been experienced in modern history will be considered.
Non-scientist specialists have begun to examine the hazard from their own perspectives, nonetheless. A prominent example is the recent publication of a thorough analysis of the national and international legal framework in which the impact hazard, and its potential mitigation, must be considered (Gerrard & Barber, 1997). These environmental lawyers argue that deployment and testing of technological defensive measures, especially in situ in outer space, would likely violate several international treaties (the 1967 Outer Space Treaty, the 1963 Partial Test Ban Treaty, and perhaps the 1973 Anti-Ballistic Missile Treaty as well as the 1992 U.N. Nuclear Power Principles). However, they argue, an emergency response to an identified oncoming asteroid might be justified by the inherent rights to national self-defense which are embodied in the U.N. charter. They also concluded that telescopic surveys are legally legitimate.
A comprehensive and more optimistic analysis of the legal framework for planetary defense has been made by Air Force Lt. Col. John C. Kunich (Kunich, 1998); he concludes that "all likely components of a planetary defense system, whether in the surveillance or the mitigation phase, can be supported under existing international and space law."
Congressional activities. Before the S-L 9 impacts even concluded, the Congress asked NASA to establish a new committee -- "Near-Earth Objects Survey Working Group", chaired by Gene Shoemaker -- to evaluate how the most dangerous asteroids might be found. The request was for an implementation plan. Once again, Gene Shoemaker's overcommitments in the post-comet-crash months were a major factor leading to a delay (nearly a year) in the group's report (Shoemaker et al., 1995). By that time, public fascination with Shoemaker-Levy 9 had waned. More important, the Republicans were now in charge of the Congress; although there have been no notable partisan differences on this issue, key individuals, such as Rep. Brown's staffer Terry Dawson, were no longer there. In addition, when NASA quietly transmitted the Shoemaker Committee report to Congress in August 1995, it recommended against initiation of the survey due to budgetary pressures. For all these reasons, the Shoemaker Committee's conclusions (that a 10-year program costing about $60 million, much less than what had been estimated by the earlier Morrison Committee, could discover 2/3rds of the threatening objects) languished. For example, Congress held no hearings following submission of the Shoemaker Committee report.
Congressional interest was reawakened in the aftermath of the March 1998 impact scare and new hearings were held by the House Subcommittee on Space and Aeronautics in May, just before the release of two blockbuster movies featuring impacts. Hastily arranged by Subcommittee staff, the hearings provided Subcommittee Chair Dana Rohrabacher, full Science Committee minority leader George Brown, and subcommittee members from both parties a forum to criticize NASA's inattention to the impact hazard. Several witnesses, including myself (Chapman 1998) and Greg Canavan, buttressed the Subcommittee's biases while Carl Pilcher, acting head of NASA's solar system exploration program, promised that NASA would address the Shoemaker Committee's goals. Language resulting from the hearings was incorporated into draft legislation, but the impact on NASA's FY 1999 programs is not clear as of this writing.
NASA activities. During the last few years, NASA has gradually expanded its expressed interest in the threat from NEO's, and it has even increased its direct funding for search activities. A long-planned spacecraft investigation of the large, Earth-approaching asteroid Eros -- the Near-Earth Asteroid Rendezvous (NEAR) mission -- has been touted significantly in terms of helping to better evaluate the impact hazard. Eros has recently been shown to have a roughly 5% - 10% chance of ending its existence by crashing into the Earth (Michel et al., 1998), although this is likely to be many millions of years from now and cannot possibly happen in the near future. The NEAR mission was conceived with scientific goals and it is doubtful that the hazard issue significantly enhanced its standing in NASA's queue of future missions.
