Janus is one of the inner satellites of the ringed planet Saturn, the sixth planet from the sun. It is the tenth largest moon of Saturn. Janus is the god of time in Roman mythology. Janus has one head that faces the past and another head that faces the future. It is also called S10 and Saturn X, where X is the Roman numeral for ten.
Janus was discovered on December 15, 1966 by Audouin Charles Dollfus. Dollfus was a French astronomer who was born on November 12, 1924 and died recently on October 1, 2010. The asteroid 2451 Dollfus was named after him.
Janus was temporarily named S/1966 S2. Another satellite with approximately the same orbit was discovered by Richard Walker three days after Janus was discovered on December 18, 1966. It was not known that there were two satellites until 1977 when Stephen Larson and John Fountain demonstrated it. The other satellite is now called Epimetheus. Voyager 1 confirmed that there are two satellites. They are called Siamese twins because their orbits are so similar. They might have formed from one moon many years ago during the configuration of Saturn’s moons.
Pioneer 11 observed Janus on September 1, 1979. Voyager 1 made a flyby of Janus on November 12, 1980. On August 25, 1981 Voyager 1 took a picture of Janus. The Cassini Solstice project made a close approach of Janus on June 1, 2008. It came within 8,925 miles of the moon.
The orbits of Janus and Epimetheus are within 30 miles of each other. They are located between the F and G rings of Saturn. The higher moon exchanges position with the lower one every four years. The most recent exchange took place last year (2010). They are the fifth and sixth closest moons to Saturn, the positions changing every four years.
Janus is 94,089 miles from Saturn. Its orbit takes only 16 hours, forty one minutes. Its mass is 4.19×10^18 pounds. The mean diameter is 111 miles. Janus is shaped like a potato, so the dimensions of Janus are 122x119x93 miles. Its surface gravity is 0.014 meters per second squared. The orbital eccentricity is 0.007. The orbital inclination is 0.16 degrees. One side of Janus always faces Saturn because its orbit takes about the same time as it takes it to revolve on its axis (the same as the moon of the Earth).
The goals for studying Janus are to determine the geological statistics, physical composition, internal structure, and its interaction with the rings of Saturn. Janus might be covered with icy material. We do know that Janus is covered with craters, some larger than 30 kilometers. Some of the named craters on Janus are Castor, Phoebe (not the moon of Saturn), Idas, and Lynceus.
