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.