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Book Review by Clark R. Chapman.
The Earth orbits the Sun amid errant asteroids and comets. Despite the general emptiness of interplanetary space, occasional collisions occur. The cratered lunar maria bear witness to impacts accumulated over the last 3.5 b.y. Due to the power-law-like size distribution of the smaller fragments of comets and asteroids, ranging down to dust, we can see meteors on any clear night we choose. We may be lucky enough to witness a meteorite fall. In a diligent well-planned search, we may even find a meteorite.
However, as the word "Doomsday" in this book's title implies, its subject is not quirky events in the nighttime sky, nor even the scientific study of asteroids, comets, and meteors. Rather, Steel writes about past cataclysms and the possibility that future disasters may strike us from the heavens. As a co-author (with David Morrison) of the first review of this topic in the refereed literature (Nature 367, 33-40, 1994), I admit that I am personally fascinated by the subject.
Scientists and commentators legitimately debate whether the impact hazard is one that individuals, or society as a whole, should take seriously. The hazard is real. Yet, because of the infrequence of major impacts, it is modest (e.g. in terms of annualized fatality rate) compared with such omnipresent dangers as war, disease, automobile accidents, homicide, or even typhoons and earthquakes.
Less debatable, and gaining acceptance by Earth scientists generally, is the significant role of cratering in shaping the face of our planet. It may be no accidental coincidence that Chicxulub, one of the three largest terrestrial impact craters yet discovered -- found by geologists exploring for oil decades before others demonstrated its impact origins -- is associated with one of the major sources of oil in a major petroleum-producing country. Indeed, impact cratering may have been the dominant geological process on Earth during the Late Heavy Bombardment, an epoch 4 b.y. ago recorded by the saturated craters of the lunar highlands. Such cratering may have established the boundary conditions from which the modern tectonic style of our planet developed.
It should be even less debatable that the continuing rain of projectiles, with effects mediated by Earth's atmosphere, must occasionally cause havoc to our planet's fragile ecosphere. Given the possibility, on a time scale of aeons, that a giant comet might even sterilize our planet, it is hardly inevitable that life has survived for as long as it has. However, many paleontologists and evolutionary biologists still steadfastly resist even the possibility that the sudden, dramatic changes in the diversity of species in the geological record could be due to impact-generated upsets to ecological stability.
So I strongly support one major purpose of Steel's book, which is to bring together the diverse evidence for the significance of comets and asteroids to our planet's history, our own history, and -- perhaps -- our future. Steel is well qualified to write such a book. He is an expert on meteor streams and, for several years, he has been involved in the astronomical search for asteroids and comets, using the Anglo-Australian Observatory. He is one of the most visible and enthusiastic explicators of science on popular television Down Under and, as this book exemplifies, he is a gifted writer.
"Rogue Asteroids..." is not a technical monograph, yet it is a book that professional meteoriticists should be aware of because it promulgates a disturbing thesis of "coherent catastrophism" which includes unusual views about the nature of comets and meteorites. I will return to these technical issues later. First, however, since the volume is aimed at a lay audience, I will address whether this book is, in fact, a good popularization of science. Reading the first 60 pages, or so, I thought that it might be. Steel accurately describes asteroids and comets, discusses what happens when they strike, and puts the hazard into context. He even broaches one of the most divisive controversies within the tiny field of impact hazard -- the one highlighted on the cover of Newsweek a few years ago concerning Brian Marsden's (false) announcement that Comet Swift-Tuttle has a chance of striking the Earth in the 22nd century -- and presents a dispassionate account that should offend neither side.
But then the book goes down hill. One of the most important things that a popularizer of science can do, especially in a cultural era of public fascination with New Age mysticism, pseudo-science, and the occult, is to portray science authoritative- ly and objectively. Especially for a book about a topic that is still overcoming the "Chicken Little giggle factor" and is struggling for respect among doubting paleontolo- gists, it is doubly important that it be credible and rigorous. Indeed, that is just what Arthur C. Clarke, the eminent science fiction author who has treated impact catastrophes in several novels, promises in his Foreword to this book:
"If there is interest in a fictional account of humankind being saved from a calamitous impact, then surely there must also be interest in the facts upon which my writings are based. In this volume, Steel sets out those facts as we understand them today."
Indeed, Steel's account is factual in chapters 1 - 3, the first part of chapter 4, chapter 5, the first part of chapter 7, and most of chapters 9 - 13. But he could not resist letting his speculations get out of control. Some are innocuous, like his parochial view about why the atmosphere of Venus differs so much from Earth's (he mentions reasons, dealing with comets, that are pertinent as far as he goes, but he ignores the broader evidence and more generally discussed theories).
