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Saturn has 18 confirmed and named natural satellites: Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas, Enceladus, Tethys, Telesto, Calypso, Dione, Helene, Rhea, Titan, Hyperion, Iapetus, and Phoebe (in order of increasing distance from the planet). Twelve additional satellites, with orbits at least 9.3 million mi (15 million km) from the planet's surface, were reported in 2000 and 2001. If confirmed, the findings will raise the number of Saturn's satellites to 30. More than a dozen others have been reported and given provisional status; nine of these were derived from the analysis of data from the 1981 flyby of the Voyager 2 space probe, and seven were identified from Hubble Space Telescope photographs taken during the 1995 ring plane crossings. It is possible that some of these are actually the same as some of the known satellites or duplicates of one another. The data probably will not be sorted out until the arrival of the Cassini probe, which is designed to do a detailed study of Saturn, its rings, its magnetosphere, its icy satellites, and Titan.
All but two of Saturn's moons form a regular system of satellites; that is, their orbits are nearly circular and lie in the equatorial plane of the planet. The exceptions are Iapetus, whose orbit is inclined almost 15°, and Phoebe, whose orbit is inclined 175°. Except for Hyperion, which has a chaotic orbit, and Phoebe, all the satellites are believed to have synchronous orbits; that is, their orbital and rotational periods are the same so that they keep the same face turned toward Saturn, just as the moon keeps the same face turned toward the earth. The largest satellite, Titan, is 3,200 mi (5,150 km) in diameter and has the size and cold temperatures necessary to retain an atmosphere; it is the only natural satellite in the solar system with a substantial atmosphere.
Saturn has six major icy satellites that can be easily seen through earth-based telescopes. The most prominent feature of heavily cratered Mimas, the innermost of the six, is a large impact crater about one third the diameter of the satellite. Certain broad regions of Enceladus are uncratered, indicating geological activity that has somehow resurfaced the satellite within the last 100 million years. Tethys also has a very large impact crater, as well as an extensive series of valleys and troughs that stretches three quarters of the way around the satellite. Both Dione and Rhea have bright, heavily cratered leading hemispheres and darker trailing hemispheres with wispy streaks that are thought to be produced by deposits of ice inside surface troughs or cracks. Iapetus, the outermost of the large icy satellites, has a dark leading hemisphere and a bright trailing hemisphere.
The remaining eleven satellites, some sharing orbits with others, are smaller. The two largest of these, the dark-surfaced Phoebe and the irregularly shaped Hyperion, orbit far from the planet; the outermost satellite, Phoebe, orbits with retrograde motion, i.e., opposite to that of the planet's rotation. The smallest, ranging from c.12 to 20 mi (20 to 32 km) in diameter, are Pan and Atlas, the satellites closest to the planet, and Telesto, Calypso, and Helene. Prometheus and Pandora, c.55 mi (90 km) in diameter, share an orbit, as do Epimetheus and Janus.
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