The close approach to Earth of Comet Hyakutake in early 1996 and the even more spectacular apparition of Comet Hale-Bopp in spring 1997 heightened the interest of the space agency in the issue of small bodies approaching the Earth. Hale-Bopp, which came nowhere near Earth, nevertheless did penetrate within the Earth's mean distance from the sun; had it struck, it would have been vastly more devastating than even the K/T boundary impact. This interest played into a new reconsideration by NASA of its basic goals, under direction from Administrator Dan Goldin. Following discovery of possible fossil life in a Martian meteorite and other related discoveries, the concept of "Origins" as a major focus of NASA's explorations has emerged. At a May 1997 meeting of NASA managers and advisors in Breckenridge, Colorado, the concept of "Destiny" was added to "Origins", with the potential impact by a comet or asteroid high on the list as an element of the prospective destiny of mankind. The current Strategic Plan of NASA's Office of Space Science now includes the goal of discovering 90% of potentially hazardous asteroids larger than 1 km diameter within 10 years. Yet adoption of this new goal has resulted, so far, in little new funding of NEO searches, perhaps because of reported misunderstandings among some officials within NASA and the Office of Management and Budget that NASA's current program will meet the goal (it will not: see below).
It has been estimated recently that NASA has recently been spending about $1.4 million (to as much as $1.8 million) per year on NEO search programs. These funds come primarily from NASA's Planetary Astronomy Program, a basic research and analysis grants program totalling about $10 million/year, designed to fund scientifically oriented telescopic observation of planetary bodies (including asteroids, comets, and satellites) throughout the solar system. The impact hazard goals are widely perceived by other planetary astronomers competing for scare funds as having a non-basic-research character, which they feel is inappropriate.
Yet NASA's contribution to the NEO search efforts has been niggardly by some measures. For example, NASA currently supports only 25% of the costs of the Spacewatch program at the Univ. of Arizona; most of the remaining three-quarters of Spacewatch funding is from private donations! For more than a decade, the Planetary Society (a popular membership organization) has funded Eleanor Helin and other international individuals engaged in NEO discovery. In 1997, it inaugurated an NEO grants program named in honor of Gene Shoemaker. Also in 1997, businessman Jim Benson, an entrepreneur trying to raise interest in a privately-funded deep-space mission to an NEA ("Near-Earth Asteroid Prospector"), offered a prize to amateur asteroid watchers. Most of the efforts of the Shoemaker-Levy observing program at Palomar, until its discontinuation a couple of years ago, were based on voluntary help, including that of Carolyn Shoemaker and David Levy; the Shoemakers have reported receiving no funding at all from NASA during the year they discovered the famous Shoemaker-Levy 9 comet, despite NASA's later taking public credit for the discovery.
Early in 1998, the acting director of NASA's Solar System Exploration Program, Carl Pilcher, decided to try to increase the agency's interest in NEO's. A panel was appointed to hear presentations regarding present NEO searches and to make recommenda- tions for future enhancements to the program. The day selected for the meeting (March 17, at the Lunar and Planetary Institute in Houston) was, serendipitously, just a few days after the March 11, 1998, erroneous announcement of a potential Earth impact by 1997 XF11, so the agenda was augmented to include preliminary evaluation of what went wrong in that case. Meeting attendees made the case that a program with annual operating costs of about $4 million (not including as-yet-incomplete capital investments in upgrading the existing equipment nor including the costs of existing Air Force GEODSS telescopes that would be used to complete the system) might suffice to meet the goals of the strategic plan, albeit in 20 years, not 10. (A much more costly alternative, involving construction of larger telescopes, would more assuredly meet the survey's goals, but probably not much sooner than 20 years because while the survey could be completed in 10 years, it might take as long as 8 years to construct the new telescopes before the observations could begin.) NASA has publicly announced that it will develop a more serious attempt to meet the goals of its strategic plan, by roughly doubling its investment in telescopic searches for NEO's. No concrete funding augmentations have happened yet, so far as I am aware, despite NASA's July 14th announcement of the creation of an NEO office at the Jet Propulsion Laboratory. Moreover, there remains a mismatch of something like a factor of two between NASA's promised funding levels and what many experts believe is required to meet the stated goals.
Military programs. Edward Teller has backed away from his advocacy of "experimentation" in the near future. In a lesser way, however, the proposed Clementine 2 space mission (a follow-on to the first SDIO-initiated Clementine mission, which failed to achieve its asteroidal objective after successfully observing the Moon), would perform actual cratering experiments on a target asteroid. Besides scientific interest in such experiments, there is little doubt that the promoters of the project also see it as the first step in developing the logistics to "deal with" a dangerous asteroid, as well as to begin direct assessment of the physical properties of objects that would be required before a deflection system could be reliably implemented. Clementine 2 is currently in limbo after President Clinton killed it in late 1997 with a line-item veto; that new Presidential power is currently wending its way up to the Supreme Court, where it may be reversed. (It is unclear whether the veto was based on informed consideration of Clementine 2's merits by presidential advisors.)
