Big space rocks could strike Earth more frequently

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There are scads of building-size, potentially hazardous asteroids lurking in Earth's immediate neighborhood, and they may be colliding with the planet 10 times more often than scientists have previously believed, according to a new study published Wednesday that examined the air burst of a 25 million-pound asteroid earlier this year near the Russian city of Chelyabinsk.

Three studies released Wednesday, two in the journal Nature and one in the journal Science, have provided the most detailed description and analysis of the dramatic event on the morning of Feb. 15.

Scientists now estimate the diameter of the object at just a hair under 20 meters, or about 65 feet. Undetected by astronomers, the rock came out of the glare of the sun and hit the atmosphere at 43,000 mph. As it descended through the atmosphere, it broke into fragments, creating a series of explosions with the combined energy of about 500 kilotons of TNT, making it more than 30 times more powerful than the atom bomb that destroyed Hiroshima in 1945, although the energy in this case was spread out over a much broader area.

The shock wave blew out windows in nearly half the buildings in Chelyabinsk. It knocked people off their feet; dozens were sunburned by the flash, which at its peak was 30 times brighter than the sun. About 1,200 people were hurt, most by broken and flying glass, but no one was killed.

One chunk the size of a love seat landed in frozen Chebarkul Lake, leaving a circular hole, as if shot with a bullet from space. That fragment, which weighed about 1,900 pounds, was retrieved months later.

The scientific investigation relied to a great degree on video imagery obtained by "dashcams," the cameras Russian drivers often use to document car crashes and potentially abusive law enforcement. Scientists visited 10 locations where the footage had been taken by stationary cameras, and used landmarks to create a map of the asteroid's trajectory.

Taken together, the new information on Chelyabinsk does not suggest that the sky is falling (no one has ever been killed by an asteroid in all of recorded human history). But it may shift the overall risk profile of asteroids, making Chelyabinsk-size events look more probable.

That's the conclusion of Peter Brown, a professor at Western University in London, Ontario, who reexamined decades of data compiled by scientific and military sensors. The scientific orthodoxy said a Chelyabinsk-size event ought to happen every 140 years or so, but Mr. Brown saw several such events in the historical record.

Famously, a large object exploded over the Tunguska region of Siberia in 1908. But there have been less-heralded impacts, including one on Aug. 3, 1963, when an asteroid created a powerful air burst off the coast of South Africa.



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