Natural satellite
A natural
satellite is a non-
man-made object that orbits a
planet or other body larger than itself. They are often called moons. The term
natural satellite may also refer to a
planet orbiting a
star, as is the case with planets orbiting the
Sun and even the
Sun itself as it orbits the galactic center of the
Milky Way. There are 240 known moons within the
Solar system, including 162 orbiting the planets , 4 orbiting
dwarf planets, and dozens more orbiting small solar system bodies. Other stars and their planets also have natural satellites.
Encyclopedia
A
natural satellite is a non-
man-made object that orbits a
planet or other body larger than itself. They are often called
moons. The term
natural satellite may also refer to a
planet orbiting a
star, as is the case with planets orbiting the
Sun and even the
Sun itself as it orbits the galactic center of the
Milky Way. There are 240 known moons within the
Solar system, including 162 orbiting the planets , 4 orbiting
dwarf planets, and dozens more orbiting small solar system bodies. Other stars and their planets also have natural satellites.
The large
gas giants have extensive systems of moons, including half a dozen comparable in size to Earth's moon. Of the inner planets, Mercury and
Venus have no moons at all; Earth has one large moon ; and
Mars has two tiny moons, Phobos and Deimos. Among the
dwarf planets,
Ceres has no moons ,
Eris has one, Dysnomia, and
Pluto has three known satellites,
Nix, Hydra, and a large companion called Charon. Charon is also unusual in that it does not orbit Pluto, but rather both bodies orbit a
barycenter in empty space, making Pluto-Charon a binary system.
Origin
Moons orbiting relatively close to the planet on prograde orbits are believed to have been formed out of the same collapsing region of
protoplanetary disk that gave rise to its primary. Most
irregular satellites are thought to be captured asteroids possibly further fragmented by collisions.
Many special scenarios exist including including origin in a break-up of the planet. For example in the case of Earth's
Moon a portion of the planet itself blasted into orbit by a large impact.
A very different scenario required to explain the origin of the large Neptunian satellites: circular but retrograde orbit of Triton and extremely eccentric but prograde orbit of Nereid.
Orbital characteristics
Tidal locking
Most regular moons in the solar system are tidally locked to their primaries, meaning that one side of the moon is always turned toward the planet. Exceptions include
Saturn's moon Hyperion, which rotates chaotically because of a variety of external influences.
In contrast, the outer moons of the gas giants are too far away to become 'locked'. For example Jupiter's moon Himalia, Saturn's moon Phoebe and Neptune's Nereid have rotation period in the range of 10 hours to compare with their orbital periods of hundreds of days.
Satellites of satellites
No "moons of moons" are known. It is uncertain whether such objects can be stable in the long term. In most cases, the tidal effects of their primaries make such a system unstable; the gravity from other nearby objects would perturb the orbit of the moon's moon until it broke away or impacted its primary. In theory, a secondary satellite could exist in a primary satellite's Hill sphere, outside of which it would be lost due to the greater gravitational pull of the planet that the primary satellite orbits. For example, the Moon orbits the Earth because the Moon is 370,000 km from Earth, well within Earth's Hill sphere, which has a radius of 1.5 million km . If a Moon-sized object were to orbit the Earth outside its Hill sphere, it would soon be captured by the Sun and become a
dwarf planet in a near-Earth orbit.
Trojan satellites
However, two moons are known have small companions in their L
4 and L
5 Lagrangian points of their orbits, about sixty degrees ahead of and behind their positions. These companions are called Trojan moons, because their positions are comparable to the positions of the
Trojan asteroids relative to
Jupiter. They are Telesto and Calypso, the leading and following companions respectively of Tethys; and Helene and Polydeuces, the leading and following companions of Dione).
Asteroid satellites
The recent discovery of
243 Ida's moon Dactyl confirms that some
asteroids also have
moons. Some, like
90 Antiope, are double asteroids with two equal-sized components. The asteroid 87 Sylvia has two moons. See
asteroid moon for further information.
Moons of the Solar system
The largest moons in the solar system are Earth's
Moon,
Jupiter's
Galilean moons ,
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.
The definition of a moon
There has been some debate about the precise definition of a moon. This debate has been caused by the presence of orbital systems where the difference in mass between the larger body and its satellite, are not as pronounced as in normal systems . Two examples include the Pluto-Charon system, and the Earth-Luna System. The presence of these systems has caused a debate about where to precisely where to draw the line between a
double body system, and a main body-satellite system, the most commonly agreed upon definition is whether the barycentre is below the surface of the larger body, though this is not official. Another defintion is whether the
Sun or the larger body in the system has the most gravitational influence over the satellite in question. At the other end of the spectrum there are many ice/rock clumps that form the ring systems on the
Solar System's gas giants and there is no set point to define when one of these clumps is large enough to be classified as a moon, though the term moonlet may be sometimes used to refer to extremely small objects in orbit around a larger body, there is again no official definition.
See also
Natural satellites of planets
Notes and references
External links
Jupiter's moons
Saturn's moons
Neptune's moons
All moons
