CHAPTER 13: ASTEROID DEFENSES
Regarding nuclear explosives, we have enough knowledge. But regarding the interaction of these explosives with comets and asteroids, we have much too little knowledge. I believe that calculations are important, but I believe that experiments are much more important. We must carry out experiments to determine how nuclear explosions of various kinds will react with heavenly bodies of various kinds. We cannot find out about these in any way other than by carrying out experiments. -- Physicist Edward Teller in a speech at the Space Protection of the Earth conference, Snezhinsk, Russia, September 1994.
In the months following the Los Alamos meeting, the astronomers and the weapons builders continued their dialogue. In time, both the United States Air Force and the Russian nuclear establishment became interested, as we describe in this chapter. An influential participant in all these discussions was Edward Teller, who had adopted the asteroid defense issue as a major theme for international cooperation in the post-cold-war world.
At Los Alamos, Teller had followed Lowell Wood's lead and focused on the problem of defending against small asteroids, which could not be detected more than a few days in advance of their collision with the Earth. This concept led to a "shoot first" defense strategy centered on a fleet of nuclear-tipped anti-asteroid missiles held at the ready. However, Teller's attitudes soon began to evolve. Later in the spring of 1992, when he and Morrison gave back-to-back keynote lectures at a meeting of the National Space Society in Colorado Springs, Teller had accepted the argument that small projectiles were not a problem and that some sort of comprehensive asteroid survey should be carried out. At that time, however, he advocated conducting a space-based search for NEOs using Star Wars technology. A series of small satellites would be placed in Earth orbit to scan the sky for asteroids, transmitting their observations to the ground to analysis.
Teller's primary argument for a space-based NEO search was that serendipitous astronomical discoveries might result from wide-field space telescopes. At Colorado Springs, Morrison countered that a ground-based search was much cheaper and more cost-effective: the job could be done for a hundred million dollars from the ground rather but would cost more than a billion dollars from space. At that meeting, the two could only agree to disagree. But by the time of the next hazard meeting, in January 1993, Teller was apparently reconciled to the cost-effectiveness of a ground-based asteroid survey like Spaceguard.
Looking ahead to the problems of deflection, however, Teller expressed concern about our lack of knowledge of the physical properties of comets and asteroids. Such knowledge would be necessary if we were to plan a comprehensive defense. He pushed for a major program of spacecraft visits to explore the comets and asteroids, using small and relatively inexpensive probes based on the "brilliant eyes" surveillance system being proposed by Wood at Livermore. He also suggested that initial spacecraft visits should be followed by active testing, including the experimental deflection of asteroids and comets to develop the technology that might someday be needed for defense against a real impact threat. Teller's proposal to start nuking asteroids stimulated a strong response from Carl Sagan and other astronomers, which we will discuss in a later chapter.
Although he remained in the background during the early interactions between astronomers and weapons scientists, one of the most influential figures in the impact hazard debates was Col. Pete Worden of the U.S. Air Force. A handsome career military officer with a Ph.D. in astrophysics, Worden was a long-time "space cadet". A severe critic of NASA, which he viewed as technically timid and excessively bureaucratic, Worden had embraced Reagan's Star Wars program as a means to develop new space technology and perhaps, if NASA faltered, to create the basis for an alternative space exploration program. Having served on the staff of the National Space Council under Vice President Quayle, in 1992 he was in the Pentagon managing advanced technology for the Star Wars program. There his small cadre of Air Force enthusiasts sometimes called themselves the "space Nazis".
With Teller's support, Pete Worden decided to organize another conference on asteroid defenses for the spring of 1993. This meeting would be everything that Rather's Los Alamos conference should have been. Focusing on near-term issues, Worden's meeting would exclude the fringe elements and their impractical "new technology" schemes. His meeting would be international, with the participation of leaders from the Russian defense establishment. And far from banning the press, Worden invited not only science reporters but also leading critics of the Star Wars program from a number of Washington space and peace advocacy groups. Bringing together 50 of the leading astronomers, weapons scientists, and their critics in an environment conducive to open discussion and compromise, Worden sought to forge a consensus in support of asteroid defense. For a location, he chose the picturesque town of Erice, perched atop a mountain on the west coast of Sicily. We both attended.
Erice, an ancient walled hill-town with narrow cobbled streets and glorious views of the Mediterranean, was developed in the 1970s as a site for international science conferences and summer courses, complete with modern meeting facilities, classrooms, and dormitories, all constructed in the renovated interiors of ancient monasteries. Erice was well known to Teller, Worden, and other insiders of the U.S. defense establishment. A series of annual "Seminars on Nuclear War" held there had provided opportunities for high-level military and civilian defense strategists from the U.S. and the Soviet Union to exchange views on a regular basis. With the end of the Cold War, these seminars had been broadened and retitled as "Seminars on Planetary Emergencies". The subject of Worden's Erice seminar in May 1993 was "The Impact on Earth of an Asteroid or Comet".
