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Threat of killer asteroids downgraded

Hits that could destroy cities predicted every 1,000 years

By Richard Stenger

Artist's concept of a killer asteroid strike
Artist's concept of a killer asteroid strike

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(CNN) -- Space boulders big enough to wipe out a major city slam into the Earth about once every 1,000 years, much less frequently than previously thought, scientists said Wednesday.

The revised calculations come from an unprecedented examination of years of data from U.S. Defense Department satellites that scour the ground for nuclear explosions.

Previous ground-based observations had suggested that asteroids or comets of comparable size pound the planet once every two or three centuries.

"This new method of obtaining information redefines our range of knowledge about how and when asteroids may hit the Earth," said University of Western Ontario astronomer Peter Brown, lead author of a report in the November 21 edition of the journal Nature.

During the past 8.5 years, the military orbiters spotted almost 300 light bursts high in the atmosphere caused by the explosions of meteors between one and 10 meters (3.3 and 33 feet) in diameter.

The satellite scan is an ideal method to track the minor space slabs, which for the most part had eluded scientific scrutiny, according to Robert Jedicke, an astronomer at the University of Arizona.

"In between the harmless little meteors and the civilization-ending flying mountains lies an intermediate regime of objects," Jedicke said in a companion Nature article.

"These objects are too small to be detected with telescopes and too uncommon in their rate of impact to be regularly visible from the planet's surface when they ignite in the upper atmosphere," he wrote.

Bigger strikes occur less often

Based on the impacts, the research team, which included U.S. Energy Department scientists and an Air Force general, predicted the strike frequency of gradually larger objects.

A small asteroid pierces the atmosphere and releases the energy equivalent of five kilotons (5,000 tons) of TNT about once a year. In comparison, the explosive power is almost half that of the bomb dropped on Hiroshima, Japan during World War II.

Larger objects packing the power of one megaton (one million tons) of TNT, which could cause considerable damage on the ground, arrive about once a century.

An object big enough to do more destruction, like the approximately 40-meter-wide (130-foot-wide) asteroid or comet that exploded over Tungushka, Siberia in 1908, hits about once a millennium.

The blast, equivalent to 10 megatons of TNT, flattened 500,000 acres (2,000 square km) of uninhabited forest. Had the bull's-eye been on New York City, it would have been destroyed, the scientists said.

Some asteroid experts were hesitant to embrace the new prediction rates.

"The new impact rate estimate is based on the assumption that the asteroidal and cometary flux remains more or less constant over time, (but) the data set used is fairly limited and the timespan rather short," said Benny Peiser, director of a U.K.-based group that monitors asteroid threats.

Nevertheless, Peiser welcomed the wealth of additional data.

"The new research substantiates that we are constantly bombarded by cosmic debris large enough to be misinterpreted as a nuclear attack. The findings are a compelling warning that we need to start scanning the skies for those small but potentially harmful intruders."

Killer asteroids at least 0.6 miles (1 km) wide, which could cause planetwide catastrophes, plow into Earth every million years or so, scientists have theorized.

The one thought to have hastened the end of the dinosaurs 65 million years ago was an estimated 6 miles (10 km) in diameter.

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