A Fateful Breakup
Shoemaker-Levy 9 (SL9) did not always exist as bits and pieces of a comet. Back in July 1992, the comet came too close to the massive planet, Jupiter. Comets are very fragile objects. SL9, for example, started drifting apart when the powerful forces of Jupiter's gravity began pulling at it. Jupiter, exerting what is called a "tidal force," pulled more strongly at the side of the comet closest to the planet. The uneven pull was just too much for the tiny comet. Its internal forces could no longer hold it together, and it fell apart.
It's Happened Before!
Tidal forces, such as those our moon exerts on the Earth's oceans, cause other objects to break apart and possibly hit more massive objects that have a greater gravitational pull. Fragments of celestial bodies that were torn apart by tidal forces have hit Jupiter's system of moons in the past. Callisto, one of Jupiter's moons, bears the scars of such damage.
A chain of craters on our own moon was caused when an object passed too close to Earth at the wrong angle and speed. Our planet's gravity, exerting a tidal force on the object, pulled it apart, and some of its pieces collided with the moon's surface.