Natural satellite

A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Enceladus and Triton both have active features resembling geysers, although in the case of Triton solar heating appears to provide the energy.

Some authors consider the Pluto-Charon system to be a double (dwarf) planet. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify non-artificial satellites of planets, dwarf planets, and minor planets. As of September 2008, 335 bodies are formally classified as moons.

Io is the most volcanically active body in the Solar System, while Europa, Enceladus, Titan and Triton display evidence of ongoing tectonic activity and cryovolcanism. The Pluto-Charon system is unusual in that the center of mass lies in open space between the two, a characteristic of a double planet system. The natural satellites orbiting relatively close to the planet on prograde orbits (regular satellites) are generally believed to have been formed out of the same collapsing region of the protoplanetary disk that gave rise to its primary.

The nineteen bodies massive enough to have achieved hydrostatic equilibrium are in bold in the chart below and labeled on the chart at right, though a few of the smaller ones are not visible at the scale of the chart. In addition to the moons of the various planets there are also over 80 known moons of the dwarf planets, asteroids and other small solar system bodies.

Minor planets suspected but not proven to have achieved a hydrostatic equilibrium are italicized in the table below. The first known natural satellite was the Moon (luna in Latin). For example, Jupiter s moon Himalia, Saturn s moon Phoebe, and Neptune s moon Nereid have rotation period in the range of ten hours, while their orbital periods are hundreds of days. No moons of moons (natural satellites that orbit the natural satellite of another body) are known.

The only known exception is Saturn s moon Hyperion, which rotates chaotically because of the gravitational influence of Titan. In contrast, the outer moons of the gas giants (irregular satellites) are too far away to have become locked. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process.

For smaller moons see the articles on the appropriate planet. These are predominately Greek, except for the Uranian moons, which are named after Shakespearean characters.

They include 167 orbiting six of the eight planets, 6 orbiting three of the five dwarf planets, 104 asteroid moons, and 58 satellites of Trans-Neptunian objects, some of which will likely turn out to be dwarf planets. Every body with an identified orbit, some as small as a kilometer across, has been identified as a moon, though objects a tenth that size within Saturn s rings, which have not been directly observed, have been called moonlets.

He derived the term from the Latin word satelles, meaning guard , attendant , or companion , because the satellites accompanied their primary planet in their journey through the heavens. As additional moons of Saturn were discovered the term moon was abandoned. It has been suggested that a few moons, notably Europa, one of Jupiter s Galilean moons, may harbour life, though there is currently no direct evidence to support this claim. Among the dwarf planets, Ceres has no moons (though many objects in the asteroid belt do).

Saturn has an additional six mid-sized moons massive enough to have achieved hydrostatic equilibrium, and Uranus has five. Until the discovery of the Galilean satellites in 1610, however, there was no opportunity for referring to such objects as a class.

When it is necessary to avoid both the ambiguity of confusion with the Earth s moon on the one hand, and artificial satellites on the other, the term natural satellite (using natural in a sense opposed to artificial ) is used. There is no established lower limit on what should be considered a moon. Titan and Triton have significant atmospheres; Titan also has methane lakes, and presumably rain.

The terms man-made satellite or artificial moon were very quickly abandoned in favor of the simpler satellite, and as a consequence, the term has come to be linked primarily with artificial objects flown in space – including, sometimes, even those which are not in orbit around a planet. As a consequence of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with satellite, even in scientific articles. Planets around other stars are likely to have natural satellites as well, although none have been observed. The large gas giants have extensive systems of moons, including half a dozen comparable in size to Earth s moon: the four Galilean moons, Saturn s Titan, and Neptune s Triton.

When the masses of two orbiting bodies are similar enough that one cannot be said to orbit the other, they are described as a double body rather than primary and satellite. In 1957, however, the launching of the artificial object Sputnik created a need for new terminology.

In the first three cases, the geological activity is powered by the tidal heating resulting from having eccentric orbits close to their gas giant primaries. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition as to what constitutes a moon.

Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites. The following is a comparative table classifying the moons of the solar system by diameter. Galileo chose to refer to his discoveries as Planetæ ( planets ), but later discoverers chose other terms to distinguish them from the objects they orbited. Christiaan Huygens, the discoverer of Titan, was the first to use the term moon for such objects, calling Titan Luna Saturni or Luna Saturnia – Saturn s moon or The Saturnian moon , because it stood in the same relation to Saturn as the Moon did to the Earth. The first to use of the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iovis sattelitibus erronibus ( Narration About Four Satellites of Jupiter Observed ) in 1610.

Four of the largest moons, Europa, Ganymede, Callisto, and Titan, are thought to have subsurface oceans of liquid water, while smaller Enceladus may have localized subsurface water. Satellite orbits are called regular or prograde if they are in the same direction as the planet s rotation otherwise they are called irregular or retrograde (The term irregular can also refer to the shape of a satellite). Small asteroid moons, such as Dactyl, have also been called moonlets. The upper limit is also vague.

Haumea has two moons, and Eris has one. The moons of the planets are named after mythological figures.

In most cases, the tidal effects of the primary would make such a system unstable. However, calculations performed after the recent detection a phenomenon normally associated with shepherd moons. Two moons are known to have small companions at their L4 and L5 Lagrangian points, sixty degrees ahead and behind the body in its orbit. Some, such as 90 Antiope, are double asteroids with two comparably sized components. The largest natural satellites in the Solar System (those bigger than about 3000 km across) are Earth s moon, Jupiter s Galilean moons (Io, Europa, Ganymede, and Callisto), Saturn s moon Titan, and Neptune s captured moon Triton.

Giovanni Domenico Cassini sometimes referred to his discoveries as planètes in French, but more often as satellites. The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of moon . Most of the major moons in the solar system have regular orbits (Triton being the exception) while most of the small moons have irregular orbits The regular natural satellites in the solar system are tidally locked to their primaries, meaning that the same side of the moon always faces the planet.

The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting moons. Triton is another exception, which although large and in a close, circular orbit, is thought to be a captured dwarf planet. Of the nineteen known moons massive enough to have lapsed into hydrostatic equilibrium, several remain geologically active today.

Pluto has three known satellites, the rather large Charon and the smaller Nix and Hydra. In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions.

The column on the right includes some notable planets, dwarf planets, asteroids, and Trans-Neptunian Objects for comparison. (This mechanism would have also operated on Triton in the past, before its orbit was circularized.) Many other moons, such as Earth s Moon, Ganymede, Tethys and Miranda, show evidence of past geological activity, resulting from energy sources such as the decay of their primordial radioisotopes, greater past orbital eccentricities (due in some cases to past orbital resonances), or the differentiation or freezing of their interiors.

Some 150 additional small bodies were observed within Saturn s ring system, but they were not tracked long enough to establish orbits. The most common dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, as it relies on distance as well as relative mass. Earth The Moon Mars Phobos · Deimos Asteroid moons Binary asteroids List of asteroid moons Mimas · Enceladus · Tethys (Telesto • Calypso) · Charon · Nix · Hydra Namaka · Hiʻiaka Dysnomia .

The Trojan moons are Telesto and Calypso, which are the leading and following companions respectively of Tethys; and Helene and Polydeuces, the leading and following companions of Dione. The discovery of 243 Ida s moon Dactyl in the early 1990s confirmed that some asteroids have moons; indeed, 87 Sylvia has two. Of the inner planets, Mercury and Venus have no moons at all; Earth has one large moon, known as the Moon; and Mars has two tiny moons, Phobos and Deimos.

The Earth s Moon are exceptions among large bodies in that they are believed to have originated by the collision of two large proto-planetary objects (see the giant impact hypothesis). These companions are called Trojan moons, as their orbits are analogous to the Trojan asteroids of Jupiter.