SOLAR SYSTEM POT-STIRRER
With its great mass, Jupiter exerts a major influence on the orbital mechanics of the asteroids and comets that pass near it. In the early Solar System, it prevented any planet from forming in the region that is now the asteroid belt by winnowing the population of large planetesimals in that vicinity. (Planetesimals are the small bodies that coalesced to form the planets.)
Today's asteroid belt has gaps in it where few objects orbit. Any that wander into these areas are soon removed by perturbations—changes in their orbit caused by the gravity of Jupiter.
Scientists are also starting to hold Jupiter largely responsible for the creation of the Oort Cloud of comets. The theory is that as comet nuclei move inward from the Kuiper Belt through perturbations with Neptune, they can end up near Jupiter. At that point, Jupiter's gravity may bend their orbits into elongated paths that approach the Sun and Earth. More often, though, it will fling them outward, to the Kuiper
Belt again, into the Oort
Cloud, or perhaps even out of the Solar System altogether.
Jupiter can also destroy comets, as the world saw in
July 1994 with Comet
Shoemaker-Levy 9. This hapless object passed close enough to Jupiter that tidal effects broke the comet apart. Jupiter then sent the comet on one last looping orbit around itself.
When the comet returned to
Jupiter, its 21 pieces crashed into the planet's southern hemisphere, leaving Earth-size dark splotches that could be seen even in small telescopes.