Natural satellite

A satellite ( colloquially usually moon) is in astronomy, a naturally formed area of ​​astronomical object that is in orbit around another object - is - like a galaxy, a planet, a dwarf planet or a small body such as an asteroid. This contrasts with the artificial satellite in space. Natural satellites of planets and smaller objects are also called " moons " (after Earth's Moon ), or " satellites " ( Slavic: " companion " ) refers; see for more details the concept of origin below in section history.

For galaxies that are satellites of another star system, astronomers speak of satellite galaxies.


On the Origin of natural satellites or moons exist, depending on specifics of their size or its web properties different ideas. Best known are mainly engaged in the past, controversial approaches to the creation of the Earth- Moon system - mainly either from a common accretion disk, by capture, by splitting or as a result of a major collision of protoplanets.

For the regular satellites of the giant planets, the accretion applies from surrounding particles. For relatively very large satellites like the Earth's Moon and Pluto - Charon the idea of ​​a major collision has gained the upper hand. For declining circling satellites, that is against the sense of rotation of the main body, the capture of a fully trained on a formerly independent body orbit is assumed to be Herkunftsweg. For the Martian moons ( Phobos and Deimos ) an origin is thought to be captured asteroids because of their very irregular shape and the proximity of the asteroid belt. For small and very irregular asteroid, its rotational speed will be affected by the YORP effect, this effect can potentially result in the event of an acceleration in the course of several million years, to the heavenly bodies break and remain individual fragments as satellites.

Other developments

About a possible demise of satellite, there are also different ideas. With the oldest of which is that of the intrusion into the Roche limit of a planet. From this critical near the tidal forces are so strong that larger satellites are torn from them. This view has become known primarily as an explanation of the main countries of origin of Saturn's rings. There is also the theory that the planets rings of the gas planets are composed of residual material of the protoplanetary gas and dust disk, which could not accumulate into satellites there within the Roche limit. The ring systems are composed of ice and dust of very fine particle size to house-sized chunks. For the distinction between the smallest moons and the largest ring chunks there is no defined size limit; the distinction is purely conventional, since the rings have been photographically not yet resolved into individual components.

Many individual rings within a ring system are apparently fed from a satellite: In the case of Saturn's moon Enceladus by rising fountains of Kryovulkanismus, and of the moon Mab, whose orbit around Uranus with the highest density of the My- ring zone coincides, which may is caused by the out striking effect of meteorite impacts. While the attraction of larger celestial body ensures that the debris fall back mostly on the surface, it does not suffice in the case of small and next to low-mass satellites even fine particles distributed annularly in orbit. As in the case of the two small moons Nix and Hydra, Pluto is thought by scientists that the impacts of micrometeorites on an astronomical period of time can lead to a dust ring to the attrition of a smaller satellite.


After the invention of the telescope, the discovery of the Galilean moons of Jupiter in 1610 was the first observation of astronomical objects, which obviously does not revolve around the earth. This was an argument against the geocentric and heliocentric world view and thus that the Moon is the only celestial body that really revolves around the Earth. His name is now used as a general synonym for all natural satellites of planets and smaller bodies.

Johannes Kepler confirmed Galileo's discoveries with his 1611 published in Frankfurt font narratio de observatis a se quatuor Iovis Satellitibus erronibus ( "Report on the four observed by him wandering companion of Jupiter" ) whilst supporting its conclusions. In this document, for the first time the Latin word satelles ( " Bodyguard ", in the plural satellites also " wake " ) for this type of celestial bodies used which called Galileo still Sidera Medicean ( " Medicean planets "). Christiaan Huygens used in its publication " Systema Saturnium " of 1659 for the Saturn discovered by him companion Titan first the word Luna (Moon).

German scholars of the 18th century referred to as moons "In addition to the planet," next to the " home planet ".

More details

In a planetary system of the respective main body runs along with the moon in orbit around the central star.

There is no known case in which a natural satellite has its own natural satellites, for such orbits within the Hill sphere of a satellite are not stable for long.

In total there are over 300 known natural satellites in the solar system.

Moons of planets

The planets of the solar system have a total of 173 previously known moons, two of which are larger ( but not massive ) than the planet Mercury. Of the eight planets have six one or more moons, the inner, terrestrial planets and the outer most two, Jupiter-like planets throughout many have.

The number in detail, specifying the largest specimens or those with special features ( in descending order according to their mass ):

  • Mercury - 0
  • Venus - 0
  • Earth - 1 moon ( in relation to its planet's largest known satellite)
  • Mars - 2: Phobos ( with the smallest distance from its planet ) and Deimos; both probably captured asteroids.
  • Jupiter - 67: inter alia, the four Galilean moons: Ganymede (the largest and richest massive moon in the Solar System), Callisto, Io ( moon with the highest density and the most volcanically active body ) and Europe; the little Metis has the highest known average path velocity.
  • Saturn - 62: inter alia, titanium ( the second largest and the only moon in the solar system with a dense atmosphere), Rhea, Iapetus ( body with the most contrasty Albedounterschieden ), Dione (with two Trojan moons ), Tethys ( also Trojan with two companions ), Enceladus, Mimas, Hyperion (the only known moon with chaotic rotation) and Phoebe (the only large moon with regular, but not bound rotation); the co-orbital satellites Janus and two small Epimetheus regularly exchange their orbit.
  • Uranus - 27: inter alia, Titania, Oberon, Ariel, Umbriel and Miranda ( zerklüftetster satellite)
  • Neptune - 14: inter alia Triton (largest decline moon), Proteus (the largest moon of irregular shape ) and Nereid ( most eccentric orbit ); the little Naiad has the shortest known orbital period and the small Neso the largest orbit with the longest orbital period and the smallest average path velocity.

Moons of extrasolar planets have not yet been observed. See: Extrasolar moon.

Moons of dwarf planets

Even with dwarf planets are known satellite ( by number order):

  • Pluto - 5: Charon (the largest dwarf planet moon), Nix, Hydra, Kerberos, and Styx
  • Haumea - 2: Namaka and Hiiaka
  • Eris - 1: Dysnomia

Satellites of asteroids

Even some asteroids have satellites. There are currently over 100 asteroids with one to two satellites known - a total of 134 such satellites ( as of 11 May 2007). The smallest two satellites at all under the objects 2003 and 2004 SS84 DC with a diameter of about 60 meters. The Galileo spacecraft photographed in 1993 for the first time an asteroid moon, the satellite Dactyl of the asteroid Ida. 1999 could be detected by a ground-based telescope at Eugenia first time an asteroid satellite.

Satellites of comets

The discovery of the announced in 2006 satellite Echidna of the asteroid Typhon has the question of possible satellites of comets raised, because Typhon belongs to the asteroid class of the Centaurs, with highly elliptical orbits in the outer solar system, and which are considered likely " extinct " and some also than anything else active comet nuclei. The distinction between comets and ice-rich but distant sun and that there was therefore inactive asteroid is also substantial ambiguous. A "real comet moon " has not yet been found.