Worden set the tone for the meeting in his introductory talk, expressing hope for a consensus based on recognition of the reality of the impact hazard and a commitment for an international effort to deal with it, initially through support for the Spaceguard Survey. When we reported our work on the nature of the risk, emphasizing asteroids 1 km or more in diameter, we found none of the hostility that had characterized Los Alamos. The discussions were constructive, and gradually agreement emerged among the participants, in spite of their different backgrounds. The Russians seemed especially anxious to share their decades of experience with nuclear weapons in the service of a new, international cause.
On the final day of the Erice conference, we all gathered in the main monastery building to agree upon a statement that summarized the conclusions we had reached. The first four points of the statement were relatively straightforward. The group agreed that "cosmic impact is an environmentally significant phenomenon which has played a major role in the evolution of life on Earth", and that "the threat is real and requires further internationally coordinated public education efforts." They also agreed that "gathering of additional physical knowledge of NEOs and their effect on the Earth is a scientifically and socially important endeavor," and that "dedicated international astronomical facilities similar to the proposed Spaceguard System should be developed."
On a final question, however, consensus broke down. Worden did not want to include any discussion of defense systems, but Teller insisted that this issue be addressed. Teller was convinced that nuclear experiments on deflecting real asteroids and comets were necessary. He had two reasons. One was technical: the need to learn how to accomplish such a task in advance of any actual emergency. His second reason was political. He argued that it would be extremely difficult to obtain international agreement for any use of nuclear devices in space, and that we had better start now to work out ways to deal with this problem. Teller considered the social experiment of forging international consensus in support of experimental deflections to be more challenging than the technical problems of accomplishing the deflection once it was approved.
In the Erice environment, Teller did not automatically get his way. Opposed by Worden, Shoemaker, and all of the Spaceguard astronomers attending the conference, Teller found himself engaged in a real debate. Most of us wanted to say explicitly that we did not favor experimentation at this time, but Teller stood his ground. He would not agree to any statement that called for deferring experiments. The majority would not sign any statement that endorsed such experiments. After heated arguments, the stalemate was resolved in the waning minutes of the conference and compromise wording was reluctantly approved: "The study of potential mitigation systems should be continued. Many of us believe that unless a specific and imminent threat becomes obvious, actual construction and testing of systems that might have the potential to deflect or mitigate a threat may be deferred because technology systems will improve." The "many" referred to in the statement was nearly everyone at the meeting except Teller.
In spite of this last-minute dispute, Pete Worden had accomplished his goal. Everyone at last agreed on the general nature of the impact threat. The Russian attendees were enthusiastic about joint efforts that might make use of their capabilities, including unused tracking telescopes that they had available. And, perhaps most remarkable, all of the critics who attended were now converts and supporters of continued efforts to deal with the impact threat.
Science is founded on the principle that observations, experiments, and calculations can be reproduced and verified. The conclusions of an individual scientist or a team should be checked through repetition by others. In practice, however, one cannot check or reproduce the vast amount of data and interpretation that are presented at meetings and published in technical journals. Individual scientists therefore develop a sense of who and what can be trusted. We learn, through a combination of experience and intuition, who are the most reliable observers, the most precise experimentalists, and the cleverest theorists. When the NASA astronomers and the physicists from the nuclear weapons labs first came together in January 1992, they had no basis for trust, no way to know who in the other camp was reliable or brilliant and who might not be. Between Los Alamos and Erice, a sort of Darwinian process eliminated the fringe elements and allowed some members from each community to gauge the worth of the others. One of the products of this interaction was a sense of trust and shared values.
* * * * * *
The emergence in Russia of concern about the impact hazard happened at the same time as in the United States, with Russian astronomers and weapons builders playing roles that paralleled those of their American counterparts. Many Russian scientists were interested in defense against asteroids for the same reasons as Americans: a mixture of scientific curiosity, desire to perform a socially useful purpose by promoting protection of the Earth, and self-interest associated with retaining their jobs and research funding. They sponsored the first international meeting specifically devoted to the NEO impact hazard, held in October 1991.
At just the moment the Soviet Union was disintegrating, astronomers from the Institute for Theoretical Astronomy of the USSR Academy of Sciences invited their colleagues from Russia and abroad (ourselves included) to meet in St. Petersburg (then newly renamed from Leningrad). Few of the Russian attendees were previously involved with the impact hazard, but after three days of open and vigorous discussion, they endorsed an asteroid survey. The organizers were particularly interested in making a case for Russian government funding for a new search telescope and for follow-up studies of NEO orbits, a scientific specialty of the Institute for Theoretical Astronomy. Following the meeting they formed an international organization on the impact hazard to support the legitimacy of their funding requests.
While the Russian astronomers continued to exchange information with their counterparts from the Spaceguard team in the United States, a quite different group of Russians began discussions with Teller and his colleagues from the U.S. weapons laboratories. For many years the defense strategists and weapons builders from the U.S. and Russia had worked together, first as cold-war rivals and more recently in a spirit of international cooperation. These scientists were especially interested in applying their experience with nuclear testing to problems associated with the effects of impact explosions. Since the USSR had set off much larger atmospheric tests than had the Americans, they were able to make unique contributions to this research. Within Russia the astronomers and the weapons scientists initially pursued their interests independently, but by 1994 they had begun to talk to each other, largely at the initiative of small, dapper nuclear physicist Vadim Simonenko, an energetic, outgoing leader of the new post-Soviet generation of Russian defense experts.
