These jets can be strong enough to change a comet's orbit. Many comets have probably slammed into Earth during our planet's history, causing global destruction. In 1994, Comet Shoemaker-Levy 9 rammed into Jupiter. Jupiter's powerful gravity pulled the comet apart long before it reached Jupiter, so the planet was pelted by almost two dozen impacts, which created Earth-sized scars in Jupiter's cloudtops. Some of these dark markings lasted for several months.
Astronomers hope to learn more about comets through several spacecraft.
Perhaps the most exciting mission is Stardust 1, which will pass through the tail of Comet Wild 2 in 2004. It will catch comet dust on a sticky paddle and bring the material back to Earth for study. Deep Space 1, scheduled for launch in 1998, will visit a different comet. And yet another spacecraft, called Contour, will fly past three comets early in the next century. Comets are giant snowballs, but as they approach the Sun, some of their ices vaporize.