Everything about Natural Satellite totally explained
A
natural satellite or
moon is a
celestial body that
orbits a
planet or smaller body, which is called the
primary. Technically, the term
natural satellite could refer to a planet orbiting a
star, or a
dwarf galaxy orbiting a major galaxy, but it's normally synonymous with
moon and used to identify non-artificial
satellites of planets,
dwarf planets, and
minor planets. (There are no known natural satellites of moons.)
Two hundred and forty bodies, all in the
Solar System, are classified as moons. They include 166 orbiting the eight planets, 4 orbiting dwarf planets, and dozens more orbiting
small solar system bodies. Other stars and their planets are very likely to have natural satellites, although none have yet 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. Saturn has an additional six mid-sized moons massive enough to have achieved
hydrostatic equilibrium, and
Uranus has five. Of the inner planets,
Mercury and
Venus have no moons at all; Earth has one large moon (the
Moon); and
Mars has two tiny moons,
Phobos and
Deimos. Among the dwarf planets,
Ceres has no moons (though many objects in the asteroid belt do),
Eris has one,
Dysnomia, and
Pluto has three known satellites,
Nix,
Hydra, and a large companion called
Charon. 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.
Origin
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. 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 Earth's Moon and possibly Charon are exceptions among large bodies in that they're believed to have originated by the collision of two large proto-planetary objects (see the
giant impact hypothesis). 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. As opposed to planetary-sized bodies,
asteroid moons are thought to commonly form by this process. Triton is another exception, which although large and in a close, circular orbit, is thought to be a captured
dwarf planet.
Geological activity
Of the nineteen known moons massive enough to have lapsed into hydrostatic equilibrium, several remain geologically active today.
Io is the most volcanically active body in the Solar System, while
Triton and
Enceledus have geysers.
Titan and Triton have significant atmospheres; Titan also has
methane lakes, and presumably rain. 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. Many other moons, such as
Tethys, show evidence of past geological activity.
Orbital characteristics
Tidal locking
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 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. 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.
Satellites of satellites
No moons of moons (natural satellites that orbit the natural satellite of another body) are known. In most cases, the tidal effects of the primary would make such a system unstable.
However, calculations performed after the recent detection of a possible
ring system around Saturn's moon
Rhea indicate that Rhean orbits would be stable. Furthermore, the suspected rings are thought to be narrow, a phenomenon normally associated with shepherd moons.
Trojan satellites
Two moons are known to have small companions at their L
4 and L
5 Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions are called
Trojan moons, as their orbits are analogous to the
Trojan asteroids of
Jupiter. 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.
Asteroid satellites
The discovery of
243 Ida's moon
Dactyl in the early 1990s confirmed that some
asteroids have moons; indeed,
87 Sylvia has two. Some, such as
90 Antiope, are double asteroids with two comparably sized components.
Natural satellites of the Solar System
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. For smaller moons see the articles on the appropriate planet. 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. 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. The column on the right includes some notable planets, dwarf planets, asteroids, and Trans-Neptunian Objects for comparison.
It is normal for natural satellites to be named after mythological figures, (predominately Greek), however
Uranus's moons are named after Shakespearean characters. The nineteen bodies massive enough to have achieved
hydrostatic equilibrium are in bold. Those suspected but not proven to have achieved a hydrostatic equilibrium are italicized.
Terminology
The first known natural satellite was the
Moon (
luna in
Latin). Until the discovery of the
Galilean satellites in 1610, however, there was no opportunity for referring to such objects as a class.
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.
As additional moons of Saturn were discovered, however, this term was abandoned.
Giovanni Domenico Cassini sometimes referred to his discoveries as
planètes in
French, but more often as
satellites, using a term derived from the Latin
satelles, meaning "guard", "attendant", or "companion", because the
satellites accompanied their primary planet in their journey through the heavens.
The term
satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, however, the launching of the artificial object
Sputnik created a need for new terminology. 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. When it's 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.
The definition of a moon
moonlets. Small asteroid moons, such as
Dactyl, have also been called moonlets.
The upper limit is also vague. When the masses of two orbiting bodies are similar enough that one can't be said to orbit the other, they're described as a
double body rather than primary and satellite.
Asteroids such as
90 Antiope are considered double asteroids, but they've not forced a clear definition as to what constitutes a moon. Some authors consider the Pluto-Charon system to be a double (dwarf) planet. 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.
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