Phoebe (astronomy)

Phoebe (astronomy), major moon of the planet Saturn. Phoebe is one of the planet’s most distant moons. Phoebe orbits Saturn at an average distance of 13 million km (8 million mi). The moon completes an orbit about once every 550 Earth days and rotates once every 9.60 hours. Phoebe is four times more distant from Saturn than the next major moon, Iapetus. Phoebe’s orbit is tilted about 5° to Saturn’s equator and is elliptical, or oval-shaped. The moon has a retrograde orbit, which means that it moves clockwise as seen from Saturn’s north pole. Most moons and planets in the solar system travel counterclockwise as seen from the north pole of the body they orbit.

Most of our knowledge about Phoebe comes from the Cassini spacecraft. The spacecraft flew within 2,000 km (1,200 mi) of the moon in 2004. Phoebe is roughly spherical, measuring about 220 km (about 132 mi) across, about one-fifteenth the diameter of Earth’s moon. Its surface is heavily pocked with craters.

Phoebe is extremely dark, reflecting only about 6 percent of the light that falls on it. Phoebe’s surface is covered by a layer of dark-reddish, powdery material. Fresh craters reveal brighter material below this dark layer. This brighter material is thought to be ice, the moon’s main component material. Phoebe has no atmosphere.

Some planetary scientists believe that Phoebe was captured by Saturn’s gravity from a population of objects called centaurs, which orbit the Sun near Saturn. The best known of these objects is Chiron, which is sometimes called a giant comet. Chiron occasionally passes near Saturn, as Phoebe might have done before it was captured and became Saturn’s most distant known moon.

Phoebe was discovered in 1898 by American astronomer William Pickering. The moon is named after a character in Greek mythology who was the virgin goddess of the hunt and of the moon.