Asteroid anniversary recalls Earth's rocky history

Summertime — a time for sunny days, beach weekends and of course, leisurely reflections on the end of the world and the monster asteroids that could smack into us. The centennial anniversary of the last big impact, the 1908 Tunguska blast that rocked Siberia, falls Monday, June 30, bringing with it a reminder of the very slight chance that a hunk of space rock out there might have Earth's number.

The Tunguska "event" leveled nearly 800 square miles of swampy woodland in Siberia, traveling from the northwest to deliver a 5-megaton blast seen by hundreds of witnesses, including one who created a postage stamp of the explosion. A space rock about 50 yards long had zoomed into the Earth's atmosphere and exploded in mid-air.

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