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Natural satellite

The term moon (never capitalized) is used to mean any natural satellite of the other planets. There are, at least, 100 moons within Earth's solar system, and presumably many others orbiting the planets of other stars. Typically the larger gas giants have extensive systems of moons. Mercury and Venus have no moons at all, Earth has one large moon, Mars has two tiny moons, and Pluto a large companion called Charon (sometimes considered to be a double planet).

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 foreign objects, fragments of larger moons shattered by large 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 distant observations through probes or telescopes, most theories about them are still uncertain.

Most moons in the solar system are tidally locked to their primaries; an exception is Saturn's moon Hyperion, which rotates chaotically due to a variety of external influences. No moons have moons of their own; the tidal effects of their primaries make orbits around them unstable. However, several moons have companions in their Lagrangian points (eg, Saturn's moons Tethys and Dione).

The recent discovery of Ida's moon Dactyl confirms that some asteroids also have moons.

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 appropriate planets.

A comparative table classifying the moons of the solar system by diameter, also including a column for some notable asteroids:

Diameter(km) Earth Mars Asteroids Jupiter Saturn Uranus Neptune Pluto
5000+


Ganymede Titan


4000-5000


Callisto



3000-4000 Luna

Europa
Io




2000-3000





Triton
1000-2000

1 Ceres
Iapetus
Rhea
Dione
Tethys
Ariel
Umbriel
Titania
Oberon

Charon
500-1000

4 Vesta
2 Pallas





100-500

(Too many to list) Amalthea Phoebe
Hyperion
Enceladus
Mimas
Janus
Epimetheus
Sycorax
Miranda
Puck
Portia
Proteus
Nereid
Larissa
Galatea
Despina

50-100

(Too many to list) Himalia
Thebe
Pandora
Prometheus
Setebos[?]
Prospero
Stephano
Caliban
1986U10
Belinda
Rosalind
Juliet
Desdemona
Cressida
Bianca
Cordelia
Ophelia
Thalassa
Naiad

10-50
Phobos
Deimos
(Too many to list) Sinope
Pasiphae
Carme
Ananke
Elara
Lysithea
Leda
Adrastea
Metis
Helene
Calypso
Telesto
Atlas
Pan




Solar system:
Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets



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