Many of us think of large meteor strikes, like the one which formed Quebec's Manicouagan Reservoir (pictured left, as seen from low earth orbit), as events that occur in geological time - that is, many tens of thousands of years, if not many millions of years ago. The possibility of such an event occurring in our lifetimes, outside of a movie theater at least, seems remote. In a new article in The Atlantic, however, Greg Easterbrook suggests that the danger of a large (1+ kilometers) asteroid hitting the earth in the near future is actually fairly high. He observes that in the last 25 years astronomers have found several thousand more near-earth asteroids than they expected, and that several geologists, notably Dallas Abbott of Columbia, have found evidence of many more terrestrial meteor strikes than they expected - mainly by looking for craters in the ocean floor and elevated iridium levels in the soil. (Iridium is an element commonly found in asteroids but relatively scarce on earth.) Some of these strikes are quite recent: a 300-meter asteroid apparently impacted in the Gulf of Carpentaria off northern Australia in 536 AD, leading to cold temperatures and crop failures for the next couple of years. Another asteroid hit the western Indian Ocean around 2800 BC, producing tsunamis and severe flooding and perhaps contributing to the Mesopotamian and Biblical flood myths. And several researchers from the Lawrence Berkeley National Laboratory believe, based on elevated soot and iridium levels in the soil, that 1 or 2 large meteors exploded in the air over North America about 12,900 years ago, causing climatic stress that may have contributed to the decline of the Clovis Paleo-Indian culture and the extinction of the wooly mammoth, along with most other large mammals in North America. These are all quite recent findings, and there are sure to be more.
Easterbrook believes that the odds of another large asteroid strike in the next century are probably 1 in 10, and that the results will be predictably catastrophic. An ocean impact will produce tsunamis and destroy coastal cities; a land impact will generate huge dust clouds and intense acid rain; both will lead to hard winters and possibly a mini-ice-age. He also believes that it is well within the capability of human beings to prevent such a disaster from happening: assuming astronomers can detect an incoming asteroid (a big assumption, but feasible), the United States or another spacefaring country could simply launch a "gravitational tractor," a large probe which would match the asteroid's velocity and use its own tiny natural gravitational force to deflect the asteroid from its earth-intercepting trajectory. The probe would not have to be manned, but I suppose it could be - it would be nice to give Bruce Willis something to do in his dotage. At any rate, NASA is reluctant to devote any of its scarce resources to asteroid defense, which is considers too science-fictional, and prefers to concentrate instead on practical and down-to-earth projects, like a Moonbase.