A small but growing number of astronomers are arguing that the risks of comets or meteors hitting Earth are much higher than past estimates suggest.

Some of these objects may be going unnoticed in space, the researchers say, and scientists may need to begin new studies tailored to finding them.

But advocates of the earlier estimates are shooting back that the evidence doesn’t warrant revising the figures drastically.

The traditional estimates, based on sky surveys and other techniques, vary.

But in general, they conflict with the number of objects actually found to have visited Earth’s neighborhood, according to David J. Asher of the Armagh Observatory in Northern Ireland and three colleagues.

These numbers may provide better estimates, they wrote in the October issue of the research journal Observatory, because they “represent observational ‘ground truth’ so far as impact statistics are concerned.”

To re-estimate risks from these limited data, the team made a simple—but to some critics, questionable—assumption.

Basically, if a given type of object struck once in the past two centuries, they assumed it hits once every two centuries on average. A somewhat longer, but essentially similar calculation translated known near-misses into impact probabilities.

They analyzed in this way three well-studied types of object either seen to have passed near Earth or believed to have hit recently. They also took into account other flybys whose occurrence they deduced from meteorites.

Their revised estimates in the Observatory paper were as follows:

• Impacts greater than 10 megatons (about 500 Hiroshima bombs), like a 1908 explosion in remote Siberia widely attributed to a space object, would occur at least every 300 years. That’s almost tenfold the rate generally accepted estimates suggest. The blast in Tunguska, Siberia, felled an estimated 60 million trees.
• Strikes of a hundredfold greater destructive energy or more would happen every 3,000 years or so, more than 10 times as often as prevailing estimates.
• Objects known as active comets packing a 100-million-megaton punch—an event like what may have killed off the dinosaurs along with more than half the Earth’s other species—would strike once every few tens of millions of years. That’s about 100 times more often than a conventional figure of every 3 billion years. The team didn’t present revised estimates for another class of comets, inactive comets, thought to hit every 150 million years or so.

“We’re not trying to make a precise estimate,” Asher said. “We’re trying to point out that if you accept all the current estimates, then the three or four single objects we’ve mentioned,” these together don’t fit prevailing views.

The work was in part a follow-up on earlier findings by researchers at Cardiff University in Cardiff, U.K., one of whom also co-authored the Observatory paper.

The earlier study, which appeared in the Nov. 2004 issue of the research journal Monthly Notices of the Royal Astronomical Society, proposed that most potentially dangerous comets are unseen because they’re too dark.

But some astronomers are skeptical of all this.

Donald K. Yeomans of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., wrote in an email that Asher’s team employed what he called “the statistics of one,” the unreliable practice of drawing statistical conclusions from one object.

Asher countered that although he sometimes used one object to represent a class of objects, he repeated this for several different classes, with similar results. “It seems to be stretching the imagination to say they’re all just coincidences,” he said.

Yeomans, lead author of a major 2003 NASA report on the impact risks, also said any dark “stealth comets” would emit strong infrared light, a type of light invisible to the eye. Past space surveys would have detected this, he added. Asher said those surveys haven’t been extensive enough, and wider ones may be warranted.

In any case, Yeomans said, not all past assessments have been as rosy as the ones Asher’s team criticized. His own report, Yeomans argued, contains numbers that aren’t as far from those of Asher as other papers are.

Asher conceded at least one point: his study doesn’t explain just why all these objects would be going unseen.

Yeomans argued that all known comets have nearly the same reflectivity, which largely determines their brightness, and that these data contradict the dark comet proposal. Asher acknowledged the lack of known dark comets is puzzling in the context of his study, but suggested something else may explain why objects are going unseen.

One possibility is that they’re breaking up into smaller, less-noticeable pieces, he added. Either way, he wrote in an email, “there remain unsettling discrepancies in our understanding of these matters.”

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