Janus (moon)

Janus is an inner satellite of Saturn. It is also known as Saturn X. It is named after the mythological Janus. This natural satellite was first identified by Audouin Dollfus on December 15, 1966, although it had been unknowingly photographed earlier by Jean Texereau. Further observations led to the realization that Janus shares a unique orbital relationship with another moon, Epimetheus. The discovery of these two moons' peculiar co-orbital configuration was later confirmed by Voyager 1 in 1980.

Discovery
Janus was identified by Audouin Dollfus on 15 December 1966 and given the temporary designation S/1966 S 2. Previously, Jean Texereau had photographed Janus on 29 October 1966 without realising it. On 18 December, Richard Walker observed an object in the same orbit as Janus, but whose position could not be reconciled with the previous observations. 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, Walker is now credited with the discovery of Epimetheus. Voyager 1 confirmed this orbital configuration in 1980.

Observational history
Janus was observed on subsequent occasions and given different provisional designations. Pioneer 11 three energetic-particle detectors detected its "shadow" when the probe flew by Saturn on 1 September 1979 (S/1979 S 2). Janus was observed by Dan Pascu on 19 February 1980 (S/1980 S 1), and then by John W. Fountain, Stephen M. Larson, Harold J. Reitsema and Bradford A. Smith on 23 February 1980 (S/1980 S 2).

Name
Janus is named after the two-faced Roman god Janus. Although the name was informally proposed soon after the initial 1966 discovery, it was not officially adopted until 1983, when Epimetheus was also named.

Orbit
Janus's orbit is co-orbital with that of Epimetheus. Janus's mean orbital radius from Saturn was, as of 2006, only 50 km less than that of Epimetheus, a distance smaller than either moon's mean radius. In accordance with Kepler's laws of planetary motion, the closer orbit is completed more quickly. Because of the small difference, it is completed in only about 30 seconds less. Each day, the inner moon is an additional 0.25° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction increases the inner moon's momentum and decreases that of the outer moon. This added momentum means that the inner moon's distance from Saturn and orbital period are increased, and in exchange the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that the moons effectively swap orbits, never approaching closer than about 10,000 km. At each encounter Janus's orbital radius changes by ~20 km and Epimetheus's by ~80 km: Janus's orbit is less affected because it is four times as massive as Epimetheus. The exchange takes place close to every four years; the last close approaches occurred in January 2006, 2010, 2014, 2018, and 2022 and the next in 2026. This is the only such orbital configuration known in the Solar System.

The orbital relationship between Janus and Epimetheus can be understood in terms of the circular restricted three-body problem, as a case in which the two moons (the third body being Saturn) are similar in size to each other.

Physical characteristics
Janus is extensively cratered with several craters larger than 30 km and has few linear features. Janus's surface appears to be older than Prometheus's but younger than Pandora's. [[File:Animation_of_Epimetheus_orbit_-_Rotating_reference_frame.gif|right|thumb|250x250px|Animation of Epimetheus orbit – Rotating reference frame

··]] Janus has a very low density and relatively high albedo, meaning that it is likely icy in composition and structurally a rubble pile object.

Features
Craters on Janus, like those on Epimetheus, are named after characters in the legend of Castor and Pollux.

Interactions with rings
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 their surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.

Along with Epimetheus, Janus acts as a shepherd moon, maintaining the sharp outer edge of the A Ring in a 7:6 orbital resonance. The effect is more obvious when the more massive Janus is on the resonant (inner) orbit.

In fiction
Janus is the location of the plot in the Alastair Reynolds science-fiction novel Pushing Ice.