Phoebe [FEE-bee] is the last of the known satellites of Saturn, at a distance of 12.952 million kilometers (8 million miles). Phoebe orbits Saturn in a retrograde direction (opposite to the direction of the other satellites' orbits) in a plane much closer to the ecliptic than to Saturn's equatorial plane. Voyager 2 found that Phoebe has a roughly circular shape, and reflects about 6 percent of the sunlight. It also is quite red. Phoebe rotates on its axis about once every nine hours. Thus, unlike the other Saturnian satellites (except Hyperion), it does not always show the same face to the planet.
Scientists believe that Phoebe 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. These asteroids 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. Phoebe also might be the source of dark material on Iapetus.
Discovered by William Henry Pickering
Date of discovery 1898
Mass (kg) 4.0e+18
Mass (Earth = 1) 6.6934e-07
Equatorial radius (km) 110
Equatorial radius (Earth = 1) 1.7247e-02
Mean density (gm/cm^3) 0.7
Mean distance from Saturn (km) 12,952,000
Rotational period (days) 0.4
Orbital period (days) -550.48
Mean orbital velocity (km/sec) -1.71
Orbital eccentricity 0.1633
Orbital inclination (degrees) 175.3
Escape velocity (km/sec) 0.0697
Visual geometric albedo 0.06
Magnitude (Vo) 16.45
|View of Phoebe|
This image of Phoebe was acquired by the Voyager 2 spacecraft on September 4, 1981. (Courtesy Calvin J. Hamilton)
Map of Phoebe
This map of Phoebe is based upon pictures taken by Voyager 2 in 1981. As Voyager 2 did not get close to Phoebe, the best images were only about 11 pixels wide. From these images it was learned that Phoebe was a dark, 220 kilometers wide round object and with a period of rotation of about 9.4 hours. In 2004, if all goes well, the Cassini Orbiter will fly by Phoebe at a distance of about 50,000 kilometers before going into orbit around Saturn. The best images will have a resolution of about 350 meters per pixel. (Courtesy A. Tayfun Oner - adapted from a figure in an article by Simonelli and colleagues)
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