Natural Satellite

The major natural satellites of the solar system to scale relative to the Earth.

A natural satellite is an object that orbits around a planet or other object larger than itself and not of human origin, as opposed to artificial satellites. They can be of great size and look like small planets. Such objects are also called moons, by analogy with the Moon , the natural satellite of Earth.

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

Origin

It is assumed that natural satellites orbiting relatively close to a planet on an orbit prograde formed in the same region of protoplanetary disk at the origin of this planet. In contrast, irregular satellites (generally orbiting on distant orbits, inclined , eccentric or retrograde ) would be captured foreign objects and possibly fragmented during collisions.

The birth of a satellite

We can cite three causes for the creation of a satellite:

• Accretion :

During the formation of a planet, there are pieces of rock, ice and dust swirling gases in a disk around. The bits of rock combine to form a clot, which under the impact of other rock fragments creates a sphere of rock that grows and absorbs any lump neighbors. She ended up dominating the disc and left alone in orbit, giving rise to a satellite.

• Capture :

The most encountered: it takes two asteroids (rarely one), orbiting one another are approaching near enough to a planet's gravitational field is not negligible. Therefore, the closest asteroid or heavier and perhaps Pluto-Charon would derive their origin from the collision of two large proto-planetary objects. The ejected material orbiting the central body would have formed one or more objects by accretion. We also think that satellites of asteroids are formed mainly by this process.

Definition

The term "satellite" has no scientific definition. In particular, the existence of couples Pluto - Charon and Earth - Moon , where the mass ratio between the central and satellite is not as pronounced as in most other systems, makes it difficult to set a limit between a satellite system a double planet. A common definition assumes that a satellite system must have a centroid located beneath the surface of the body wider, but it is not official and is arbitrary.

At the other end of the scale, systems ring around the outer planets of the solar system are composed of small pieces of ice and rock and there is no size limit setting from which this piece is large enough to be considered a satellite of its own.

• 

  Comparison of Earth and Moon.

• 

  Comparison of Jupiter and its four largest satellites.

Terminology

The first known natural satellite was the Moon. Until the discovery of the Galilean satellites in 1610, no opportunity was thus presented to characterize such objects. Galilee chooses for its part, the Latin term planet ("Planets") to designate them. That Kepler who appoint "satellites" in 1611, Latin satellites meaning "guardian" or "companion", the satellite seems to accompany the world in his travels.

Christian Huygens , the discoverer of Titan , was the first to use the word "moon" for this type of object, calling Saturn or Titan Luna Luna Saturnia ("Saturn's moon" or "the Saturnian moon").

Over the discoveries, the term was abandoned, Jean-Dominique Cassini sometimes used the term "planet" for his discoveries, but more often than "satellites."

The term "satellite" became the standard for describing an object in orbit around a planet, to avoid the ambiguity of "moon". However, in 1957, the launch of Sputnik 1 , the first artificial object in orbit around the Earth, made it necessary to distinguish between artificial satellites and natural satellites. The simple term "satellite" tended to refer primarily to man-made objects and the word "moon" was often used again, but, for example, still called 'satellite' new body detected around the giant planets of the solar system (their provisional designation is "S" followed by number) or asteroids.

Moons of the Solar System

General

Currently (April 2007), about 240 known moons in the solar system. 166 satellites orbiting the planets of the solar system: Jupiter into account 63, Saturn 60, Uranus 27 Neptune 13, March 2 and the Earth 1.
6 known moons orbiting dwarf planets. Dozens more have been discovered around asteroids and other small bodies. Some studies estimate that 15% of trans-Neptunian objects have at least one satellite.

The largest moons in the solar system (with a diameter greater than 3 000 km) are the Moon from Earth, the Galilean moons of Jupiter ( Io , Europe , Ganymede and Callisto ), Titan , moon of Saturn , and Triton , the moon of Neptune. All these moons are larger than Pluto. Ganymede and Titan are larger than Mercury , the smallest of the planets of the solar system.

The gas giants have entire systems of natural satellites, of which half have a size comparable to the Moon. Among the inner planets , Mercury and Venus have no satellites, the Earth has a single large (the Moon) and Mars two tiny moons ( Phobos and Deimos ). Among the dwarf planets, Ceres has none (unlike many other objects in the asteroid belt ), Eris has one; Haumea two; Pluto three ( Nix , Hydra , Charon ) (90482) Orcus one; and (50000) Quaoar one.

Synchronous Rotation

Most moons are similar in synchronous rotation with the body around which it rotates, which means they turn on themselves so long they make a complete revolution around the planet and thus have always same face to the world (eg the case of the Moon ). The exceptions include Hyperion , a moon of Saturn, rotates chaotically because of several influences.

In contrast, outer satellites of gas giants are too remote to be in synchronous rotation. For example, Himalia (moon of Jupiter), Phoebe (moon of Saturn) and Nereid (moon of Neptune) have a rotation period of 10 h and an orbital period of hundreds of days.

Satellite Satellite

There are no known natural satellite of another natural satellite. It is unclear whether such objects are stable in the long term. In most cases, the tidal effects caused by the primary would make such an unstable system. In theory, a secondary satellite could exist inside the Hill sphere of a primary satellite, but no such object has been detected. Research was conducted to find a satellite of the Moon , without success .

But even if no natural satellite has been discovered around another satellite objects along a path of quasi-satellite can orbit a satellite around temporarily. This was made the Soviet Union with their probe Phobos 2 in 1989 around the Martian satellite of the same name .

The co-orbital satellites

Two small moons have companions in their Lagrangian point L ₄ and L _5, called co-orbital moons by analogy with the Trojan asteroids of Jupiter :

• Telesto precedes Tethys , while Calypso follows.
• Helene precedes Dione and Pollux follows;

However, only the satellites Saturn present a case of resonance between them. During the twentieth century, research has been inconclusive in finding such cases with the Moon .

Satellites of asteroids

An asteroid moon is an asteroid orbiting another asteroid. It is generally assumed that they are formed by the debris of an impact involving the primary asteroid. Other systems may have been formed by small objects captured by the gravity of a larger body. In the early 1990s, the discovery of the small moon Dactyl orbiting (243) Ida confirmed that the asteroids may also have natural satellites. Some body like (90) Antiope , are double asteroids formed from two components of similar size. (87) Sylvia even has two satellites, Romulus and Remus. Asteroid moons are more common, we begin to discover triple systems .

List

The following table lists all the moons in the solar system by mean diameter and the body around which they orbit. The right column includes some other notable objects (planets, dwarf planets, asteroids, trans-Neptunian) for comparison.

See also

Internal Links

• Trojan asteroid
• Chronology of discovery of natural satellites
• Satellite galaxy
• Asteroid moon
• Co-orbital moon
• Extrasolar moon
• Quasi-satellite
• Inner satellite
• Irregular satellite
• Moons by object:
  • Moons of Mars
  • Moons of Jupiter
  • Moons of Saturn
  • Moons of Uranus
  • Neptune's natural satellites
  • Moons of Pluto

References

Science & Vie Junior, The moons of the solar system: Formation of the satellites, Mathilde Fontes, Paris, 1913, ^No. 230, November 2008

Science & Vie Junior, bing bong The moon, Fabrice Nicot, Paris, 1913, ^No. 162, March 2003

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