Natural satellite

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Image:Moons of solar system.jpg

The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. There are at least 140 moons within Earth's solar system, and presumably many others orbiting the planets of other stars.

The large gas giants have extensive systems of moons. Mercury and Venus have no moons at all, Earth has one large moon ("The Moon"), Mars has two tiny moons, and Pluto has a large companion called Charon (Pluto and Charon are sometimes considered a double planet).

Contents 1 Origin 2 Orbital characteristics 3 Moons of the Solar system 4 See also 5 External links 5.1 Jupiter's moons 5.2 Saturn's moons 5.3 Neptune's moons 5.4 All moons

Origin

Most moons are assumed to have been formed out of the same collapsing region of protoplanetary disk that gave rise to its primary. However, there are many exceptions and variations to this standard model of moon formation that are known or theorized. Several moons are thought to be captured asteroids; others may be fragments of larger moons shattered by impacts, or (in the case of Earth's Moon) a portion of the planet itself blasted into orbit by a large impact. As most moons are known only through a few observations via probes or telescopes, most theories about their origins are still uncertain.

Orbital characteristics

Most 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 are Saturn's moon Hyperion, which rotates chaotically due to a variety of external influences, and the outermost moons of the gas giants, which are too far away to become 'locked' (an example is Saturn's moon Phoebe).

It is not possible for a moon to have moons of its own: the tidal effects of their primaries would make such a system unstable. However, several moons have small companions in the Lagrangian points of their orbits (e.g., Saturn's moons Tethys and Dione).

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 (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.

The following is a comparative table classifying the moons of the solar system by diameter. The column on the right includes some notable planets, asteroids and Kuiper belt objects for comparison.

Diameter(km)	Earth	Mars	Jupiter	Saturn	Uranus	Neptune	Pluto	Other objects
5000-6000			Ganymede	Titan
4000-5000			Callisto					Mercury
3000-4000	Luna		Io Europa
2000-3000						Triton		2003 UB313 Pluto
1000-2000				Rhea Iapetus Dione Tethys	Titania Oberon Umbriel Ariel			90377 Sedna 90482 Orcus 50000 Quaoar 20000 Varuna 28978 Ixion
100-1000			Himalia Amalthea	Enceladus Mimas Hyperion Phoebe Janus Epimetheus Prometheus	Miranda Sycorax Puck Portia	Proteus Nereid Larissa Galatea Despina	Charon	1 Ceres 2 Pallas 3 Juno 4 Vesta (and many others)
50-100			Thebe Elara Pasiphaë	Pandora	Caliban Juliet Belinda Cressida Rosalind Desdemona Bianca	Thalassa Naiad S/2002 N 4		(Too many to list)
10-50		Phobos Deimos	Carme Metis Sinope Lysithea Ananke Leda Adrastea	Siarnaq Atlas Helene Albiorix Telesto Pan Paaliaq Calypso Ymir Kiviuq Tarvos Ijiraq	Ophelia Cordelia Setebos Prospero Stephano S/1986 U 10 S/2001 U 2 S/2001 U 3 S/2003 U 3 Trinculo S/2003 U 1 S/2003 U 2	S/2002 N1 S/2002 N 2 S/2002 N 3 S/2003 N 1		(Too many to list)
less than 10	Cruithne¹		At least 47, see Jupiter's natural satellites for a listing.	Erriapo Narvi Skathi Mundilfari Suttungr Thrymr Pallene Polydeuces Methone S/2004 S 3 Daphnis				(Too many to list)

¹ It is debatable whether Cruithne counts as a real moon; it is mainly placed here for comparison's sake.

In addition to the moons of the various planets there are also over 30 known asteroid moons, asteroids that orbit other asteroids.

See also

• Jupiter's natural satellites
• Saturn's natural satellites
• Uranus' natural satellites
• Neptune's natural satellites

• Timeline of natural satellites
• Naming of natural satellites

• Quasi-satellite

External links

Jupiter's moons

• Data on Jupiter's satellites
• Jupiter's new moons (discovered in 2000)
• Jupiter's new moons (discovered in 2002)
• Jupiter's new moons (discovered in 2003)

Saturn's moons

• Saturn's new moons (discovered in 2000)
• Saturn's new moon (discovered in 2003)

Neptune's moons

• Neptune's new moons (discovered in 2003)

All moons

• Moons of the Solar System (The Planetary Society)
• Scott Sheppard's page
• JPL's Solar System Dynamics page
• Moon of an Object? First Photo of Satellite Beyond the Solar System

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