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The battered surface of Phoebe as viewed by the Cassini spacecraft in June 2004.
Click on image for full size
Image courtesy of NASA/JPL/Space Science Institute.

Phoebe

Phoebe is a medium-sized moon of Saturn. An American astronomer named William Henry Pickering discovered Phoebe in 1898. The name "Phoebe" comes from Greek mythology. In the Greek pantheon, Phoebe was the daughter of Uranus and Gaia and the grandmother of Apollo and Artemis.

Phoebe has a diameter of 220 km (137 miles), making it the ninth largest of Saturn's moons. Phoebe's albedo is around 0.08, which means the moon's surface is very dark. This is unusual, for most of Saturn's moons are quite bright. Some of this dark surface material may have been blasted into space by micrometeorite impacts and may have later settled on Hyperion and the leading hemisphere of Iapetus, darkening the surfaces of those moons.

Phoebe orbits much further from Saturn than do any of Saturn's other medium-sized moons. At a distance of nearly 13 million km (8 million miles) Phoebe is almost four times as distant from the ringed planet as is the next furthest medium-sized moon, Iapetus. Phoebe's orbit is also odd because it is retrograde - the moon moves around Saturn in a direction opposite the planet's rotation and the direction that most of Saturn's other moons orbit. Finally, Phoebe's rotation is also unusual. Most of Saturn's moons are locked into synchronous rotation periods - they turn on their axis once per orbit, always keeping one side pointed towards their home planet (as does Earth's moon). Of Saturn's medium-sized or larger moons, only Phoebe and Hyperion are not locked into synchronous rotation. Phoebe spins on its axis about once every 9.4 hours and orbits Saturn once every 548 days.

Astronomers believe, based on Phoebe's strange surface and orbital motion, that Phoebe may be an asteroid or Kuiper Belt Object that was captured into orbit around Saturn by the large planet's strong gravitational field. Our only good pictures of Phoebe, taken by the Voyager 2 spacecraft in September 1981 from a distance of 2.2 million km (1.4 million miles), are quite fuzzy and show few details. Scientists are anxious to get a better view of this strange moon. Hopefully they will get one soon, for the Saturn-bound Cassini spacecraft is scheduled to make a close flyby of Phoebe at a range of just 2,000 km (1,243 miles) on June 11, 2004.


Table of Moons of Our Solar System

Image of the Medium-sized Moons of Saturn

Last modified October 8, 2009 by Randy Russell.

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