Post by Chicago Astronomer - Astro Joe on Mar 22, 2006 1:00:25 GMT -6
Space rock risk underreported, researchers argue
March 21, 2006
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.
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.
Full story here: www.world-science.net/exclusives/060320_impactfrm1.htm
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Nothing is set and to adhere to old-school methodologies is folly...
