In astronomy, a satellite , also called a moon or a satellite , is a compact, naturally created astronomical object that is in an orbit around another, significantly more massive object. Natural satellites of planets are mostly called moons . The planet orbiting a moon is also called the mother planet . In the past, the moons of planets were also called minor planets .
The gravitation of a planet becomes noticeable as tidal forces on a moon orbiting it . The moon is slightly deformed. This causes the rotation of the moon to gradually adapt to its orbital frequency around the planet. The moon then always turns the same side to the planet. Conversely, the gravitation of a moon causes tidal forces on the planet and thus has repercussions on its rotation. Since the moon and planet differ significantly in mass, these tidal forces are generally not sufficient to align the rotation of the planet with the orbit of the moon.
The gravity of moons has a significant influence on the shape and stability of the ring system of a planet.
There are different ideas about the formation of natural satellites or moons depending on the characteristics of their size or their orbital properties. The best known are the approaches to the formation of the earth-moon system , which have been controversially discussed in the past - mainly either from a common accretion disk , by capture, by splitting off or as a result of a large-scale collision of protoplanets .
For the regular satellites of the gas planets, accretion from surrounding particles applies. For relatively particularly large satellites such as the Earth's moon and Pluto moon Charon , the notion of a major collision has gained the upper hand. For satellites that are circling backwards , that is, against the sense of rotation of the main body, the path of origin is assumed to be the capture of a body that was completely formed in a formerly independent orbit. For the moons of Mars ( Phobos and Deimos ) an origin as captured asteroids is assumed due to their very irregular shape and the proximity of the asteroid belt . In the case of small and very irregular asteroids, the speed of which is influenced by the YORP effect , this effect can, if accelerated over several million years, cause the celestial bodies to break apart and individual fragments to remain as satellites.
There are also different ideas about a possible sinking of satellites. One of the oldest of these is that of penetrating the Roche boundary of a planet. From this critical proximity, the tidal forces become so strong that larger satellites are torn apart by them. This view is best known as an explanation of the primary origin of Saturn's rings . There is also the theory that the planetary rings of the gas planets consist of residual material from the protoplanetary gas and dust disk , which could not aggregate into satellites within the Roche boundary. The ring systems consist of ice and dust of very fine particle size to house-sized chunks. There is no defined limit size for differentiating between the smallest moons and the largest ring chunks; the distinction is made purely conventionally, especially since the rings have not yet been photographically broken down into individual components.
Some individual rings within a ring system are apparently fed by a satellite: In the case of the Saturn moon Enceladus by rising fountains of a cryovolcanism , and by the moon Mab , whose orbit around Uranus coincides with the zone of highest density of the My ring, which is possibly caused by the impact of meteorite impacts . While the gravitational pull of larger celestial bodies ensures that most of the debris falls back onto the surface, it is not sufficient for small ones and, in addition to low-mass satellites, fine particles are also distributed in a ring in the orbit. As in the case of the two small Pluto moons Nix and Hydra , scientists suspect that the impacts of micrometeorites over an astronomical period of time can lead to a dust ring up to the abrasion of a smaller satellite.
After the invention of the telescope , the discovery of the Galilean moons in 1610 on Jupiter was the first observation of astronomical objects whose orbit clearly does not orbit the earth. This was seen as an argument against the geocentric and for the heliocentric worldview . In the course of further observations it was discovered that the moon is the only celestial body that really orbits the earth. In analogy to the Earth's moon, the larger objects orbiting other planets were also called moons .
Johannes Kepler confirmed Galileo's discoveries with his work Narratio de observatis a se quatuor Iovis Satellitibus erronibus (“Report on the four wandering companions of Jupiter observed by him”) published in Frankfurt in 1611 and at the same time supported his conclusions. In this script, the Latin word satelles ("bodyguard", in the plural satellites also "entourage") is used for this type of celestial body, which Galileo called Sidera Medicea ("Medicean stars"). In his publication Systema Saturnium from 1659, Christiaan Huygens was the first to use the word Luna (moon) for the Saturn companion Titan, which he discovered .
German scholars of the 18th and 19th centuries also referred to moons as "secondary planets", in addition to the "main planets".
