Roche limit

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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 ringsring,
in astronomy, relatively thin band of rocks and dust and ice particles that orbit around a planet in the planet's equatorial plane. All four of the giant planets in the solar system—Jupiter, Saturn, Uranus, and Neptune— have rings, although only those of Saturn
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, 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 MetisMetis
, in astronomy, one of the 39 known moons, or natural satellites, of Jupiter.
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 and Saturn's moon PanPan,
in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn XVIII (or S18), Pan is 12.5 mi (20 km) in diameter, orbits Saturn at a mean distance of 83,000 mi (133,583 km), and has an orbital period of 0.575 earth days.
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 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 cometcomet
[Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun or another star. Comets visible from the earth can be seen for periods ranging from a few days to several months.
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, 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.

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Roche limit

(rosh) The minimum distance from the center of a body at which a satellite can remain in equilibrium under the influence of its own gravitation and that of its primary. Assuming that the satellite is held together only by gravitational attraction, i.e. it has zero tensile strength, and that it has the same density as the primary, the Roche limit is about 2.45 times the radius of the primary. Inside the Roche limit a satellite would be torn apart by tidal forces. It is thought that the ring systems of the giant planets were formed when a satellite wandered inside the planet's Roche limit. The expression for the limit was discovered in 1849 by the French scientists Edouard Roche.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006