The Roche limit
. A large satellite (top) that moves well within a planetís Roche limit (dashed curve) will be torn apart by the tidal force of the planetís gravity. The side of the satellite closer to the planet feels a stronger gravitational pull than the side farther away, and this difference works against the self-gravitation that holds the body together. A small solid satellite (bottom) can resist tidal disruption because it has significant internal cohesion in addition to self-gravitation.
Copyright 2010, Professor Kenneth R. Lang, Tufts University