Janus

Saturn X

Janus Facts

• Janus is the sixth of Saturn's known satellites:
  • distance from Saturn: 151,472 km
  • diameter: 178 km (196 x 192 x 150)
  • mass: 2.01e18 kg
  a.k.a. 1980 S 1

• Pronounced "JAY nus"

• 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. (Walker discovered it independently but his telegram arrived a few hours after Dollfus's.) 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 the Voyager spacecraft.

• 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's but younger than Pandora's.

Pictures

1. (above) Janus 43k gif
2. Janus low res 14k gif

3. Janus 15k gif

4. Map of Janus 102k gif

More about Janus

• from LANL

Open Issues

• Little is known about Janus.
• Is the co-orbital relationship with Epimetheus stable?