Science Fair Project Encyclopedia
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. 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).
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 appropriate planets.
|50-100||Thebe||Pandora||Caliban||Thalassa||(Too many to list)|
|10-50||Phobos||Carme||Siarnaq||Ophelia||S/2002 N1||(Too many to list)|
|less than 10||Cruithne¹||At least 47, see Jupiter's natural satellites for a listing.||Erriapo||(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.
- Jupiter's natural satellites
- Saturn's natural satellites
- Uranus' natural satellites
- Neptune's natural satellites
- 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)
- Moons of the Solar System (The Planetary Society)
- Scott Sheppard's page
- JPL's Solar System Dynamics page
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