The optimists deride the historical and anthropological signs that impacts spelled doom for civilizations as exceedingly flimsy. The pessimists say that since many smallish impacts probably occurred over the past 10,000 years, it makes sense to search for their effects, one of which might be the sudden demise of civilizations.

The optimists demand to see the evidence, but the pessimists counter that the most convincing signs -- craters -- are misleadingly rare. Most of the Earth is covered by water, where craters don't form, and only about 3 percent of asteroids are iron. The remainder, like the Tunguska object, are stony objects that break up in the atmosphere. Thus, as asteroid hunter Gehrels says, "3 to 4 percent of asteroids would come through -- that's an accurate number."

Furthermore, as he observes, craters that do form are eventually wiped out by erosion or plate tectonic movement.

The optimists say that for any individual, the chance of asteroidal assassination is equivalent to the chance of dying in a jet crash. The pessimists say that a civilization is, well, a precious thing to waste, so it only makes sense to look for objects large or small before they come looking for us. Both sides agree that a well-aimed Tunguska object could obliterate any city on the planet.

Aristotle 'n asteroids
Although the debate can sink to the level of a hissing match, both sides point to some wiggle room in the argument. Are the pessimists blowing things out of proportion? Lewis, who compares the risk to flying in an airplane, says the "whole mindset may turn out to be justified, but at the moment, it should be treated as speculation." Indeed, Morrison, who's never been associated with the pessimists, says "It's a legitimate scientific debate" about whether societies have been obliterated in the past. I don't know how you resolve it easily."

Are the optimists whistling past the graveyard, ignoring gargantuan threats to society? Perhaps, but even Peiser does not claim proof that asteroids have played such an important role in history: "I don't say these theories are correct. I say it's possible that [the hazard estimate of] the NASA establishment is off by orders of magnitude."

Perhaps it's not a case of scientific disagreement but rather a philosophical dispute. As Morrison says, "Given the same data, people will still come to different conclusions about the correct resources to devote to the search [for various types of asteroids]. It's a perfectly legitimate social question about what risks you want to deal with."

Gehrels says completing the search for 100-meter objects would "be a reasonable goal," and notes that Spacewatch is attempting a statistical survey of these smaller but still deadly objects. Nevertheless, he sees signs of progress in the ongoing searches. With carefully chosen words, he says, "The asteroid hazard is the most serious thing that humanity faces, because it's the only one that can eliminate society at once. But we are on the way to taking care of it."

Astronomer Steel, however, is less sanguine. Asked via e-mail if current search programs are adequate, he fired back: "I take it you are joking. There is NO SOUTHERN HEMISPHERE SEARCH PROGRAMME [emphasis his].

Ipso facto, current efforts are not sufficient. However, the only real Northern Hemisphere search projects -- wonderful efforts operated from the U.S. -- are only covering (i) a minor portion of the northern sky; (ii) Not covering that sky to an adequate faintness limit: They are using only one-meter telescopes, and the laws of physics, which may not be contravened, dictate that telescopes with apertures larger than two meters are required... The great scientists are doing the best they can with very limited funds, but globally we are doing rather less than 10 percent of what needs to be done lest we go the way of the dinosaurs."