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.
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