Undetectable Asteroids Could Destroy Cities, Experts Say

An estimated 1,100 to 1,500 asteroids of more than one kilometer across (about a thousand yards) are believed to be circulating in the solar system. None of these "Earth killers," like the one drawn here, is known to be on a collision course with our planet, although astronomers have identified only about 700 of them.

A more likely threat is a strike by asteroids smaller than the Earth killers, scientists say. There may be as many as ten million smaller bodies, and some of them could hit Earth with very little notice.

Image courtesy NASA

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