ROGUE ASTEROIDS AND DOOMSDAY COMETS

Duncan Steel is a research astronomer at the Anglo-Australian Observatory and a research fellow at the University of Adelaide. He has served on both the Detection Committee and the Intercept Committee created by NASA. Thus, he is well qualified to write a book on the subject of hazards to Earth from rocks falling out of the sky.

In this book, he presents the case for such impacts in a logical, step-by-step manner. Chapter 2 discusses the telescopic detection of asteroids and comets, and estimates the likelihood of catastrophic events for various classes of objects as impacts per year. Chapter 3 describes the probable nature of the catastrophes in rather broad terms, with reference to the sizes and compositions of the objects and the amounts of energy involved. Simple equations are used. These events fall into two groups: local and global catastrophes. An example of the former is the Tunguska impact, which released energy of about 20 megatons and devastated 500(?) square miles of Siberian wasteland in 1908. It is not hard to imagine the effect if a similar body fell on a large city today. It is also true that a "local" event could devastate a wide area of coastline through tsunamis, if the body fell in the ocean.

Global catastrophes require a body of over 1 km in diameter. The possible effects are much longer-lasting. They include: worldwide firestorms caused by infalling secondary debris; loss of a growing season caused by dust in the atmosphere (much like the "nuclear winter" described in the TTAPS Study); and acid rain due to large quantities of nitrogen and sulfur oxides.

Chapter 4 discusses the effects of a large strike in more detail, and introduces a new hazard that I have not seen mentioned in the scientific literature before — the collection in Earth's atmosphere of large quantities of dust from a comet through whose orbit it passes frequently. This, Steel argues, could induce a "winter" scenario without any catastrophic impact.

Chapter 5 examines the geological evidence for large impacts in the past — craters. It seems from this record that the view that major impacts belong only to the distant past is possible only because no one looked very hard for craters until recently. Indeed, up until the 1930s, it was thought by many scientists that the Barringer Crater in Arizona — perhaps the "cleanest" crater on the planet — was due to volcanic eruption.1

Chapter 6 introduces the concept of periodicity of major impacts, for which the well-known "Nemesis" theory was one explanation. Steel reviews several theories, presented in a large number of scientific papers. And here I encountered the first error in the book (other than mere typos). Steel claims that, in its 250 million year passage around the center of the Milky Way, our solar system passes through the plane of the Galaxy four times, or once every 60 million years. I find this hard to credit. (Twice, I could accept.) He also says that diffuse clouds of dust could disrupt the orbits of comets in the Oort Cloud at such times. That they have enough mass to affect comet orbits, I grant; that they only exert their attraction at the times of plane-crossing seems impossible. Such clouds are very large; Steel himself says that they are light-years in extent. Thus they must have substantial extent in the vertical dimension as well. Could this be an example of two-dimensional thinking?

Chapters 7 and 8 present the case for what Steel calls "coherent catastrophism" — that is, destructive showers of large meteors which occur at regular intervals. He presents evidence from the historical record (for example, a "fire on the Moon" witnessed by the monk Gervase in Canterbury, England, 1178 A.D.) and from pre-history. This latter evidence consists of legends from various cultures, such as the oral tradition of Australian aborigines that large rocks fell from the sky and caused horrible fires, and also the construction of artifacts such as Stonehenge.

This section of the book is a departure from the solid scientific reasoning about orbital perturbations and telescopic searches. Indeed, Steel's use of myths and legends recalls the writings of Immanuel Velikovsky, and in Chapter 8 he takes pains to distance himself from that sort of pseudoscience. The core of his thesis is that, about 5,000 years ago, the Earth's orbit crossed that of Comet Encke, the source of the Taurid meteor showers. Playing devil's advocate, he asserts that Stonehenge and the pyramids of Egypt and Central America were built to propitiate these "angry gods". The shape of the pyramids, he conjectures, mimics the zodiacal glow, which would have been much brighter then because of the dust traveling with the comet. The dating of these constructions fits reasonably well with computational backtracking of Encke's orbit. Steel's argument is not conclusive — but it is persuasive. If he is right, we can expect the next bombardment somewhere around the year 3000. If we're not ready by then, IMNSHO, we deserve whatever we get.

