Review by Duncan Lunan
Incoming! Or Why We Should Stop Worrying and Learn to Love the Meteorite (2011) by Ted Nield, Granta, hardback, £20.00.
First published online in different form, Concatenation, Winter 2011-2012.
Ted Nield is a doctor of geology and editor of the journal Geoscience. Written from that perspective, his book is an excellent introduction to meteoritics (the science of meteorites and impacts) as well as to the ongoing dynamic processes which shape the Earth. In his account of how humanity has reacted to impact events during recorded history, other reviewers have perhaps made too much of the incident where a meteorite briefly became the chief god of ancient Rome, but it is a high spot in a generally good read. One recurring point is the sheer unwillingness of the emerging natural sciences community to recognise that such events occurred, regardless of the evidence, until they were witnessed by scientists in person and could no longer be ignored.
Coming to modern times, Nield gives us the evolution of the idea that asteroids and comets could have caused mass extinctions in the past, and still pose a global threat today. One point he doesn’t make, but freely acknowledged on his 2011 visit to Glasgow, is that in the early 1960s the idea that impacts could cause such harm was so controversial that some of the earliest papers were published by science fiction magazines. In a painting for Willy Ley’s The Conquest of Space (1949), the artist Chesley Bonestell imagined an event on the scale of the Barringer Meteorite crater occurring on Manhattan Island. Bad as it looked, streets were still recognisable. Bridges were still standing, though some were broken, and ships could be seen at some of the piers. In some versions of the painting, the Ferris Wheel on Coney Island is still standing. When Isaac Asimov realised that destruction on that scale could be caused by a much smaller nickel-iron meteorite weighing a few dozen tons, the size of a large desk, perhaps it was natural for him to publish in his regular column for Fantasy & Science Fiction (‘The Rocks of Damocles’, March 1966; in Asimov on Astronomy, Macdonald & Jane’s, 1974). But when writers such as Robert S. Dietz and J.E. Enever went on to consider the destructive effects of impactors a mile in diameter, they pretty well had to publish in the monthly science feature of Analog because more reputable outlets would have none of it. (R.A. Hall, ‘Secondary Meteorites’, Analog, Jan. 1964; J. E. Enever, ‘Giant Meteor Impact’, Analog, March 1966, Fig. 2)
Having picked up on those articles at the time, and having highlighted the issues in my own book New Worlds for Old (David & Charles, 1979, Fig. 3), I remember the feeling of relief later that year when the BBC’s lunchtime radio news ran an item on the suggestion, by Walter and Luis Alvarez, that the discovery of a thin layer of iridium, at the Cretaceous-Tertiary (K-T) geological boundary, indicated a giant impact coinciding with and probably causing the extinction of the dinosaurs – now associated with the Chicxulub crater off Yucatan. Respectability at last! But having followed and reviewed the issues in detail, Ted Nield now takes a different view. Scientists who say the Chicxulub impact didn’t kill the dinosaurs are now finding it hard to get a hearing, by their account, and while Nield points out that nevertheless their views are known, he feels a degree of sympathy. In his view, attributing the extinction of the dinosaurs and other lifeforms at that time to a single cause is too simplistic, and at odds with what is known about other such events. He believes it has a great deal more to do with the near-simultaneous but prolonged and complex effects of the volcanic plume which formed the Deccan Traps in India (Fig. 4). The K-T impact may have contributed, may even have provided the coup de grace, but wasn’t the major cause.
One major reason for his scepticism is that no other mass extinctions have been correlated with impacts. To that I would reply that there are known impact events which have still to be dated, particularly a very large crater under the Antarctic ice which was discovered by the Soviets in the 1960s. (F.Yu. Zigel, The Minor Planets, NASA Technical Translation F-700, US Govt. Printing Office, 1972.) And the late Carl Sagan suggested in The Cosmic Connection (Doubleday, 1973, Fig. 5) that only large impacts at sea could explain an apparent connection between reversals of the Earth’s magnetic field and ‘megadeaths’ of marine organisms. (The argument is that the shock of a big impact disturbs the currents in the Earth’s core, with a 50-50 chance of which way they’ll settle out afterwards. As I understand it, the last two reversals coincided with big impacts on land.) Of the various books attempting to describe the destructiveness of the K-T impact, the most lurid is The Great Extinction by Michael Allaby and James Lovelock (Secker & Warburg, 1983, Fig. 6). But the horrors they describe are largely overkill when Enever’s ‘Giant Meteor Impact’ (above) provides enough damage to cause mass extinctions every time something a mile across hits the sea.
So we might argue that the Chicxulub impact could have caused the mass extinction on its own, even if as it happens, it didn’t. Here the biggest revelation of the book comes from Sweden, where geological conditions have preserved the record of a sustained bombardment of the Earth, lasting up to 15 million years but with a peak of one to two million years (Fig. 7), during the Ordovician Period (488-444 million years Before Present).
The falling bodies stemmed from the ‘Gefion family’, a group of asteroids which were formed when a body 100-150 km in diameter was a shattered by a collision, roughly 485 million years ago. Supporting evidence in the form of micrometeorites is now turning up all over the world, along with craters formed by larger bodies, and statistically it’s to be expected that at least one of those events would be in the range of the Chicxulub impact. Yet far from being associated with a Great Dying, the period represents one of the most prolific phases in the evolution of life on Earth – some of the changes probably triggered when ecological niches were freed by locally destructive events. (There was a mass extinction at the end of the Ordovician, but that was due to global cooling as the Gondwana supercontinent drifted across the south pole.)