Simonenko worked at the Institute of Technical Physics of the Russian Federal Nuclear Center -- one of two major centers for research on nuclear weapons and the Russian equivalent of Livermore National Laboratory in the United States. For four decades the existence of this Federal Nuclear Center had been a military secret, and the city in the southern Urals where it was located did not even have a name -- just a postal code, Chelyabinsk-70. It was a major milestone in the emergence of this secret city when Simonenko obtained permission to host an international conference on asteroid defense there. The conference was named "Space Protection of the Earth 1994", and both Teller and Wood accepted invitations to attend, as did five other Americans, including Tom Gehrels and one of us (Morrison).
The American party arrived at the Ekaterinburg Airport in the small hours of the morning of September 25, having crossed 12 time zones in the flight from California. Soon we were in a bus, following the blinking blue light of a police escort through endless miles of white birch trees to our destination: a closed city that appears on no map of Russia. Only during the preceding year had it obtained a name: Snezhinsk, which means "snowy" in Russian. When we finally arrived, we entered Snezhinsk through high barbed wire fences guarded by soldiers with automatic weapons and unsheathed bayonets. Here, isolated from the rest of the world, lived the 15,000 workers at the nuclear institute and their families. A huge bronze statue of Lenin still dominated the central square, and several of us spoke with students in the schools who had never before seen or met an American. These talented people, who have spent two generations building nuclear bombs in this closed society, were looking for alternatives. Shooting down asteroids seemed like a possibility.
The Space Protection meeting was attended by about 150 Russians, the majority of whom had not previously worked with foreigners. These scientists and engineers were experts in nuclear weapons and missile systems, including space tracking and surveillance. For a week they met to discuss asteroids, with particular emphasis on schemes for interception and nuclear deflection or destruction of NEOs. Teller, as "father of the H-bomb", was idolized by the Russian nuclear community, and he and Wood had come to Russia with a message that this audience was happy to hear. The Teller-Wood thesis was simple: we must build an international defense system against cosmic impacts, and an urgent part of that effort is to conduct nuclear tests to learn how to deflect or destroy NEOs. Teller emphasized that such an experimental program was the only way to obtain the required data, and that nuking asteroids represented the most cost-effective kind of experimentation. In case anyone missed the message, Wood also told the audience that there were no international prohibitions against nuclear explosions in space, since the existing treaties dealt only with "weapons of mass destruction", not peaceful uses of nuclear explosives intended to develop a capability to protect the Earth. Teller added that only fear-mongers opposed the peaceful use of nuclear explosives.
When Tom Gehrels told the conference that the majority of American scientists opposed nuclear testing in space, he was fiercely attacked by both Teller and Wood for his efforts to "politicize" the meeting. These attacks seemed particularly inappropriate directed toward Gehrels, who had broad international experience and personal cultural ties to Europe and Asia as well as America. For good measure, Wood also publicly accused Morrison and the Spaceguard scientists of intentionally minimizing the impact risk and lying to the American Congress. Here, on Russian soil, the apparent consensus among the Americans evaporated, and Teller and Wood seemed less inhibited about their advocacy of nuclear testing in space. This was certainly a message that the Russians welcomed, desperate as they were to find a future for their nuclear profession in a world increasingly committed to disarmament and reduction in international tensions.
The Russian and American weapons builders have held two additional defense meetings after the Space Protection of the Earth 94 conference, at Livermore in 1995 and again at Snezhinsk in 1996, but these meetings were anticlimactic. Without funding, there was little technical progress on developing defense systems, only a recycling of the same ideas and proposals. In fact, not one scientist from either the Livermore or Los Alamos National Laboratories bothered to attend Simonenko's Space Protection of the Earth 96 meeting in the Urals. Meanwhile, the major Russian aerospace industries have taken up the NEO defense issue in their efforts to find new customers. The Lovochkin, Khrunichev, and Makeyav companies all made presentations at the 1996 Space Protection meeting favoring immediate deployment of a "Space Shield" defense system, based on the concepts of a short-range defense that had been discredited in the United States ever since the Los Alamos workshop in 1992. A.V. Zaitzev of Lavochkin went so far as to say that he could reveal for the first time to this audience the previously-secret development in Russia of all the elements of such a Space Shield. He said that their proposal was based on the Zenit rocket which can be readied for launch in just 90 minutes, the Mars-96 spacecraft which has the capacity to carry multi-megaton nuclear explosives, and the ASTRON space telescope which can be modified to serve as a NEO surveillance system. All this sounds pretty scary, but even the Russian scientists did not take this Space Shield proposal very seriously.