Occasional assertions in this book are odd, or downright wrong, and would mislead the naive, lay reader about planetary science. These include Steel's view (repeated twice) that Saturn's rings were formed by a tidally disrupted comet; his statement that there is a single Canyon Diablo meteorite, found several kilometers away from Meteor Crater, distinct from meteorites found around the crater; his stance that terrestrial crater ages "certainly" demonstrate a periodicity; and the assertion that Mercury has craters like the Moon because both bodies lack an atmosphere (rather than because both bodies have been geologically inactive).
More disappointingly, in Chapter 8, Steel strays far from his expertise and writes what can only be called nonsense. He argues that Stonehenge and the Egyptian pyramids were constructed in response to a devastating pummeling of our planet by a fusillade of comets beginning 5,000 years ago. I am no archaeologist, but in the 1960's I did work within the Harvard Radio Meteor Project for Gerald Hawkins, who was then researching the statistics of the alignments of Stonehenge (published in Hawkins' 1965 book, "Stonehenge Decoded"). Eschewing the objective rigor of archaeoastronomy, Steel pictures these ancient monuments as we might perceive elephants and other fantastic shapes among the clouds in the sky. He relies on nothing more substantial than visual metaphor and timing "coincidences" of plus- or-minus 500 years. Stonehenge seems to be Steel's personal Rorschach test, in which he sees his personal passion -- meteor storms -- writ large. His speculations might be amusing at a cocktail party, but they have no place in a popular book about science.
Although Steel disarmingly introduces Chapter 8 as devil's advocacy that "many will find bizarre," he has a more serious purpose: it is to gain some acceptance (in this case concerning the distant past, which will always be shrouded in uncertainty) of a theory, promoted by Steel's British colleague Victor Clube, for which the word "bizarre" would be an understatement. Clube believes that the modern history of Western civilization (one example is the Protestant reformation) has been shaped chiefly by comet storms which, furthermore, he believes are increasing at the present time. Indeed, Steel himself approvingly quotes Clube's theory that the Dark Ages commenced as a result of "conflagrations" due to the so-called Taurid complex of meteors and comet fragments.
Here is where professional meteoriticists and planetary scientists should take note. Steel attributes many impact occurrences (including the Farmington meteorite and the Tunguska blast) to the Taurid complex, an ill-defined and inherently difficult- to-observe heterogeneous belt of debris, orbiting in the inner solar system, derived from a giant comet that he believes was disrupted about 5,000 years ago. Encke's comet and several meteor showers are the primary observables. A theme that runs through this book, and pops up in subdued form even in some of the chapters I label as being authoritative, is that coherent clusters or streams of cometary debris -- the Taurid complex is a particular, current example -- somehow dominate the terrestrial impact rate, and invalidate the average rates (and hence hazard of future impacts) deduced by Gene Shoemaker and others.
There is more than a nugget of truth to the Taurid story. Undoubtedly the complex exists, in some form. Indeed, it may well contain unknown numbers of objects of sizes (ranging from meters to hundreds of meters) that are inherently difficult for astronomers to detect. Among the Earth-approaching objects generally, several dozen have been discovered within this size range, chiefly by the Spacewatch telescope in Arizona; but they represent the barest tip of the iceberg of the whole population, so we really don't have observational data concerning what fraction may be within as-yet-undispersed comet trails and meteor streams (like the Taurids), as distinct from randomly orbiting in the inner solar system.
Furthermore, I agree with Steel that many astronomers may be prejudiced against the existence of such objects, simply because they haven't been seen -- despite the fact that they hardly could have been seen with current observational techniques. The history of scientific study of the Saturnian system illustrates the bias. Astronomers first thought that the Saturn system had (a) dust particles, of a characteristic size, and (b) moons several hundred kilometers across, and larger -- but nothing of intermediate sizes. It took Voyager's camera to show that smaller moons are embedded in the rings and it took Voyager's radio occultation experiment to reveal that ring particles ranged up to at least house-sized.
Before Voyager, longer wavelength infrared and radar observations of the rings had begun to show that the rings include objects larger than dust grains. The same techniques, applied to comets, are beginning to challenge traditional assumptions in cometary science. For decades, there was a paradigm that comet nuclei are composed of a mixture of dust and ices, for astronomers could confidently estimate "gas-to-dust" ratios in cometary comae. More recently, IRAS studies (mentioned by Steel) and radar observations of comets (strangely omitted) have shown that comets shed objects much larger than mere dust. It remains unknown what fraction of cometary debris is in chunks meters to hundreds of meters in size, and whether such chunks would simply be pieces of ice/dust conglomerate or, instead, are the more coherent building blocks from which comets accreted, according to some theories. To this extent, Steel has a point.
