Except for the atmosphere and slow movement of the surface by plate tectonics, Earth would look as battered as our moon, seen here in a false-color image taken by the satellite Galileo in 1992.
Galileo satellite, NASA.
The asteroid Ida is covered with craters. Its irregular shape also indicates a history of larger impacts.
Courtesy NASA.
He said, she said
Listening to the debate over asteroid hazards, it's hard to remember that both sides are talking about projections based on the same data. On the one hand, the optimists say that impacts that would cause a global catastrophe (defined as killing more than one-quarter of humanity), will occur about every 330,000 years (see p. 59 of "Hazards... " in the bibliography).
On the other hand, Tom Gehrels, who edited "Hazards..." and founded Spacewatch, an asteroid search at the University of Arizona, says impact is "a horrible problem. Do I lie awake at night? Yes."
Gehrels is not alone. Radio astronomer Gerrit Verschuur of the University of Memphis writes that while the asteroid hunters are concentrating on 1-kilometer objects, "It is the smaller impacts that pose the greater danger, and they occur more frequently" (see p. 108 in "Impact!" in the bibliography).
We asked astronomer Duncan Steel, of the University of Salford in the United Kingdom, whether, as the Paine simulation indicates, the crater record understates the true number of impacts. He wrote back, "This is what one can infer to have happened, based simply upon our knowledge of the influx (i.e., physics). Now we need to look into the historical record and also the archeological record in this light. In the past scholars have dismissed tales of exploding rocks from space and vast cataclysms because 'such things do not happen'. But now we know they do."
Anthropologist Peiser is one of those who suspect that history and anthropology have suggestive evidence of impacts. If, he asks, the computer simulation was correct, and 350 Tunguska-size objects struck the Earth within the last 10,000 years, how would that have affected human society? He says the decline of many ancient civilizations, including that of the Roman Empire, "may be associated with episodes of increased cosmic activity. Up to eight major [social] events in the last 10,000 years were associated with major impacts," Peiser maintains. He argues that estimates of asteroid activity reflect present conditions, even though Earth may have occupied far more hazardous regions of space in the past.
Trying to carry arguments across disciplines is risky, writes Verschuur. "The reaction to any proposal that we have anything of geological, paleontological or astronomical value to learn from history is a direct parallel to the reaction met by the Alvarez team when they first dared to suggest a connection between astronomical events and mass extinctions. The prejudice of scientists against anything that does not come from within their own discipline ... is considerable."
I'm from Missouri... Show me
At about this point, the optimists on the other side of the issue start to echo a famous ad campaign: "Got data?" To Peiser's assertion that, "Some 100 surface impacts, including more than a dozen oceanic impacts, have repeatedly devastated whole regions, small countries and early civilizations around the globe" during the past 10,000 years, Clark Chapman, an asteroid expert at the Southwest Research Institute in Colorado, wrote that this reflects "historical anecdotes and the ancient versions of 'urban legends.' I regard these as having little probative [substantiating] value, unless accompanied by sound physical and geological evidence. But there are some neo-catastrophists, located mainly in Britain, who have an almost Velikovskian pseudo-scientific take on this matter and have argued that such impacts are more frequent..." Velikovsky, of course, is the guy who gave asteroid impacts such a bad name back in 1950.
Responding to the same statement, David Morrison, director of astrobiology and space research at NASA Ames Research Center, argues that the 100 impacts number is "highly speculative. I know of no convincing evidence that any 'small countries' or 'early civilizations' have been devastated by a cosmic impact. It is possible that one or more have (one thinks of the Biblical story of Sodom) but the evidence is weak. And of course there would be no way to know about most oceanic impacts."
John Lewis, the University of Arizona astronomer who created the software that cranked out the numbers so worrisome to Peiser and others, notes that if you run a simulation for a million years, you get "an astronomical number of casualties." Even if a billion people are cooked by asteroids over a million years, he says, "The total number of people who will die in that time is in the trillions." The danger of dying from an impact, he says, is "comparable to flying an airplane, it's about the same risk."
You tell me. Should we be searching harder for asteroids?