In the same week in September 1996 that the defense scientists met at Snezhinsk, world leaders at the UN signed the Comprehensive Nuclear Test Ban Treaty, adding to the Russians' desperate concern about their future in what had once been a model city, with its lovely parks and excellent schools. No one had been paid for three months, and a few days following the conference, the Director of the Federal Nuclear Center in Snezhinsk committed suicide. As we drank vodka toasts to international cooperation at the final conference banquet, flurries of snow were already falling through the birch forests, and the winter promised to be a cruel one. Whatever the future might hold for NEO defenses, these celestial threats could not solve the immediate problems of the Russian defense labs and their staffs.
3000 words (3/27/97)
CHAPTER 13: ASTEROID DEFENSES
In the months following the Los Alamos meeting, the astronomers and the weapons builders continued their dialogue. In time, both the United States Air Force and the Russian nuclear establishment became interested, as we describe in this chapter. A continuing influential participant in all these discussions was Edward Teller, who adopted the asteroid defense issue as a major theme for international cooperation in the post-cold-war world. We discuss how US Air Force Col. Pete Worden brought the US and Russian participants together for the first time in 1993 in an isolated monastery in Sicily, and the return invitation from the Russian nuclear scientists to meet in 1994 in the Siberian city of Chelyabinsk-70. At Chelyabinsk-70, a formerly secret city still surrounded by double barbed-wire fences patrolled by soldiers and dogs, little has changed since the Soviet era. There is even a huge bronze statue of Lenin still standing in the central square. This was the occasion of Teller's first visit to the Russian nuclear center, and we provide first-hand descriptions of how he used the opportunity to urge the Russians to join an international program to test nuclear weapons against Earth-approaching asteroids. In telling the history of these events, we also further describe the technology that could be used for such space defense systems.
CHAPTER 14: THE YEAR OF THE COMETS
Needs opening quote on comets
Although the category of Near Earth Objects, or NEOs, includes both comets and asteroids, we have focussed in the previous chapters on discovering, tracking, and defending against asteroids. This makes sense, because most impacting objects are asteroids, probably more than 90%. Nevertheless, comet impacts play an important role as well, and the recent appearance of several widely-publicized comets has called attention to the threat they pose. First came the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994, then in 1996 Comet Hyakutake came closer to Earth than any other comet this century, and finally Comet Hale-Bopp in 1997 was the most widely seen spectacular comet since 1910. Truly, the 1990s have been the years of the comets.
Actually, there are three classes of NEOs that we must consider, not just two (asteroids and comets). The long-period comets are the simplest and least-ambiguous category: loosely compacted icy and rocky objects (dirty snowballs) that enter the inner solar system on highly elongated orbits. By definition, a long-period comet is one that takes more than 20 years to orbit the sun. (Some astronomers insert another category, intermediate-period comets, for those with periods between 20 and 200 years). As a matter of fact, almost all long-period comets have periods of thousands of years or more, and many of them are probably entering the inner solar system for the first time in its 4.5-billion year history.
The second category is the short-period comets, those with orbital periods of less than 20 years. These are all comets that first approached the Sun on long-period orbits, but whose orbits have been made smaller, usually by the gravitational attraction of Jupiter. Most short-period comets are said to be part of the Jupiter family of comets, since their approximately 6-year orbits take them out only as far as the orbit of Jupiter. Finally, there are the near-Earth asteroids themselves, solid objects with orbits that bring them repeatedly close to the Earth. Typically, their orbital periods are less than 3 years. Although mostly rocky and metallic in composition, the near-Earth asteroids probably include a substantial number of dead or dormant comets, which have lost their near-surface ices as a result of repeated heating by the Sun.
Thought of as a cosmic bomb, the composition of a colliding object makes little difference. The energy of an impact, not the physical or chemical make-up of the projectile, primarily determines the damage done. This energy, however, is proportional to the square of the impact speed. Therefore it matters a great deal that long-period comets, swinging into the inner solar system on highly elongated orbits, approach the Earth at higher speeds than either the asteroids or the short-period comets. Typically, an asteroid strikes the Earth at a speed of 20 km/s, while a long-period comet strikes at two times this speed or 4 times the energy (since 2-squared is 4). In the case of Comet Halley, which circles the Sun in a backward or retrograde orbit, the encounter velocity is more than 60 km/s. So Halley would generate 9 times the explosive energy of a typical asteroid of the same mass. Comet impacts are few in number, but they pack a powerful punch.
Another distinction between long-period comets and objects in smaller orbits is that current search strategies are poorly suited to find the former. For example, the Spaceguard search strategy of discovering all potential impactors years before they pose a direct threat applies only to objects whose orbits thread repeatedly through the inner solar system. To find and catalogue these objects, we depend on the fact that they will pass near the Earth thousands of times before they actually hit. If we miss one of them this year, we will pick it up on its next orbit, or the one after that. This is an excellent strategy for near-Earth asteroids and short-period comets, but it fails completely for long-period comets. If one of these interlopers is destined to hit the Earth, it will not give us decades or centuries of warning. Instead, we will have to pick it up on the way in, and even with the best telescopes the warning time is not likely to be more than two years. Protection against long-period comets requires eternal vigilance, and if there is any prospect of defending ourselves, we must act on relatively short notice. We will return to the "nightmare scenario" of a predicted comet impact in the next chapter.
