Billions of years ago, the young planets withstood a steady rain of rock and ice as they slowly assembled from a disk of debris orbiting the Sun. The shower has since let up, but occasionally a big strike occurs.

A series of such events occurred July 16-22, 1994, when 21 fragments of Comet Shoemaker-Levy 9 plunged into Jupiter, exploding with the force of millions of nuclear bombs.