Janus
Janus ( "JAY nus" 
) is the sixth of Saturn's known satellites:
orbit: 151,472 km from Saturn
diameter: 178 km (196 x 192 x 150)
mass: 2.01e18 kg
Janus was the god of gates and doorways, depicted with two faces looking in opposite directions; also the root of the English word "January".
Discovered by the French astronomer Audouin Dollfus in 1966.
Dollfus is credited with the discovery of Janus but it's not really certain whether the object he saw was Janus or Epimetheus and his observations led to a spurious orbit. (Walker discovered it independently but his telegram arrived a few hours after Dollfus'.) Larson and Fountain determined in 1978 that there are in fact two moons at about 151000 km from Saturn. This was confirmed in 1980 by Voyager 1.
Janus and Epimetheus
are "co-orbital".
The orbital radii of Janus and Epimetheus differ by only 50 km, less than
the diameter of either. Their orbital velocities are thus very nearly
equal and the lower, faster one slowly overtakes the other. As they
approach each other they exchange a bit of momentum the end result
of which is to boost the lower one into a higher orbit and to drop the
higher one to a lower orbit. They thus exchange places. The exchange
takes place about once every four years. The orbital data given
here is as of the time of the Voyager encounters.
Janus is extensively cratered with several craters larger than 30 km but few linear features. Its surface appears to be older than Prometheus' but younger than Pandora's.
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