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Asteroids less threatening than expected

Medium-size asteroids that could flatten a city the size of New York strike Earth less frequently than previously believed, possibly only about once a millennium, according to a study aided by military satellites.

Rocky space debris created by collisions in the asteroid belt between Mars and Jupiter or chunks that break away from comets rain down on the Earth every day as meteroids, but most of the asteroid or comet pieces are tinier than grains of rice and quickly burn in the upper atmosphere as meteors.

In 1908, however, a meteor estimated to be up to 50 yards wide nearly hit the ground before it burned up over Russia, causing an explosion that flattened hundreds of square miles of forest in Siberia. The blast was estimated to be the equivalent of about 10 megatons of TNT _ or 10-million tons.

By comparison, the nuclear bomb that exploded over Hiroshima in World War II unleashed about 13 kilotons of explosive power _ or 13,000 tons.

In the new study, satellite data taken over the past eight years suggest that an intermediate-size asteroid like the one that struck Siberia occurs an average of only once every 1,000 years _ not every couple of centuries as previously believed, said Peter Brown, a University of Western Ontario astrophysicist.

His study, to be published today in the journal Nature, was based on measurements of the flashes of light created when the debris burns after hitting the upper atmosphere.