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When To Cancel ArmageddonCalculating the odds of asteroid impact.

Posted Wednesday, Sept. 3, 2003, at 12:49 PM ET

Collision course? The world penciled in a date for Armageddon Monday when astronomers reported that Asteroid 2003 QQ47 had a 1-in-909,000 chance of hitting Earth in 2014 with the force of 20 million Hiroshimas. Further observations quickly showed the asteroid is even less likely to hit Earth, and scientists have reduced its rating from 1 to 0 on the Torino scale of threat measurement. In July 2002, Brendan I. Koerner's "Explainer" tackled the question of how to calculate the odds of such collisions and why they change so quickly.

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Photograph of asteroid heading for Earth © Denis Scott/Corbis.
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