The 10-meter-diameter chunk of rock that exploded over western Siberia yesterday had nothing to do with the 45-meter asteroid whizzing close by Earth today, scientists say. But it does provide a more dramatic reminder of the incessant rain of cosmic debris that the planet endures. Such a meteoric detonation tens of kilometers high happens on average every 10 years or so. This one just happened to strike over a populated area, injuring several hundred people, mostly by sending window glass flying. That pales beside the destruction wreaked by the detonation of a 40-meter asteroid over an unpopulated part of Siberia in 1908; that so-called Tunguska event  leveled 2000 square kilometers of forest. The object's energy—it was traveling at thousands of kilometers per hour—was released in an explosion when it shattered and atmospheric friction burned up the bits in a moment. Airbursts of Tunguska size probably happen every 1200 years on average somewhere on a mostly empty Earth. One of these days, a bigger bit of cosmic debris will make it to the ground intact.
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