From all of that, Dr. Nield concludes that impacts or even bombardments are not the threat to life on Earth that has been supposed. I wouldn’t go that far: it strikes me, for instance, that if there was only one Ordovician impact in the Chicxulub range, but it happened on land, things could have been very different had it fallen at sea. But even if he’s right, his subtitle takes a very long view of what’s good for life on Earth and what’s not: I was reminded of Dougal Dixon’s After Man, A Zoology of the Future (Granada, 1981, Fig. 8), which imagined that we take ourselves out and most of the higher species with us, but after 50 million years all the evolutionary niches (except ours) have filled up again, so that’s all right. In running a project to consider options for deflecting or mitigating asteroid threats, I was surprised, not to say distressed, by the number of people who believe or claim to believe that we should do nothing, letting nature take its course with us.
Incoming! mentions proposals to deflect impactors only twice, and then only to say that the Bruce Willis scenario with nuclear weapons is probably a bad idea, which it is. But while Ted Nield was in Glasgow, Astronomers of the Future invited him to give us a talk and put the question, ‘What would you want done if we knew there was going to be an impact?’. Dr. Nield conceded that if he had to share in its consequences, if it happened, he would just as soon that some efforts were made to prevent it.
Afterword (May 2023)
When I met Dr. Nield at Glasgow’s ‘Aye Write!’ festival in 2011, the book based on the discussion project I was chairing had the working title ‘Incoming Asteroid!’, which I’d first used on articles for New Moon and Space Policy in 1992. He was kind enough to say he didn’t mind my using it, despite the similarity. Mine was published by Springer (New York) in November 2013 as Incoming Asteroid! What could we do about it? (Fig. 9), and I included Dr. Nield’s points and my discussions with him, in it.
In the intervening ten years there has been little change in opinion about the Chicxulub event and its consequences. It’s still not entirely clear whether the impactor was an asteroid or a comet, but the abundances of the elements in the ‘Iridium layer’ worldwide tend to support the asteroid hypothesis (Fig. 10). From ongoing analysis of the soot layer within the iridium layer, it is clear that the ejecta from the impact did cause wildfires over a very large area, if not worldwide (Fig. 11).
Dramatic evidence for the huge groundshock caused by the impact has been found on the shores of a former inland sea in North Dakota, where the water went into seiche, surging back and forth with the power of an inland tsunami. Among the torn-up vegetation and dead fish around the perimeter was the upside-down body of a stegosaurus, and if it was possible for a fossil to have an annoyed expression, that one undoubtedly would (Fig. 12). The paths of the marine tsunami radiating from the impact have been charted, and they really were over a mile high at the outset (Fig. 13). The freshwater caves and sinkholes in the Yucatan peninsula, concentric with the impact (Fig. 14), have been mapped in increasing detail and it’s clear that nothing could have survived their creation, or for a long way beyond.
Deep-water drilling on the central peak (Fig. 15) has revealed that the impacted strata would have released acid rain, far more toxic and widespread than any human-created pollution or volcanic eruption’s. While the environmental consequences of the Deccan Traps plume were huge, they were long-drawn-out, but the Chicxulub impact’s were sudden and catastrophic. The Ordovician Period has nothing to match them.
One of the unanswered questions in Incoming Asteroid! was the effect of water vapour, ice and dust in the upper atmosphere after the impact. There was evidence that the subsequent global downpour, global darkness, nuclear winter and ice age might have lasted not just for months but for decades. The aftermaths of the Chelyabinsk upper atmosphere explosion, where dust remained in circulation for over a year, and the Shoemaker-Levy impacts on Jupiter, where water vapour and ice remained over the sites for 20 years, were not encouraging. But recent research suggests that nevertheless the global darkening was comparatively brief, more like the aftermath of the Krakatoa eruption. Our shrew-like ancestors could survive it in hibernation, but the dinosaurs had no chance (Fig. 16).
Probably the most encouraging developments have been in the area of detection and prevention of impacts, the field that Dr. Nield said little about. There’s still room for concern about the gaps in the search programme, but it is going well and continuing, with very nearly all the objects more than a kilometre in diameter now identified and tracked, none of them currently posing threats in the next thousand years, though that could change as their orbits evolve (Paul M. Sutter, ‘Astronomers Prepare for the Next Thousand Years of Hazardous Asteroid Impacts’, Universe Today, 17th May 2023). The DART impact with the asteroid moon Dimorphos proved much effective than expected, hurling huge quantities of material into space and altering the moon’s orbital period by over 30 minutes (Fig. 17). It looks as if the deflection techniques discussed in Incoming Asteroid!, such as solar reflectors, mass drivers and impacts, could indeed be effective, and the misgivings I voiced in Incoming Asteroid! and last year in Orkney News were incorrect.
Given sufficient time and sufficient spacecraft mass, the gravitational tractor technique of altering an incoming asteroid’s trajectory still looks most effective. Our study showed that the ships of a manned expedition to a body 1 kilometre in diameter, suitably clustered, might be able to deflect it even if other active methods failed. We supposed a crash programme putting NASA’s Space Launch System into mass production, and although Jay Tate of Spaceguard Centre UK doubted if it would ever fly at all, the first Artemis mission to the Moon has been a success. Ariane V has been withdrawn from the arsenal of smaller launch vehicles, but Ariane VI is replacing it, Falcon 9 is now regularly flying crews into space, Falcon Heavy is now available, Vulcan will soon have similar capabilities, Musk’s Starship has had its first launch, and the Chinese and Japanese space programmes are similarly moving ahead, with India not far behind. If we were threatened by an asteroid now, given sufficient notice, we could probably cope.
Comets are another matter. As the late Dr. Arthur Hodkin pointed out at our Spaceguard seminar, to the disquiet and even the annoyance of his audience, of the comets visited so far by spacecraft, no two are alike; we know far less about them; and to deal with what we do know, Star Trek technology on the level of matter-antimatter drives and tractor beams will be required. We just have to hope that if anything threatens us meantime, it will be something we can cope with.