Although it is reported that major proposed military programs to deal with small bodies have not been funded, the majority of NEO discoveries in early 1998 are facilitated by the DoD. The most effective program has recently become the Lincoln Laboratory LINEAR program, operated in New Mexico. During mid-1998, LINEAR was responsible for more than half of the worldwide discoveries of NEO's. And the NEAT program involves collaboration between the Jet Propulsion Laboratory (JPL) and the Air Force.
In other respects, there has been continued interest on the part of the American military about developing a "planetary defense" program to deal with the threat. For example, an Air Force study resulted in the October 1996 publication of a report ("Planetary Defense: Catastrophic Health Insurance for Planet Earth", Urias et al. 1996) that advocates deployment of a planetary defense system by the Department of Defense. It would consist of "a detection subsystem, command, control, communications, computer, and intelligence (C4I) subsystem and a mitigation subsystem." Given overall DoD secrecy, I am not up-to-date concerning specific DoD programmatic initiatives in this area.
International developments. At a September 1995 meeting in Vulcano, Italy, of the International Astronomical Union's NEO committee, it was decided to create an international "Spaceguard Foundation" (SGF) to solicit funding from various sources and to coordinate international efforts in mounting a detection program similar to that advocated by the Morrison and Shoemaker committees. In March 1996, with Andrea Carusi as President, the Foundation was incorporated in Italy and began several efforts.
One of Carusi's enterprises, initiated prior to formation of the Foundation, was to arrange for the Parliamentary Assembly of the Council of Europe to pass a resolution, on March 20, 1996, on "the detection of asteroids and comets potentially dangerous to humankind." The resolution urges the governments of member states and the European Space Agency to support international programs that would inventory NEO's larger than 0.5 km in size and contribute to international efforts to evaluate and deal with potential impacts. A current initiative of the SGF, for which United Nations endorsement has been obtained, is to build a large NEO survey telescope in the southern hemisphere, perhaps in Namibia.
National affiliates of the Spaceguard Foundation have been founded in Japan, the U.K., Germany, and elsewhere. Meanwhile, Australia has backed away from its once significant role in detection of NEO's. Spaceguard Foundation ex-Vice President Duncan Steel's observation effort was closed down in 1996, and remains inoperative despite international pressure that has been mounted to reverse that action. The Australian program not only capitalized on its southern hemisphere location but also was productive on the important but unrewarding effort of making follow-up positional measurements to ensure that newly discovered NEO's are not lost.
A series of American-Russian bilateral meetings, held in the once-secret Soviet city of Chelyabinsk-70 and at Livermore, California, primarily engaged the military community, although there were a few representatives from the civilian/academic community. In late 1996, Vadim Simonenko, of the Russian Federal Nuclear Center, established a private "Space Shield Foundation" in Russia to promote international programs on hazard analysis, identification of NEO's, and planetary protection. On the other hand, continuing financial problems within Russia are threatening both the scientific and technological programs in that country.
Significant discovery and/or follow-up observing programs are conducted in a number of countries. A recent addition to the effort is the Xinglong program in China. Another important program (ODAS), based in southern France, is partially supported by Germany. In April 1998, the Japanese government announced unding for a 1-m NEO search telescope. Amateur astronomers in countries like Japan and Italy play important roles, as well. These observations are received, digested, and disseminated to the astronomical community by the International Astronomical Union's Minor Planet Center (MPC), associated with the IAU's astronomical circulars office at the Harvard/Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. Directed by Brian Marsden, the MPC has received no funds from the IAU since 1994 but more recently has received support from NASA for the salary of its chief assistant, Gareth Williams.