For now, let us agree on the terminology we will use. When we say asteroid, we are including the short period comets, which have similar orbits and impact speeds and can be catalogued in a Spaceguard survey just like the asteroids. When we speak of comets, we are referring to the long-period comets, those with orbital periods of more than 20 years. These are the ones that hit us fast and don't provide much warning. Comet, in this chapter, always means long-period comet. NEO, as before, includes both asteroids and comets with orbits that cross the orbit of the Earth.
While almost all astronomers agree that comets pose less of a hazard than asteroids, they don't agree how much less. One reason we are uncertain is that it is very difficult to measure the size of a comet's nucleus, which we must do in order to estimate its mass and hence the energy it would release if it hit the Earth. Radar and infrared techniques have both been applied to this problem, but to date few comets have revealed their size. The best measurement was made for Comet Halley in 1986, when the European Giotto spacecraft flew within 600 km of Halley's nucleus and photographed it directly. Irregular and elongated, the dark nucleus turned out to be 11 km long and about 7 km wide, roughly the size of the island of Manhattan south of 116th Street. The largest measured nucleus is that of Comet Hale-Bopp, revealed from both infrared measurements and direct photography with the Hubble Space Telescope to have a diameter of between 20 and 40 km. Halley is somewhat smaller than the K-T impactor that blasted out Chicxulub crater, and Hale-Bopp is substantially larger. Several other measured comets (including Hyakutake) have diameters of between 2 and 7 km.
Based or our current rather meager knowledge, it appears that comets make up about 5% of the impactors in any given size range. Since on average each comet hits with about four times the energy of an asteroid of its same size, comets make up about 20% of the hazard. These estimates are most secure for relatively small impacting objects, such as the 2-km objects that constitute the primary threat to Earth. At larger sizes, there is some indication that asteroids become under-represented relative to comets. As one example, note that there is no present Earth-crossing asteroid as large as Comet Hale-Bopp. Gene Shoemaker thinks that at the magnitude of the K-T impact (hundreds of millions of megatons), comets may be at least as important as asteroids. But fortunately, such major mass-extinction events take place only once in 100 million years or more. Thus we estimate that asteroids make up 80% of the contemporary NEO threat, comets 20%.
There is another proposal made by a few astronomers that would elevate cometary impacts to a much more important role. We mentioned in Chapter 7 that a handful of British neo-catastrophists, including Victor Clube and Duncan Steel, believe that most terrestrial impacts result from the breakup of giant comets entering the inner solar system for the first time. When one of these pristine comets, perhaps more than 100 km in diameter, passes close to the Sun, it can spawn thousands of Halley-size or smaller fragments, called a comet shower. The result might be several large terrestrial impacts within a period of only a few thousand years, a hypothesis Clube and Steel call coherent catastrophism. These astronomers believe that such a giant-comet break-up took place in the solar system between 10 and 20 thousand years ago. They suggest that we are currently in a lull, but that in about a thousand years the heavy cometary bombardment will begin again. The ideas of coherent catastrophism are explained in detail in two recent popular-level books by Duncan Steel (Rogue Asteroids and Doomsday Comets) and Geritt Vershuur (Impact: The Threat of Comets and Asteroids), and they have received considerable press coverage, especially in Britain and Australia. We don't agree with these arguments, and we see no evidence of such a comet shower taking place now or in the historical past. But if they are correct, then comets dominate in impact risk, and indeed the risk itself is (or soon will be) hundreds or thousands of times greater than we have portrayed in this book.
Comet orbits present more of a challenge than those of asteroids. Through a telescope, an asteroid is a tiny star-like point that can be measured with precision relative to the background of stars. And asteroids move inexorably according to Kepler's Laws. In contrast, the solid nucleus of a comet is generally invisible within the extended glow of its atmosphere and tail, so position measurements have large errors. In addition, comets have the disconcerting habit of shifting their orbits slightly. The problem is not a breakdown of classical gravitational theory, but rather the rocket effect produced on comets when they are near the Sun. The evaporation of material from the surface of a comet is concentrated in jets or fountains. These jets of gas and dust exert a tiny reaction force on the nucleus, gradually changing its orbit just as if a rocket engine had been fastened to it. The problem is that the location and force of these jets change from time to time, so this rocket effect is virtually unpredictable. Thus many comets diverge gradually from their predicted paths, and while these effects are small, they can be important if we are trying to figure out whether a comet might hit the Earth. As we saw in discussing ways to deflect an asteroid or comet headed for the Earth, very small changes in speed can make large positional differences after several years. The inherent unpredictability of comet orbits means that effective defense strategies would require rapid response. It could be difficult, even with very precise astronomical observations, to predict whether or not a comet will hit the Earth. It might not be until the last couple of months before impact that we could be sure of our fate.
