Phoebe

With the exception of Iapetus and Phoebe, Saturn's satellites all travel in circular prograde orbits coplanar with both the planet's equator and the ring system. Phoebe has an elliptical, inclined and retrograde orbit. Voyager 2's best imagery from a range of 2.2 million kilometres had a resolution of 20 kilometres per pixel, just enough to establish that this moon is spheroidal with a diameter of about 220 kilometres, rotates in 9.5 hours in a prograde manner, is cratered, and that while its mean albedo is 6 per cent there are several relatively 'bright' patches.129 Neither Voyager flew sufficiently close for the perturbation of its trajectory to yield an estimate of the moon's mass. The fact that its orbit is almost in the plane of the ecliptic strongly implies that it is a captured asteroid or comet, and as such it is a respectable size. Its surface, which is much darker than any of the icy Saturnian satellites,130 has spectral traits indicative of'primitive' carbonaceous material; hence, if it is an asteroid, it is a 'C' type relic of the solar nebula. Apart from short periods spent in Saturn's magnetotail, Phoebe spends almost all of its time in the solar environment.

As the Space Age dawned, it was reasoned that stray asteroids were unlikely to be able to reach Saturn without being intercepted by Jupiter. It was thought that this was why several families of moonlets only a few dozen kilometres in diameter had been spotted in irregular orbits around Jupiter whereas none had been found orbiting Saturn.131 Nevertheless, it was acknowledged that Saturn was farther from

In recent years, many moonlets have been discovered in irregular orbits ranging far from Saturn, several of which pursue retrograde orbits which suggest that they are fragments chipped off Phoebe by a major impact. (Adapted from 'Saturn saturated with satellites', D.P. Hamilton. Nature, vol. 142, p. 132, 2001.)

In recent years, many moonlets have been discovered in irregular orbits ranging far from Saturn, several of which pursue retrograde orbits which suggest that they are fragments chipped off Phoebe by a major impact. (Adapted from 'Saturn saturated with satellites', D.P. Hamilton. Nature, vol. 142, p. 132, 2001.)

the Sun and if such tiny objects were present they would be difficult to identify. In recent years, however, advances in detector technology and automated analysis have prompted a rash of such discoveries.132 In one year, a dozen tiny moons were spotted in irregular orbits ranging far from Saturn. However, the fact that these split into three or four groups implied that they are fragments of a small number of objects that strayed into the Saturnian realm and were subsequently disrupted. Interestingly, one group has retrograde orbits which suggest that they are fragments chipped off Phoebe by a major impact. The 'bright' areas on Phoebe may mark the sites of these impacts.

If ever humans explore the Saturnian system, Phoebe will make a superb vista point, its inclined orbit offering the most spectacular views of the ring system. In the event that its retrograde motion proves inconvenient, then Hyperion will serve almost as well. A base on Titan would not offer much of a view, at least not in the sky. Although bland, Saturn would be overwhelmingly large in the sky from one of the inner satellites, and because they travel in the ring plane they would not offer much of a view of the ring system, which would form a bright line crossing in front of and projecting out to each side of the globe. However, until the first humans settle the Saturnian system, we will have to rely upon robotic probes to explore it by proxy.

A montage of the Saturnian system using Voyager imagery.

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