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Inner moon
In astronomy, an inner moon is a natural satellite following a prograde, low-inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed ''in situ'' at the same time as the coalescence of the original planet. Neptune's moons are an exception, as they are likely reaggregates of the pieces of the original bodies, which were disrupted after the capture of the large moon Triton.〔 Inner satellites are distinguished from other regular satellites by their proximity to the parent planet, their short orbital periods (usually under a day), their low mass, small size, and irregular shapes.
==Discovery==
Thirty inner satellites are currently known, found orbiting around all four of the giant planets (Jupiter, Saturn, Uranus and Neptune). Because of their small size, and glare from the nearby planet, they can be very difficult to observe from Earth. Some, such as Pan and Daphnis at Saturn, and Naiad at Neptune, have only ever been observed by spacecraft.
The first inner satellite to be observed was Amalthea, discovered by E. E. Barnard in 1892. Next were the Saturnian moons Epimetheus and Janus, observed in 1966. These two moons share the same orbit, and the resulting confusion over their status was not resolved until the ''Voyager 1'' flyby in 1980. Most of the remaining inner satellites were discovered by spacecraft ''Voyager 1'' and ''Voyager 2'' during their flybys of Jupiter (1979), Saturn (1980), Uranus (1986) and Neptune (1989).
The most recent discoveries have been two moons of Uranus (Mab and Cupid), found using the Hubble Space Telescope in 2003, and Daphnis found orbiting Saturn by the Cassini spacecraft in 2005.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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