Yet another problem with comet orbits is their vulnerability to perturbations from Jupiter. None of the near-Earth asteroids has a Jupiter-crossing orbit, so they generally follow stable paths. But both the long-period comets and the Jupiter-family comets can experience dramatic orbit changes when they approach the giant planet. We have seen two recent examples of such perturbations. Prior to its 1997 appearance, Comet Hale-Bopp had an orbital period of 4400 years, but now the comet has been pulled down into a 2200 year orbit. The fate of Shoemaker-Levy 9 was even more dramatic. This comet was probably first captured in 1929 into a chaotic orbit around Jupiter, then disrupted in a close Jupiter encounter in 1992, and finally consumed by the giant planet in 1994.
Given the difficulty of calculating comet orbits, it is no surprise that the first false-alarm in predicting an impact on Earth dealt with a comet. On October xx, 1992, astronomer Brian Marsden issued what appeared to be an official warning from the Minor Planet Center of the International Astronomical Union that Comet Swift-Tuttle might crash into the Earth on its next return, on August 14, 2126. Although we doubt any astronomers who received the warning expected to live so long, 2126 is practically tomorrow by astronomical standards. Indeed your grandchildren's grandchildren might still be alive, so the warning had a certain news value -- indeed it garnered immediate prominent attention by The New York Times.
Marsden is an expert in the ancient but esoteric field of dynamical astronomy, the precise computation of orbits. As director of the International Astronomical Union Minor Planet Center, Marsden is the "cop" of the astronomical community, charged with establishing orbits for newly discovered objects (such as comets, asteroids, and supernovae) and assigning them appropriate names and numbers. Amateur astronomers know him as the person who resolves issues of priority in comet discoveries, and thus decides what name each new comet will bear. Marsden had participated in the Spaceguard Working Group and had attended the Los Alamos meeting, where he played a generally conservative role as the advocate for careful observations and strong international cooperation. He was not likely to shoot from the hip, so a warning from him of a possible future impact was certain to be taken seriously.
While the name of Comet Swift-Tuttle was not itself familiar, many people knew indirectly of the comet's existence. Several times each year, as the Earth circles the Sun, its orbit intersects the orbit of one or another comet. The comet, of course, is usually nowhere near, being elsewhere on its orbital path. But since comets shed dust as they evaporate, their orbits tend to be dirty places, filled with tiny specks of cometary debris. As the Earth crosses these dusty comet trails (not to be confused with the more visible but ephemeral comet tails), we see the bits of dust burn up in our atmosphere as meteors or shooting stars. If the comet trail is very dirty, we see a meteor shower. The most well-known and dependable of the annual meteor showers is called the Perseids, peaking between August 11 and 14 of each year. The Perseid meteors, which are seen by both amateur astronomers and casual sky-watchers each August, are debris from Comet Swift-Tuttle.
Swift-Tuttle is a long-period comet, with a period of 133 years. It was discovered by American observers Swift and Tuttle during the Civil War, and it did not make its second pass by the Sun until 1992. Until it was first seen by a Japanese amateur astronomer in late September, 1992, astronomers were not sure of the orbit or period of the comet, but with new observations flowing in, Marsden and his colleagues were able for the first time to compute an orbit and predict the circumstances of its next return, in 2126.
The fact that the Earth annually encounters the dust trail of Swift-Tuttle indicates that the comet's orbit intersects that of the Earth. In 1992, however, the closest the comet came was 150 million km. A collision is possible only if the comet and the Earth occupy the same volume of space at the same time. The speed of the comet relative to the Earth is about 25 km/s, so it travels the width of the Earth in about 600 seconds, or 10 minutes. Thus to predict an impact, Marsden would have to calculate the position of the comet in its orbit with a precision of 10 minutes at its next close approach, in 2126.
What Marsden actually told the press in October 1992 was that he thought the uncertainty in the Swift-Tuttle orbital prediction for 2126 was at least two weeks. For the best calculated orbit, no collision would take place, but Marsden felt that the uncertainties permitted the possibility of a collision. Speaking offhand to reporters, he guessed that the chances of a collision might be about 1 in a thousand, which is a substantial exaggeration. Even in a state of ignorance, the likelihood that the comet would arrive nearly two weeks late, and do so in the exact 10-minute interval that would lead to a collision, were far less than 1 in a thousand. In any case, many press stories reported this as an actual prediction of a collision in 2126. Since we suspect that the nucleus of Swift-Tuttle is at least as large as that of Halley, such a prediction implied a catastrophe of unprecedented magnitude.
As it happened, there was a way to resolve the issue by calculating a better orbit. Marsden had used only observations of Swift-Tuttle made in 1862 and 1992, and some of these were ambiguous. Don Yeomans, an expert on orbits from JPL, knew that more observations extending over a longer time base would lead to a much refined orbit. Projecting backward in time, Yeomans concluded that the comet must also have come past in 1737, and a search of the historical records indicated observations were made of a comet that year with an orbit consistent with Swift-Tuttle. Combining these observations with the other data, Yeomans calculated that the 2126 return of the comet would bring it to the Earth's orbit on August 5, while the Earth itself was still 20 million km away. Thus with improved orbital calculations, the impact threat disappeared.