The impact hazard continues to be used as an excuse for other international political issues, for example control of nuclear weapons. On April 27, 1996, for instance, The New York Times reported (under the headline "Chinese seek atom option to fend off asteroids") that Chinese government officials opposed signing a nuclear weapons test ban treaty with Russia because of the potential utility of such weapons against asteroids. An understanding of the impact hazard may also be important in terms of monitoring compliance with the Comprehensive Test Ban Treaty. Chyba et al. (1998) argue that large meteorite impacts occurring on a typical timescale of a decade could be misinterpreted, in seismic data, as treaty violations or potential terrorist activity.
Public interest and media coverage. About ten trade books on the topic of the impact hazard have appeared in the last several years, mostly by reputable authors. However, a few feature a neo-catastrophist perspective popular in Great Britain, which holds that the Earth is in a much more precarious state than is generally accepted in the broader scientific community. Articles about the impact hazard have appeared in national magazines; a recent, and particularly accurate and well-written, article is by Ferris (1997). A CD rom on the topic is now available (Mascoli, 1998).
In the spring of 1997, there was unprecedented television coverage of the impact hazard, including a widely promoted but inaccurate and poorly received NBC miniseries, entitled "Asteroid". Higher quality documentaries were broadcast on several networks, including rebroadcast of a 1995 PBS "Nova" program on the topic. Perhaps the best of the series was a National Geographic Special on NBC, featuring Gene and Carolyn Shoemaker. A 1996 film production on the impact hazard, primarily filmed at the 1995 Vulcano meeting, by the National Film Board of Canada has received critical acclaim as well as the Discovery Channel's Emmy Award-winning "Three Minutes to Impact." Asteroid impacts are increasingly involved in plots or subplots of science fiction TV programs like "Star Trek" and science fiction novels, as well as in commercial advertising.
Two summer blockbusters have done for asteroids and comets what "Volcano" and "Dante's Peak" did for exploding volcanoes in the summer of 1997. A Steven Spielberg/Dreamworks production, "Deep Impact," involving a comet impact, starring (among others) Morgan Freeman and Robert Duvall, opened in May 1998. A two-pronged approach to mitigation is employed by the U.S. President: (1) evacuation into shelters of a small part of the population selected by lottery and (2) a problematic attempt by astronauts to deal with the onrushing comet. Attempts to be scientifically accurate, by employing the Shoemakers and planetary scientist Josh Colwell, resulted in comparatively few errors, although the near-Earth bombing of the larger comet fragment should have resulted in a mass-extinction holocaust instead of the depicted fireworks display.
The second movie, which opened in July, is a Bruce Willis thriller entitled "Armageddon", dealing with impacting asteroids; the hazardous object, discovered just weeks before impact, is impossibly large -- the size of Texas. The expensive thriller is scientifically and technologically preposterous in almost every respect. Both movies were accompanied by widespread associated media publicity. One Associated Press reporter doggedly pursued his unfounded suspicion that Brian Marsden created the 1997 XF11 affair (see below) in order to heighten interest in one of the movies. Few people remain unaware of the threat from the skies, following the 1998 movies.
I thank David Morrison for his direct and indirect assistance. Discussions with Alan Harris, Ted Bowell, Rick Binzel, Paul Chodas, Don Yeomans, Hal Levison, David Morrison, and numerous other colleagues in astronomy, as well as participants in the two Prediction Workshops, have helped me to describe the history of this topic and formulate the issues that face us.
Ahrens, T.J. & A.W. Harris 1992. Deflection and fragmentation of near-Earth asteroids. Nature 360 429-433.
Alvarez, L.W., W. Alvarez, F. Asaro, & H.V. Michel 1980. Extraterrestrial cause of the Cretaceous- Tertiary extinction. Science 208 1095-1108.
Alvarez, W. 1997. T. rex and the Crater of Doom (Princeton Univ. Press).
Baldwin, R.B. 1949. The Face of the Moon (Univ. Chicago Press).
Binzel, R.P. 1997. A Near-Earth Object hazard index. In Near-Earth Objects: The United Nations International Conference (ed. J. Remo, Annals N.Y. Acad. Sci. 822) 545-551.
Bowell, E. & K. Muinonen 1992. The end of the world: An orbital uncertainty analysis of a close asteroid encounter. Bull. Amer. Astron. Assoc. 24, 965.
