This image shows two sides (hemispheres) of Saturn's moon Phoebe. Voyager 2 took these images on September 4, 1981, when it was 2.2 million km (1.36 million miles) from the moon. Image courtesy NASA/JPL.

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.

This picture shows two different views of Saturn's moon Phoebe. Voyager 2 took these images on September 4, 1981. Image courtesy NASA/JPL.

Phoebe is the name of 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 myths, Phoebe was the daughter of Uranus and Gaia and the grandmother of Apollo and Artemis.

Phoebe has a diameter of 220 km (137 miles). It is the ninth largest of Saturn's moons. The surface of Phoebe is very dark. That is strange. Most of Saturn's moons have bright surfaces. Its dark surface isn't the only strange thing about Phoebe.

Phoebe's orbit around Saturn is also odd. Phoebe orbits much further from Saturn than any of the other medium-sized moons. Also, Phoebe orbits backwards! It moves around Saturn in the opposite direction of most of the other moons.

Why is Phoebe so strange? Many astronomers think Phoebe used to be an asteroid or Kuiper Belt Object. They think Saturn's strong gravity captured Phoebe sometime long ago.

Astronomers want to learn more about Phoebe. Right now, our only pictures of Phoebe are very fuzzy. They were made by Voyager 2 in 1981. Soon we should have much better pictures! The Cassini spacecraft will fly very close past Phoebe on June 11, 2004.

This is a picture of Saturn's moon Phoebe. Actually, it is two different pictures of Phoebe. The pictures were taken at different times. They show different sides of the moon. Image courtesy NASA/JPL.

Phoebe is the name of a moon of Saturn. Phoebe is a fairly small moon. It is about 220 km (137 miles) across. Eight of Saturn's moons are larger than Phoebe. A whole bunch of Saturn's moons are smaller than Phoebe.

An astronomer named William Henry Pickering discovered Phoebe in 1898.

The name "Phoebe" comes from Greek mythology. In the Greek myths, Phoebe was the daughter of Uranus and Gaia. She was also the grandmother of Apollo and Artemis.

Some things about Phoebe are strange. The moon moves around Saturn in a very odd orbit. Most of Saturn's moons are very light in color, but Phoebe is very dark.

Why is Phoebe so strange? Many astronomers think Phoebe used to be an asteroid or Kuiper Belt Object. They think Saturn's strong gravity captured Phoebe sometime long ago.

Astronomers want to learn more about Phoebe. Right now, our only pictures of Phoebe are very fuzzy. They were made by Voyager 2 in 1981. Soon we should have much better pictures! The Cassini spacecraft will fly very close past Phoebe on June 11, 2004.