Janus

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In Roman Mythology, Janus was the god of gates and doorways, depicted with two faces looking in opposite directions; also the root of the English word "January".


Janus is a moon of Saturn 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 sharing one orbit. This was confirmed in 1980 by Voyager 1.

For more information about Janus and Epimetheus' unusual shared orbit, see Epimetheus.

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. From its very low density and relatively high albedo, it seems likely that Janus is a very porous icy body. There is a lot of uncertainty in these values, however, and so this remains to be confirmed.

  • Orbital radius: 151,472 km at the time Voyager 1 visited, alternately 151,422 km
  • Diameter: 178 km (196 x 192 x 150)
  • Mass: 2.01*1018 kg
  • Orbital period: 0.6945 days
  • Orbital inclination: 0.14°