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The Question

(Submitted April 19, 1998)

Did an asteroid ever hit Earth?

The Answer

Yes, it is virtually certain that asteroids have hit earth. First of all, earlier in the history of the solar system there were probably many more asteroids than there are now, so the odds of a collision were greater. More recently a large asteroid impact is credited with having led to the extinction of the dinosaurs. Other planets and their moons show many craters, which provide evidence for asteroid impacts. On earth these are destroyed by weathering. The June 1997 issue of Sky and Telescope has an article on this subject in which they estimate that a very large asteroid impact occurs once every 100 million - 1 billion years (the earth is 4 billion years old), and a large asteroid impacts once every 1 - 100 million years.

Tim Kallman
for "Ask an Astrophysicist"

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