Marsden was at first reluctant to abandon his doomsday prediction. His primary motive in releasing the prediction had been to goad astronomers into making more accurate observations of Swift-Tuttle as it sped away from the Sun and Earth in the waning months of 1992, so that its orbit could be determined with higher precision. But he was also enjoying the publicity his comments had brought. He confided to friends that he thought the Swift-Tuttle warning had served as a timely wake-up call for the impact threat -- distant enough in time to avoid panic, but real enough to command serious attention from the media and the public. Not until December 10 did he issue an official "all clear", in which he told the press that "there is very little possibility of the comet's collision with the Earth during the next millennium."
If Marsden's motive was to stimulate a public discussion of the impact hazard, he certainly succeeded. One result was a cover story in the November 23 issue of Newsweek, entitled "Doomsday Science: New theories about comets, asteroids, and how the world might end." The story faithfully summarized the conclusions of the Spaceguard Working Group and downplayed the more divisive political issues. The story concluded: "Killer asteroids and comets are out there. And someday, one will be on a collision course with Earth. Of all the species that ever crawled, walked, flew or swam on Earth, an estimated two thirds became extinct because of an impact from space. Mankind may yet meet that fate too. But we're the only species that can even contemplate it and, just maybe, do something to prevent it."
Two years later, the actual collision of Comet Shoemaker-Levy 9 with Jupiter attracted wide attention and did much to legitimize the concept of collisional hazards, as we noted in previous chapters. Because of its large size, Jupiter is a much more frequent target for impacts than is the Earth. Indeed, George Wetherill has calculated that the presence of Jupiter protects the Earth from most comet impacts. In effect, it draws the fire away from our own planet in the cosmic shooting gallery. But the fact that Jupiter was hit during our lifetimes called public attention to the potential vulnerability of the Earth to similar catastrophes.
One of the groups that was impressed by the lesson of Shoemaker-Levy 9 was the Science and Technology Committee of the U.S. House of Representatives. Committee Chairman George Brown considered this an ideal time to renew his earlier call for a Spaceguard asteroid survey, an idea that had lain largely dormant since the completion in 1992 of the two reports called for in the 1990 NASA authorization bill. With his encouragement, staffers Terry Dawson and Bill Smith drafted a new request for inclusion in the NASA bill for Fiscal Year 1995. On July xx, in the very week of the great comet crash, Brown's committee adopted the following language:
This time NASA responded quickly to the Congressional initiative, and within two weeks they appointed Gene Shoemaker to chair a new NEO detection study. We were both among the alumnae of the Spaceguard Working Group who provided continuity with previous work. The weapons labs were represented by Teller protege Greg Canavan, a senior physicist at Los Alamos National Laboratory and a former Air Force officer. The Air Force also responded positively, and they were represented on the Shoemaker committee by both Col. Pete Worden, recently appointed Commander of Falcon Air Force Base, and civilian John Darrah, Chief Scientist of the USAF Space Command.
The Congressional charge to Shoemaker's team was technically challenging but straightforward in principle. The House Committee had accepted all the essentials of the Spaceguard Survey Report, including its focus on asteroids 1 km or larger in diameter as constituting the most serious hazard. Now they wanted NASA to prepare a specific plan to carry out the Spaceguard Survey, and to complete it in just 10 years, rather than 25 years as proposed in the 1992 report. Shoemaker's team had to try to find a way to achieve results quickly. In addition, NASA wanted the price of the survey cut to a fraction of the $250 million estimated in the original report. In other words, Spaceguard should be implemented according to the "better, faster, cheaper" philosophy of NASA Administrator Dan Goldin.
The Shoemaker Committee was remarkably successful in meeting its objectives. We determined that improvements in CCD detectors and computer systems had substantially simplified the task, and that the survey could be carried out in one decade with half of the number of telescopes originally proposed. When the cost of the new Spaceguard program was added up, it came to just $5 million per year for a decade total of $50 million, only one fifth the original pricetag. However, work on the report went slowly as Gene Shoemaker, now a global celebrity, found himself traveling far and wide receiving awards and giving lectures about his comet. Deadline after deadline passed, and the report bearing the good news about a faster, cheaper Spaceguard was not submitted to NASA until May 1995. When the space agency forwarded the Shoemaker report to Congress, it stated in a cover letter that NASA did not intend to implement to proposed survey, and that no funds would be requested from Congress for this purpose.
In the year since all eyes had been turned on Shoemaker-Levy 9, the American political landscape had changed dramatically. With Republicans in control of the 104th Congress, George Brown lost his chairmanship of the House Science Committee, and Terry Dawson, the primary Spaceguard supporter on the committee staff, lost his job. As part of a bipartisan effort to downsize government and reduce the national budget deficit, NASA faced a declining budget that threatened the survival of the space program. Had the science community anticipated the call from Congress and been ready with a Spaceguard implementation plan in the fall of 1994, we might have succeeded. But by the summer of 1995, no one was listening.
