Janus (moon)

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

**Janus**
Discovery
Janus in front of Saturn, as imaged by Cassini
Discovered by	Audouin Dollfus
Discovered in	December 15, 1966
Orbital characteristics ^[1]
Epoch 31 December 2003 (JD 2453005.5)
Semimajor axis	151,460 ± 10 km
Eccentricity	0.0068
Orbital period	0.694660342 d
Inclination to Saturn's equator	0.163 ± 0.004°
Is a satellite of	Saturn
Physical characteristics
Diameter	179 (193×173×137) km ^[2]
Mass	1.912 ± 0.005 ×10¹⁸ kg ^[1]
Mean density	0.64 ± 0.06 g/cm³
Surface gravity	~0.0137 m/s²
Rotation period	synchronous
Axial tilt	zero
Albedo	0.5
Atmosphere	none

Janus (jay'-nus, IPA: [ˈdʒeɪnəs], Greek Ιανός) is an inner satellite of Saturn. It is also known as Saturn X (Roman numeral X = 10). It is named after the mythological Janus.

1 Discovery and orbit
2 Name
3 Orbital relationship between Epimetheus and Janus
4 Physical characteristics
5 Ring
6 Janus in literature
7 See also
8 External links
9 References

[edit] 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 (IAUC 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 (IAUC 1995). On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus (IAUC 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.

[edit] Name

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 OED, the adjectival form of the moon's name is Janian.

[edit] Orbital relationship between Epimetheus and Janus

Janus and Epimetheus are "co-orbital". Janus' orbital radius from Saturn is currently only 50 km less than that of Epimetheus. 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, in turn, loses an equal amount of momentum and drops into a lower orbit at the same time, effectively speeding it up. The moons thus "trade" orbits and begin moving apart again, without completely 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 last close approach was on 21 January 2006 [1], the next will be in 2010. At that time, Janus' orbital radius will increase by ~20 km, while Epimetheus' decreases 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 [2].

[edit] 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.

[edit] 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 5,000 km ^[3]. Its source are particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths ^[4].

[edit] Janus in literature

The moon Janus features prominently in the science fiction novel Pushing Ice by Alastair Reynolds. In the novel, Janus is a camoflagued alien spaceship which unexpectedly takes off in the direction of a giant structure orbiting the star Spica. The novel tells the story of the crew of a human mining ship who establish a colony on the moon as it leaves the solar system.