Chapter 9 examines the behavior of meteorites in Earth's atmosphere. This study depends mostly on computer modeling (although historical data from the 1908 Tunguska event form a useful benchmark.) The team of Christopher Chyba (NASA-GSFC), Kevin Zahnle (NASA-Ames) and Paul Thomas (U-WI/Eau Claire) has produced a simple model that probably explains why the Tunguska body never reached the ground. (Not incidentally, it also abolished such fanciful explanations of Tunguska as antimatter asteroids and flying saucers.) However, a more sophisticated model due to Jack Hills and Patrick Goda handles a much wider range of impact angles and speeds, and produces results in good accord with the actual measurements of Tunguska (sketchy as those unfortunately are.) The upshot of this chapter is that we should not assume our atmosphere will stop any body likely to hit us in the near future.

Steel turns in Chapter 10 to the search for near-Earth asteroids. The second paragraph is telling: "From the amount of media coverage that the asteroid/comet impact hazard receives," he says, "one might believe that there are whole armies of astronomers scanning the skies, hunting these things down. That might enable you to sleep better at night, thinking that while you snooze, your welfare is being looked after, but in reality nothing could be further from the truth. As David Morrison first pointed out, the total number of people engaged in this sort of work is less than the staff of an average McDonald's restaurant." He describes the Schmidt telescope and its use with photographic plates (and now CCDs) to find the traces of these objects — an arduous task, even with computerized equipment. Still, despite the difficulty and the small number of resources devoted to the problem, the detection rate has increased in recent years. But, "few scientists involved in this area believe that we have to date discovered much more than 5% of [asteroids that pass near the Earth]... Thus, if one has 'our number' on it for the year 2025, we would most likely not find it ahead of time." Sobering, that.

A hopeful note is sounded at the beginning of Chapter 11. Alerted by astronomers' increasing knowledge of comets and asteroids, the U.S. Congress in 1990 directed NASA to study ways of detecting and diverting bodies on collision courses. Two committees were set up: the Detection Committee and the Intercept Committee. Steel served on the former and interacted with the latter. He reports their efforts in this chapter and the next. The proposed detection effort — named Spaceguard after Arthur C. Clarke's fictional organization in Rendezvous with Rama (1973) — consisted of a set of six Schmidt telescopes dispersed over the planet, the whole package estimated to cost $300 million. Its mission was projected to be complete after 25 years of operation. In 1994, after the impact of Comet Shoemaker-Levy 9 with Jupiter, the Congress mandated that NASA complete the detection phase in 10 years. Regrettably, to date nothing like that level of additional funding has been made available to begin this effort. (There is, however, a program using an existing single telescope.) The possibilities for intercepting and diverting a colliding body are discussed in Chapter 12. Steel notes that, in contrast to the civilian scientists accustomed to working with minuscule budgets, the defense-contractor types involved with this aspect of the problem tended to advocate solutions costing multiple billions of dollars, even when less expensive measures would have sufficed.

Chapter 13 summarizes the situation with a prescient quotation from 1953's Target Earth by Allan O. Kelly and Frank Dachille. An Epilogue describes in detail the discovery of the fragmented comet now known as Shoemaker-Levy 9.

I found this book hard going at first. In addition to presenting a lot of complex scientific information (which I grant is necessary for understanding), Steel has a somewhat wordy and pedantic style. However, his grammar is excellent and there are very few typos. The chapter on Stonehenge seemed extraneous — though, interestingly, it was at this point that I began to feel like C. S. Lewis' metaphorical acrobat — transported, if not from Jupiter to Mercury, at least to Neptune. In other words, the rest of the book reads much faster. The information is vital, and it is quite evident that Steel knows the subject better even than many astronomers. The book is well-indexed; it also provides extensive chapter notes, a glossary and a bibliography. My only regret is that the bibliography omits Chapter 13; hence I still lack the full citation for Target Earth.2

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