Browne, M. 1998. Old photos helped refine progress of asteroid. New York Times, 14 March.
Chapman, C.R. 1998. The threat of impact by near-Earth asteroids (www.boulder.swri.edu/clark/hr.html) and Action plan statement to: House Subcommittee on Space & Aeronautics (www.boulder.swri.edu/clark/actnea.html).
Chapman, C.R. 1995. What If?... In The Great Comet Crash (ed. J.R. Spencer & J. Mitton, Cambridge Univ. Press) 103-108.
Chapman, C.R. & D. Morrison 1989. Cosmic Catastrophes (Plenum Press).
Chapman, C.R., D. Morrison, E. Bowell 1989. Hazards from Earth-approachers: Implications of 1989 FC's "near miss". Meteoritics 24 258.
Chapman, C.R. and D. Morrison 1994. Impacts on the Earth by asteroids and comets: assessing the hazard. Nature 367 33-40.
Chodas, P.W. & D. Yeomans 1997. Impact warning times for Earth Crossing Asteroids. Bull. Am. Astron. Soc. 29 960.
Chyba, C.F., G.E. van der Wink, C.B. Hennet 1998. Monitoring the Comprehensive Test Ban Treaty: possible ambiguities due to meteorite impacts. Geophys. Res. Lett. 25 191-194.
COMPLEX 1998. Exploration of Near Earth Objects (Committee on Planetary and Lunar Exploration, National Research Council, National Academy Press, Washington D.C.) 44 pp.
Ferris, T. 1997. Annals of science: is this the end? The New Yorker, Jan. 27, 1997, 44-55.
Gerrard, M.B. and A. W. Barber 1997. Asteroids and comets: U.S. and international law and the lowest-probability, highest consequence risk. N.Y.U. Environmental Law Journal 6 (1), 4- 49.
Gladstone, B. 1998. Report on Morning Edition, National Public Radio, 4 June 1998.
Goldman, S.J. 1998. The most dangerous rocks in space. http://impact.skypub.com/rocks.html (from Sky & Tel., June 1998, 33).
Gordon, B.B. 1998. The asteroid caper, editorial in Astronomy, July issue, pg. 6.
Harris, A.W., G.H. Canavan, C. Sagan & S. Ostro 1994. The deflection dilemma: use versus misuse of technologies for avoiding interplanetary collision hazards. In Hazards due to Comets & Asteroids (ed. T. Gehrels, Univ. Ariz. Press) 1145-1156.
Helin, E.F. and E.M. Shoemaker 1979. Palomar planet-crossing asteroid survey, 1973-1978. Icarus 40 321-328.
Hildebrand, A.R. & 6 others 1991. Chicxulub Crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico. Geology 19 867-871.
Kleiman, L.A. (ed.) 1968. Project Icarus (M.I.T. Report No. 13, M.I.T. Press).
Kunich, J.C. 1998. Posted on World Wide Web, August 1998: sac.saic.com/space_warfare/planlrev.htm.
Marsden, B.G. 1997. Overview of orbits. In Near-Earth Objects: The United Nations International Conference (ed. J.L. Remo, Ann. N. Y. Acad. Sci. Vol. 822) 52-66.
Mascoli, G. (Producer) 1998. Impact: Ground Zero (All Around Us Interactive, www.cdanddvd.com), CD-rom.
Masursky, H. (1967). Quoted in "Monitoring of asteroids urged to warn of impending collisions," New York Times, 18 Oct.
McCord, T.B. et al. 1995. Detection of a meteoroid entry into the Earth's atmosphere on February 1, 1994. J. Geophys. Res. 100 3245-3249.
Michel, P., Ch. Froeschl‚ & P. Farinella 1998. Dynamics of Eros. Astron. J., in press.
Morrison, D. (chair) 1992. The Spaceguard Survey: Report of the NASA International Near-Earth- Object Detection Workshop (JPL/NASA, Jan. 25, 1992).
Morrison, D., C.R. Chapman & P. Slovic 1994. The impact hazard. In Hazards due to Comets & Asteroids (ed. T. Gehrels, Univ. Ariz. Press) 59-91.