However, readers of Meteoritics may be interested to know that Steel proposes an extreme hypothesis, which is intimately connected with other parts of his Taurid complex story: meteorites were formed within differentiated giant comets in the outer solar system! Furthermore, to the degree any meteoritic material is in the asteroid belt, it got there because giant-comet debris in the inner solar system is being injected into the asteroid belt, which is growing -- in Steel's view -- rather than being depleted! I wonder how Steel can possibly explain asteroid families and how he thinks asteroids can avoid colliding with each other, fragmenting, and having fragments leak away from the belt through the chaotic dynamical processes that have been so-well demonstrated. "Reversing time's arrow" (in Steel's glib phrase) will no more augment the belt and reassemble asteroids than it will cause the pieces of a broken plate to fly up from the floor and reassemble themselves into a whole plate in your hand.
The central problem of Steel's thesis, however, and the one that feeds Clube's obsession with the comet-dominated theory of human history, relates to whether broken comets (like Shoemaker-Levy 9 or the Taurid complex) should change our view of impact rates. The fact is that there is excellent agreement, within a factor of two (certainly no more than a few), between (a) the currently observed population of near-Earth asteroids, comets, and meteoroids (corrected for observational incompleteness); (b) crater ages on the Earth; and (c) the cratering record on the Moon (which extends both to smaller sizes and to older ages). Whether the integrated cratering records on the Earth and the Moon were made by bursts of impacts or randomly in time, the average rate is what we see. And it is the very same one that we expect (from well-known crater scaling relationships) would be caused by the population of projectiles that we actually observe, during this decade, in near-Earth space!
One could argue that it is a coincidence that the current population equals the long-term average. Perhaps the usual population is nearly zero but 2% of the time there are showers at 50 times the average rate, and we just happen to be on a shoulder of a peak, far above the background level but coincidentally just at the average rate. Steel, Clube, et al. apparently believe that the average impact rate over the last 5,000 years has been much greater than it appears to be this century, when comet impacts and meteorite storms have manifestly been unimportant in human affairs. So are we in a rare lull, within one of the rare impact showers? Apparently, that is not Steel's or Clube's view since they contend that we face a higher danger than the long-term average would suggest. Steel seems to feel, against all reason, that the non-random clustering of projectiles should, somehow, augment the probabilistic hazard from impacts.
I certainly agree that there must be, at some level, episodes in impact rates -- whether due to stochastic events (like large asteroid or comet disruptions) or periodic forces (for which evidence is currently debatable, despite the certainties Steel espouses in Chapter 6). And there could be a difficult-to-observe stream of Tunguska-sized impactors poised to hit us, just as there could be an as-yet- undiscovered (for want of a Spaceguard network of search telescopes) 2 km asteroid about to strike in the next century, and end civilization as we know it. However, well understood, inevitable processes involving the physical and orbital evolution of asteroids and comets readily explain the current and long-term average impact rates, and it is very unlikely (in the probabilistic sense) that we could currently be in an epoch during which the impact rate is anomalous by more than factors of a few. (Error bars in the impact hazard analysis I published with David Morrison are dominated by the even larger uncertainties of what amount of global ecological damage is done by an impactor of a given size.)
That is not to say that a Taurid disaster is impossible: as Steel notes, Jupiter was just hit by a once-in-a-millennium barrage, and there could be an unusually dense stream of objects about to ruin the next century. But before the scientific community will accept that the Taurid complex greatly augments our generalized estimates of the danger, there will have to be much more astronomy (observations characterizing the spatial distribution and size distribution of the components of the complex) and much less reliance on ancient myths and legends about alleged cataclysms. It is sensational and dishonest to promulgate a Velikovsky-like reading of obscure ancient history as factual planetary science. Furthermore, it is sloppy for a good meteor astronomer to pay such little heed to other branches of planetary science, like meteoritics, as he offers these outlandish speculations in what Arthur Clarke promises, instead, to be a book of scientific fact.
In conclusion, I recommend buying "Rogue Asteroids and Doomsday Comets" along with a pair of scissors. Cut out and throw away pp. 60-64 and 70-73 from Chapter 4, pp. 114-116 and 125-136 from Chapter 7, and all of Chapters 6 and 8. Then have an enjoyable read, being just a little wary, however, whenever the Taurid complex is mentioned.
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