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.