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Satellites of outer planets
Holman, Matthew J. Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts.
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Each of the giant planets (Jupiter, Saturn, Uranus, and Neptune) hosts three types of natural satellites: regular satellites, collisional remnants, and irregular satellites. The regular satellites follow prograde orbits (in which they move in the same direction as the planets about the Sun) that are nearly circular and lie in the equatorial plane of their host at a distance of a few tens of planetary radii. Their sizes (about 1000 km or 600 mi) rival or exceed that of the Earth's Moon. The smaller (about 10–100 km or 6–60 mi) collisional remnants also trace nearly circular, equatorial orbits, but they inhabit regions only a few planetary radii in extent and are often associated with planetary rings. At these distances, the more intense meteoroid flux, gravitationally focused by the planet, has collisionally disrupted presumably once-larger bodies. The sizes of the known irregular satellites overlap those of the collisional remnants, but the irregular satellites follow extended (hundreds of planetary radii) orbits with significant eccentricities and inclinations. These distinct orbital characteristics suggest different formation mechanisms: The regular satellites and the progenitors of the collisional remnant satellites are believed to have formed in a circumplanetary accretion disk, while the irregular satellites are generally thought to have been captured from heliocentric orbits. Although the regular satellites and collisional remnants have been and continue to be well studied by spacecraft missions, investigations of irregular satellites have been hampered by the small number of them that are known. However, in the past few years dozens more have been discovered, enlarging the known number from 11 to nearly 50. The larger number of examples of such bodies enables more detailed study of the mechanisms of their capture.
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