Most scientists agree that cosmic impacts have played a major role in Earth history and that they continue to pose a significant threat today. But interpretations vary greatly from legitimate science from pseudoscience, according to Skeptical Inquirer Magazine who recently reviewed ten trade books that span this broad range in interpretations.
It all started with the now-famous paper published in Science in 1981 by Luis and Walter Alvarez and their colleagues, who suggested that an impact sixty-five million years ago produced the mass extinction that terminated the Cretaceous era. What Luis and Walter Alvarez suggested that even relatively modest impacts might have had a catastrophic effect on the environment. Comets that come close to the Earth are called Near-Earth Objects (NEOs) and it’s obvious from the cratered surface of the moon that Earth, too, is subject to impacts. Planetary probes, starting with Mariner 4 in 1964, showed us that this sort of thing is pretty common in the universe. What the Alvarez paper suggested was that such impacts can generate global-scale wildfires and dust storms, killing most life forms and influencing the course of biological evolution. Fortunately, impacts large enough to produce mass extinctions (e.g., NEOs >15 km in size) are rare, taking place every hundred million years or so, give or take a few million. However, according to the Skeptical Inquirer, the planet is struck by a 1-km asteroid or comet every 100,000 years.
According to the Skeptical Inquirer, “the British Neo-catastrophists viewpoint is advocated in its extreme form by astronomers Victor Clube and Bill Napier, who interpret historical records as indicating that Earth has been subject to extreme battering from space within the past few millennia. In their popular books The Cosmic Serpent and The Cosmic Winter, they take the position that the emergence of astrology in the western Mediterranean, the association of gods with planets in many ancient cultures, the widespread fear of comets and belief in angels, and many other aspects of our cultural and religious history are a reflection of massive bombardment of the planet a few thousand years ago. They further conclude that more recent historical events, including the collapse of the Roman Empire, the Dark Ages, and even the English Civil War, are related to climate changes induced by exceptional deposition of cosmic dust in Earth's atmosphere. Although their historical analysis is suspiciously similar to that of Immanuel Velikovsky, Clube and Napier adamantly reject the association, arguing that unlike Velikovsky they root their explanations in sound physical and astronomical principles.” They go on to say, “Supporting Clube and Napier are British astronomers Duncan Steel and Mark Bailey, who have concluded that the solar system is currently experiencing the aftermath of the break-up of a giant comet some millennia in the past. Our planet still intersects debris from this comet in what they call the Taurid complex of dust, small comets, and asteroids. They term this theory coherent catastrophism. Steel and Bailey estimate that the present lull in impacts will end in about a thousand years, when our orbit again crosses the denser parts of the Taurid complex, at which time the impact risk will rise by at least a factor of a hundred. All of these neo-catastrophists argue that urgent action is required to prevent the collapse of civilization under the next cosmic onslaught.
"Most of us find these neo-catastrophist arguments difficult to swallow. Putting aside the issue of the Velikovskian interpretation of history and legend, the impact rate is still constrained by the cratering history of the Moon, which reflects the long-term average. If there are huge "spikes" in the frequency of impacts, produced by the break-up of giant comets, they must be compensated by much lower flux rates between peaks. Yet Clube, Steel, and their colleagues simultaneously assert that the consensus group underestimates the current impact rate, and that a big spike is coming. You can't have it both ways. If they are correct that almost all impacts occur during the spikes, then the present danger must be very low, and we have centuries to prepare to deal with the next peak. But they don't see it that way, and neither do the authors of several recent books.”
In the meantime I ran across a recent newspaper article in the Vancouver Sun, based on the findings of an international research team, who concluded that the extinction of mammoths and the decline of Stone Age people about 13,000 years ago was caused by a low-density object (probably a comet)—not by overhunting by Paleoindians, climate change or disease. They suggest that this object “exploded in the upper atmosphere and triggered a devastating swath of destruction that wiped out most of the large animals, their habitat and humans of that period.” Ted Bunch, NAU adjunct professor of geology and former NASA researcher who specializes in impact craters, said that “the detonation either fried them or compressed them because of the shock wave. It was a mini nuclear winter.” Bunch and Jim Wittke, a geologic materials analyst at NAU co-authored a paper that targets an extraterrestrial impact 12,900 years ago for the mass extinctions at the end of the Ice Age.