An inner moon of Saturn. Janus was discovered
on December 15, 1966, by Audouin Dollfus
and is also known as Saturn X. It is nearly co-orbital
(shares its orbit) with Epimetheus and
is located between Saturn's F- and G-rings. Because Janus and Epimetheus
differ in their orbital radii by only 50 km, their orbital velocities are
very nearly equal and the lower, faster one gradually catches up and overtakes
the other. As the moons approach each other they exchange a small amount
of momentum as a result of which the lower one is boosted into a higher
orbit while the higher one drops to a lower orbit. This exchange of places
happens about once every four years.
Although Dollfus is credited with the discovery of Janus, it is not 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 that of Dollfus.) Larson and Fountain determined
in 1978 that there are two moons at about 151,000 km from Saturn –
a fact confirmed in 1980 by Voyager 1.
Janus is potato-shaped and extensively cratered, with several craters larger
than 30 km (19 miles), but has few linear features. Its surface looks as
though it has been smoothed by some process. Like Pandora
and Telesto, Janus may be covered with a
mantle of fine dust-sized, icy material.
||1966, by Audouin Dollfus
||151,460 km (94,130 miles)
||193 × 173 × 137 km
(120 × 108 × 85 miles)
||0.695 day (16 hours 41 minutes)
|Janus, with Saturn in the background, seen
by Cassini's narrow-angle camera at a distance of about 145,000 km
(90,000 miles) and a Sun-Janus-spacecraft angle of 62°. The image
was taken using a filter sensitive to wavelengths of infrared light
centered at 930 nm. Image scale is 871 meters (2,858 ft) per pixel.
of Janus (Encyclopedia of History)