A total of 557 natural satellites are currently known in the solar system . (As of February 4, 2019)
Moons of planets
The planets of the solar system have a total of 205 previously known moons. Two of them are larger, but not more massive than the planet Mercury . Of the eight planets, six have one or more moons, with the inner , Earth-like planets having one or two and the outer, Jupiter-like planets consistently having many.
The number in detail with details of the largest specimens and other special features (in descending order according to their mass ):
- Mercury - 0
- Venus - 0
- Earth - 1: Moon (the largest known satellite in relation to its planet)
- Mars - 2: Phobos (the closest to its planet) and Deimos ; both likely captured asteroids.
- Jupiter - 79: u. a. the four Galilean moons : Ganymede (largest and most massive moon in the solar system and the only one with a pronounced magnetic field), Callisto , Io (moon with the highest density and volcanically most active body) and Europe ; the small metis has the highest known mean speed on the path.
- Saturn - 82: u. a. Titan (second largest and only moon in the solar system with a dense atmosphere ), Rhea , Iapetus (body with the most contrasting albedo differences ), Dione (with two Trojan moons ), Tethys (also with two Trojan companions), Enceladus , Mimas , Hyperion (the only one known moon with chaotic rotation) and Phoebe (the only larger moon with regular, but not bound rotation ); the two small co-orbital satellites Janus and Epimetheus regularly exchange orbits.
- Uranus - 27: u. a. Titania , Oberon , Ariel , Umbriel, and Miranda (most rugged satellite)
- Neptune - 14: u. a. Triton (largest retrograde moon), Proteus (largest irregularly shaped moon) and Nereid (most eccentric orbit); the small Naiad has the shortest known orbital time and the small Neso the largest orbit with the longest orbital time and the lowest mean orbital speed.
Moons of dwarf planets
Satellites are also known for dwarf planets (sorted by number):
- Pluto - 5: Charon (largest dwarf planet moon), Nix , Hydra , Kerberos and Styx
- Haumea - 2: Namaka and Hiʻiaka
- Makemake - 1: S / 2015 (136472) 1
- Eris - 1: Dysnomia
Moons from asteroids
In addition to the dwarf planets , some asteroids also have satellites among the minor planets . So far, 345 asteroids, each with one or more satellites, are known - a total of 366 such satellites. The largest asteroid moon is currently Vanth of TNO (90482) Orcus with 443 km; other large asteroid moons are Ilmarë , Xiangliu , Actaea , Hiisi and Nunam . The companions of the near-Earth asteroids (162000) 1990 OS and (363067) 2000 CO 101 , each with a diameter of around 45 meters, are considered to be the two smallest satellites . In 1993 the Galileo probe accidentally photographed an asteroid moon for the first time, the satellite Dactyl of the asteroid Ida . In 1999 Eugenia was able to detect an asteroid satellite with an earth-based telescope for the first time .
Moons of comets
The discovery of the satellite Echidna of the asteroid Typhon , announced in 2006 , raised the question of possible satellites of comets , because Typhon belongs to the asteroid class of centaurs , with strongly elliptical orbits in the outer planetary system, and which are considered to be probably "extinct" and some as well as still somewhat active cometary nuclei. The distinction between comets and ice-rich, but far from the sun and therefore inactive asteroids is also essentially ambiguous. In 2017, the companion of (300163) 2006 VW 139 discovered the first moon whose mother body is classified as both an asteroid and a comet. A “pure cometary moon” has not yet been found.
Moons of moons
Moons orbiting a satellite of a planet are not yet known. Theoretically, these lunar moons are possible within the Hill sphere of the satellite, but the interference from tidal forces can severely limit the duration of this constellation. Therefore, only moons of moons with a large orbit to the planet and with correspondingly large mass ratios can be stable over a longer period of time. This would be possible (but not currently observed) in the solar system with the following moons: Earth-Moon, Callisto as well as Titan and Iapetus. For some exoplanets, however, such a constellation is believed to be possible, such as Kepler-1625b .
- Irregular satellite
- Double planet
- Extraterrestrial ocean
- Kordylevsk clouds
- Kozai effect
- Shepherd moon
- List of moons of planets and dwarf planets
- List of moons from asteroids
- List of satellite galaxies in the Milky Way
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- What is a moon
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- Asteroids / TNOs with satellites: by date reported
- Florian Freistetter: Can a moon have a moon? In: Spektrum.de. May 31, 2020, accessed June 5, 2020 .
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