Roche limit, the closest distance that a celestial body held together only by its own gravity can come to a planet without being pulled apart by the planet's tidal (gravitational) force. This distance depends on the densities of the two bodies and the orbit of the celestial body. Inside the Roche limit, orbiting material will tend to disperse and form rings, while outside the limit, material will tend to amalgamate to form celestial bodies. The French mathematician and astronomer Edouard Roche first enunciated this theoretical limit in 1848.
If a planet and a satellite have identical densities, then the Roche limit is 2.446 times the radius of the planet. Some satellites, both natural and artificial, can orbit within their Roche limits because they are held together by forces other than gravitation. Jupiter's moon Metis and Saturn's moon Pan are examples of natural satellites that survive despite being within their Roche limits—they hold together largely because of their tensile strength. A weaker body, such as a comet, could be broken up when it passes within its Roche limit. For example, comet Shoemaker-Levy 9's decaying orbit around Jupiter passed within its Roche limit in July, 1992, causing it to break into a number of smaller pieces. All known planetary rings are located within the Roche limit, and may be either remnants from the planet's protoplanetary accretion disc that did not amalgamate into satellites or fragments from a body passed within its Roche limit and broke apart.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.