Saturn X redirects here. For the spurious moon reported in 1905, see Themis

Janus is a moon of Saturn. It is also known as Saturn X (Roman numeral X = 10). It is named after the mythological Janus.

Discovery and orbit

The following is a summary. For more detailed information about Janus and Epimetheus' unusual shared orbit, see Epimetheus.

Janus occupies essentially 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 realised 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 Dollfus, on December 15 1966 (1987). The new object was given the temporary designation S/1966 S 2. Previously, Jean Texereau had photographed Janus on October 29 1966 without realising it ([[1695]). On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus (1991).

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. (See below for a more detailed description of their unique arrangement.)

Janus 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 S 2, Tom Gehrels and James A. van Allen, IAUC 3417). Janus was observed by Dan Pascu on February 19 1980 (S/1980 S 1, IAUC 3454), and then by John W. Fountain, Stephen M. Larson, Harold J. Reitsema and Bradford A. Smith on the 23rd (S/1980 S 2, IAUC 3456). The Voyager 1 probe finally confirmed Janus' existence on March 1 1980. All of these people thus share, to various degrees, the title of discoverer of Janus.


Janus is named after Janus, the two-faced Roman god. Although the name was informally proposed soon after the initial 1966 discovery, it was not officially given this name until 1983. Epimetheus received its name at the same time.

According to the Oxford English Dictionary, the adjectival form of the moon's name is Janian.

Orbital relationship between Epimetheus and Janus

Janus and Epimetheus are "co-orbital". Janus' orbital radius from Saturn is currently 151,472 km and Epimetheus' orbital radius is 151,422 km, a separation of only 50 km. Since closer orbits have higher velocities the two moons must inevitably approach each other, and since Epimetheus' diameter is 115 km and Janus' is 178 km it would seem at first glance that a collision is also inevitable. But as the inner moon catches up with the outer moon their mutual gravitational attraction boosts the inner moon's momentum and raises its orbit, causing it to slow down. The outer moon loses an equal amount of momentum and drops into a lower orbit at the same time, speeding it up. The moons thus "trade" orbits and begin moving apart again, without overtaking each other and without even approaching each other closely (no closer than about 10,000 km). The exchange takes place about once every four years; the next closest approach is in Jan 2006. At that time, Janus' orbital radius will decrease by ~20 km, while Epimetheus' increases by ~80 km (Janus' orbit is less affected because it is 4 times more massive than Epimetheus). This arrangement is unique in the solar system, as far as is currently known.

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. Other examples of the three-body problem include Trojan asteroids and Trojan moons, the "horseshoe" orbit of Cruithne with respect to Earth, and potentially dozens of other objects in similar orbits [1].

Physical characteristics

Janus is extensively cratered with several craters larger than 30 km but few linear features. The Janian 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.

See also

External links

... | Phoebe | Janus | Epimetheus | ...

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