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Saturn's Moon Phoebe
Saturn's moon Phoebe -- Phoebe is the outermost of Saturn's known moons. Phoebe is almost 4 times more distant from Saturn than its nearest neighbor (Iapetus). It was discovered by William Henry Pickering in 1898.
Most of Saturn's moons have very bright surfaces, but Phoebe's albedo is very low (.06), as dark as lampblack. All of Saturn's moons except for Phoebe and Iapetus orbit very nearly in the plane of Saturn's equator.
Phoebe's orbit is retrograde, inclined almost 175�, and is highly eccentric. The plane of its orbit is much closer to the solar system's ecliptic than it is to Saturn's.
Phoebe is also unusual in that it does not rotate synchronously as all the other moons of Saturn except Hyperion do.
All this suggests that it may be a captured asteroid with a composition unmodified since the time it was formed in the outer Solar System. It resembles the common class of dark carbonaceous asteroids, which are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula.
Since they are so small they never heated up sufficiently to change chemical composition. If this is the case Phoebe is the first such object that has been photographed at close enough range to show shape and surface brightness.
Material knocked off of Phoebe's surface by microscopic meteor impacts may be responsible for the dark surfaces of Hyperion and the leading hemisphere of Iapetus.
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Discovery
Discovered by William Henry Pickering
Discovered in 1898
Orbital characteristics
Semimajor axis 12,952,000 km
Eccentricity 0.163
Orbital period 550d 11h 31m
Inclination 175.3�
Physical characteristics
Radius 115 x 110 x 105 km
Mass 7.2�1018 kg
Mean density 2.3 g/cm3
Surface gravity 0.039m/s2
Rotation period 9h 30m
Axial tilt 26.183�
Albedo 0.06
Atmosphere none
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NASA
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