Roche limit

The Roche limit is the distance within which a celestial body held together only by its own gravity will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction. Inside the Roche limit, orbiting material will tend to disperse and form rings, while outside the limit, material will tend to coalesce. The term is named after Édouard Roche, the French astronomer who first calculated this theoretical limit in 1848.

Related Topics:
Gravity - Tidal force - Coalesce - Édouard Roche - French - Astronomer - 1848

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The Roche limit should not be confused with the concept of the Roche lobe, which is also named after Édouard Roche. The Roche lobe describes the limits at which an object which is in orbit around two other objects will be captured by one or the other.

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Typically, the Roche limit applies to a satellite disintegrating due to tidal forces induced by its primary, the body about which it orbits. Some real 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 which are able to hold together despite being within their fluid Roche limits. They hold together partly because of their tensile strength, and partly because they are not actually fluid. In such cases, it is possible for an object resting on the surface of such a satellite to be pulled away by tidal forces, depending on where it is: tidal forces are most repulsive along the line of centers between the satellite and primary. A weaker satellite, such as a comet, could be broken up when it passes within its Roche limit. Comet Shoemaker-Levy 9's decaying orbit around Jupiter passed within its Roche limit in July 1992, causing it to fragment into a number of smaller pieces. On its next approach in 1994 the fragments crashed into the planet.

Related Topics:
Satellite - Orbit - Natural - Artificial - Jupiter - Metis - Saturn - Pan - Fluid Roche limit - Comet - Shoemaker-Levy 9 - 1992 - 1994

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Since tidal forces overwhelm gravity within the Roche limit, no large satellite can coalesce out of smaller particles within that limit. Indeed, all known planetary rings are located within their Roche limit. They could either be remnants from the planet's proto-planetary accretion disc that failed to coalesce into moonlets, or conversely have formed when a moon passed within its Roche limit and broke apart.

Related Topics:
Planetary ring - Accretion disc

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