Janus is an inner moon of Saturn and it is co-orbital with another moon Epimetheus.
Janus is the 10th largest moon of Saturn, and the 30th largest moon in the Solar System currently known.
Discovery
Janus occupies practically the same orbit as the moon Epimetheus. This caused some confusion for astronomers, who assumed that there was only one body in that orbit, and for a long time struggled to figure out what was going on. It was eventually realized that they were trying to reconcile observations of two distinct objects as a single object.
The discovery of Janus is attributed to its first observer: Audouin Charles Dollfus, on December 15, 1966. Previously, Jean Texereau had photographed Janus on October 29, 1966 without realising it. Dollfus named it at the same occasion.
On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus.
Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain realised that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits. Voyager 1 confirmed this in 1980.
Naming
The moon was given the temporary designation S/1966 S2 after discovery.
Due to uncertainty in Janus's orbital status, the moon was observed on subsequent occasions and given different provisional designations. It was observed by the Pioneer 11 probe when it passed near Saturn on September 1, 1979: three energetic particle detectors observed its "shadow" (S/1979 S2). Janus was observed by Dan Pascu on February 19, 1980 (S/1980 S1) and then by John W. Fountain, Stephen M. Larson, Harold J. Reitsema and Bradford A. Smith on the 23rd (S/1980 S2).
Janus the moon is named after Janus, the two-faced Roman god.
In ancient Roman religion and mythology, Janus is the god of beginnings and transitions, thence also of gates, doors, doorways, endings and time. He is usually a two-faced god since he looks to the future and the past.
Although the name was informally proposed soon after the initial 1966 discovery, Janus was not officially given this name until 1983.
Stats
Diameter (mean): 179 km
Semi-major axis: 151,472 km
Orbital Period: 0.695 days
Co-orbital moons
Janus and Epimetheus share their orbits, the difference in semi-major axes being less than either's mean diameter. This means the moon with the smaller semi-major axis will slowly catch up with the other. As it does this, the moons gravitationally tug at each other, increasing the semi-major axis of the moon that has caught up and decreasing that of the other.
This reverses their relative positions (proportionally to their masses) and causes this process to begin anew with the moons' roles reversed. In other words, they effectively swap orbits, ultimately oscillating both about their mass-weighted mean orbit.
Janus rotates synchronously with its orbital period, keeping one face pointed toward Saturn.
Formation
Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites, but if this is the case the disruption must have happened early in the history of the satellite system.
Physical characteristics
Janus is extensively cratered with several craters larger than 30 km but few linear features. Janus's 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 and icy rubble pile. The moon is also highly non-spherical.
Ring
A faint dust ring is present around the region occupied by the orbits of Janus and Epimetheus, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006.
The ring has a radial extent of about 5000 km. Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.
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