How quickly the situation had deteriorated. In 1995 Gene and Carolyn Shoemaker terminated their long-standing sky survey at Palomar, a project that had consumed one quarter of their time for more than a decade. Gene retired from his position at the US Geological Survey and moved to Lowell Observatory, where he began a collaboration with Ted Bowell to build a new survey telescope closer to home. A few months later the Australian government cut off funding for the photographic asteroid survey at the Anglo-Australian Observatory, and by the end of 1996 Duncan Steel was out of a job. Where four teams had been searching for asteroids at the time Shoemaker-Levy 9 was discovered, the worldwide effort was now down to just two underfunded groups, led by Tom Gehrels in Arizona and Eleanor Helin at Palomar. After rising steadily for the past two decades, the discovery rate of NEOs actually declined in 1995.
Yet there was good news too. The Shoemaker Committee really had figured out ways to simplify and accelerate the Spaceguard Survey, and NASA had increased its annual support to $1 million per year, primarily to encourage development of new instrumentation. Equally important, the US Air Force was now seriously considering NEO searches. Pete Worden was interested in all aspects of NEO studies, from searches to space missions to the development of defense systems, and he gradually converted some of his colleagues -- primarily the technically-minded Majors and Colonels of the Air Force Space Command -- to the asteroid issue.
The USAF Space Command has a substantial capability in celestial tracking and observation, dating back to satellite tracking stations built at the time of Sputnik. More recently, nearly 20 one-meter-aperture telescopes have been constructed to track Air Force satellites, particularly those secret or "black" satellites that maintain radio silence as they wait in high orbits for possible activation in time of emergency. This GEODSS (Global Electro-Optical Defense Space Surveillance) system has operated observatories in New Mexico, Hawaii, Korea, Spain, and Diego Garcia Island in the Indian Ocean. In principle, these telescopes could be instrumented to search for asteroids as well. Indeed, some Space Command staff thought an asteroid survey might provide the rationale to upgrade the GEODSS detector systems. Plans were made to carry out the upgrade of one of the New Mexico telescopes in 1995, leading to an experimental asteroid search program in 1996. But an independent NASA-USAF team beat them to the punch.
Without consulting the USAF high brass, Eleanor Helin and her JPL search group established a collaboration with Air Force scientists to carry out asteroid observations with the GEODSS telescope in Hawaii. They called themselves the Near Earth Asteroid Team (NEAT). With NASA instrument development funds and assistance from Livermore National Lab, NEAT built a new CCD-based camera and supporting software, which began sky scans early in 1996.
[***more here on NEAT accomplishments]
With two modern electronic search systems (Spacewatch and NEAT) on-line, the discovery rate of NEOs began to climb again. Impressed with the NEAT results, NASA began negotiations in 1997 with the National Science Foundation to install a second NEAT camera on an old 2-meter telescope at Cerro Tololo Interamerican Observatory in Chile, thus extending the searches to southern skies. Congress again expressed its interest, with the House Science Committee voting in April 1997 to encourage NASA to increase its spending on NEO searches from $1 million to $3 million per year.
Tom Gehrels was not idle either, and by 1997 he was well on his way to expanding the Spacewatch search to a larger 2-meter telescope. This had been his plan from the time he first proposed Spacewatch to the 1981 NASA Snowmass meeting, and 17 years later his dream was about to become a reality. Yet another modern search camera was under construction at Lowell Observatory in northern Arizona, in a NASA-funded collaboration between Gene Shoemaker and Ted Bowell. Called the Lowell Observatory Near Earth Object Search (LONEOS), this telescope differed from the Spacewatch and NEAT systems in using a wide-angle telescope, optimized for covering large sky areas and therefore for discovering the relatively large (1-5 km) asteroids. Meanwhile, the old 0.4-meter photographic telescope at Palomar, which had discovered most of the known NEOs over the previous two decades, was finally closed down for good.
NASA and the USAF also discussed expanding the NEAT survey to other GEODSS telescopes beyond the one Helin was using in Hawaii. Such a collaboration could provide the fastest way to accelerate the search, as proposed by the Shoemaker Committee in 1995. However, the Air Force was still not convinced that NEO surveys were consistent with the primary GEODSS role of tracking military satellites. Others, for their part, doubted whether if was wise for discovery of asteroids to pass from civilian to military hands. They worried about security issues that excluded most astronomers from the GEODSS sites and might encourage the classification of survey results. NASA has taken the position that if it supplies the NEAT cameras, the data from Air Force surveys must remain in civilian hands.
As a result of all these new systems, the core of a Spaceguard Survey was emerging in 1997, based on two Spacewatch telescopes (of 1 and 2 meter aperture), the 1-meter NEAT/GEODSS telescope in Hawaii, a proposed 2-meter NEAT system in Chile, and the LONEOS wide-angle camera. This extraordinary development was primarily the product of entrepreneurial efforts by individual astronomers rather than a coherent plan executed from above. Such a system should be able to catalogue all the 1-km NEOs within two decades, if not one. All that remains is to ensure cooperation among these individual searches. If each telescope scans the same part of the sky instead of working in a complementary mode, the full capability of the system will not be realized. But if the teams work together instead of competing to see who can discover the most NEOs, Spaceguard can be a reality before the end of this millennium.
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