Morrison, D. and E. Teller 1994. The impact hazard: issues for the future. In Hazards due to Comets & Asteroids (ed. T. Gehrels, Univ. Ariz. Press) 1135-1144.
Muinonen, K. 1998a. Asteroid and comet encounters with the Earth: Impact hazard and collision probability. In Proceedings of The Dynamics of Small Bodies in the Solar System: A Major Key to Solar System Studies (eds. A.E. Roy and B.A.Steves, NATO Advanced Study Institute, Maratea, Italy) in press.
Muinonen, K. 1998b. Upper bounds for the Earth-XF11 collision probability. Bull. Am. Astron. Soc., in press.
Muinonen, K. & E. Bowell 1993. Asteroid orbit determination using Bayesian probabilities. Icarus 104, 255-279.
Opik, E. 1957. In Irish Astron. J.
Park, R.L. 1992. Star warriors on sky patrol. New York Times (March 25, 1992) A19.
Powell, J.L. 1998. Night Comes to the Cretaceous. (W.H. Freeman, N.Y.)
Rather, J.D.G., J. H. Rahe, and G. Canavan (organizers) 1992. Summary Report of the Near-Earth- Object Interception Workshop (JPL/NASA, Aug. 31, 1992).
Remo, J.L. (Ed.) 1997. Near-Earth Objects: The United Nations International Conference (Annals of the N.Y. Acad. of Sci. 822), 632 pp.
SAIC 1994. Proceedings of the Scientific Exchange Meeting on Asteroids (USAF Space Warfare Center, Science Applications Intl. Corp., May 12-13, 1994).
Sagan, C. 1992. Between enemies. Bull. Atomic Sci. 48 24.
Sagan, C. and S. Ostro 1994. Dangers of asteroid deflection. Nature 369 501.
Sharpton, V.L. & P.D. Ward (eds.) 1990. Global catastrophes in Earth history (GSA Special Paper 247).
Silver, L.T. & P. H. Schultz (eds.) 1982. Geological implications of impacts of large asteroids and comets on the Earth (GSA Special Paper 190).
Shoemaker, E.M. 1963. Impact mechanics at Meteor Crater, Arizona. In The Moon Meteorites and Comets (ed. B.M. Middlehurst & G.P. Kuiper, Univ. Chicago Press) 301-336.
Shoemaker, E.M. 1983. Asteroid and comet bombardment of the Earth. Ann. Rev. Earth & Planet. Sci. 11 461-494.
Shoemaker, C.S. & E.M. Shoemaker 1995. A comet like no other. In The Great Comet Crash (ed. J.R. Spencer & J. Mitton, Cambridge Univ. Press) 7-12.
Shoemaker, E.M. et al. 1995. Report of the Near-Earth Object Survey Working Group (NASA).
Steel, D. 1995. Rogue Asteroids and Doomsday Comets. (John Wiley & Sons, N.Y.)
Toon, O.B. et al. 1997. Environmental perturbations caused by the impacts of asteroids and comets. Reviews of Geophys. 35 41-78.
Urey, H.C. 1973. Cometary collisions and geological periods. Nature 242 32-33.
Urias, J.M. et al. 1996. Planetary defense: catastrophic health insurance for planet Earth. Research paper presented to Air force 2025. [This and other recent documents are available on the "Asteroid and Comet Impact Hazards" Web site: http://ccf.arc.nasa.gov/sst/.]
Verschuur, G.L. 1998. Impact hazards: truth and consequences. http://impact.skypub.com/page1.html (from Sky & Tel., June 1998, 26-34).
Watson, F.G. 1941. Between the Planets (Harvard Univ. Press, Cambridge).
Whipple, F. L. 1951. A comet model II. Physical relations for comets and meteors. Astrophys. J. 113 464-474.
Wood, L., R. Hyde & M. Ishikawa 1990. Cosmic bombardment II: intercepting the bomblets cost- efficiently (Lawrence Livermore Natl. Lab., UCID-103771).
Available Abstracts, Preprints, Articles.
Clark
R. Chapman's Publications.
Home Page