See also: Satellite (astronomy)

A natural satellite is an object which Orbite around a Planet or of another object larger than itself and which is not human origin, in opposition to the artificial satellite . Such objects are also called the moons , by analogy with the the Moon, the natural satellite of the Ground.

Technically, the term could apply to a planet orbiting a star, or even a star orbiting a galactic center, but such a use is rare. In normal weather, it indicates the natural satellites of planets, dwarf planets and small bodies.

Origin

It is supposed that the natural satellites orbiting relatively close to a planet on an orbit Prograde were formed in the same area of the disc protoplanétaire at the origin of this planet. By opposition, the satellite irregular (generally orbiting on distant orbits, tilted, eccentric or Retrogresses S) would be foreign objects captured and possibly fragmented at the time of collisions.

There exist exceptions or variations with this standard model of formation. In particular, the couples the Ground-Moon and perhaps Pluto-Charon would draw their origin from the collision of two large proto-planet gears objects. The ejected matter orbits about it around the central body would then have formed one or more objects by accretion. It is thought in addition that the satellite of asteroids are formed mainly by this process.

Definition

The term of “satellite” does not have a precise scientific definition. In particular, the existence of couples Pluto - Charon and Ground - the Moon, where the report/ratio of the masses between the central body and its satellite is not as marked as in the majority of the other systems, returns difficult the determination of a limit separating a satellite system from a Planet doubles. A joint definition supposes that a satellite system must have a Barycentre located under the surface of the broadest body, but it is not official and remains arbitrary.

With the other end of the scale, the annular systems around the gas giant of the solar system are composed of small pieces of ice and of rock and there does not exist any limit defining a size from which such a piece is sufficiently large to be regarded as a satellite with whole share.

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