| • Science | • People | • Locations | • Timeline |
| Discovery | |
|---|---|
| Discovered by | Audouin Dollfus |
| Discovered in | 1966 |
| Orbital characteristics | |
| Semimajor axis | 151,472 km |
| Eccentricity | 0.007 |
| Orbital period | 16 h 40 min |
| Inclination | 0.14 ° |
| Is a satellite of | Saturn |
| Physical characteristics | |
| Mean diameter | 177.6 km |
| Mass | 1.98×1018 kg |
| Mean density | 0.65 g/cm3 |
| Surface gravity | ~0.017 m/s2In physics, acceleration (symbol: a is defined as the rate of change (or time derivative) of velocity. It is thus a vector quantity with dimension length/ time˛. In SI units, this is metre/second˛. To accelerate an object is to change its velocity over a |
| Rotation periodprograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 one sidereal day). | 16 h 40 min ( synchronousIn astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it) |
| Axial tiltAxial tilt is an astronomical term regarding the inclination angle of a planet's rotation axis in relation to its orbital plane. It is also called obliquity . A planet whose rotation axis were perpendicular to the orbital plane would have an axial tilt of | 0.015 ° |
| AlbedoNote: This article discusses the physical or planetological property of albedo. For other usage, see Albedo (disambiguation). The albedo is a measure of reflectivity of a surface or body. It is the ratio of electromagnetic radiation reflected to the amoun | 0.5 |
| Atmosphere | none |
Janus ("JAY nuss") is a moon of Saturn. It is also known as "Saturn X" (Roman numeral X = 10). It is named after the mythological JanusIn Roman Mythology, Janus was the god of gates, doors, beginnings, endings and doorways. The month of January was named for him. He was usually depicted as Janus Geminus (twin Janus) or Bifrons with two faces looking in opposite directions. In some places.
Janus occupies essentially same orbit as the moon Epimetheus. Astronomers assumed that there was only one body in that orbit, and accordingly had a hard time figuring out their orbital characteristics; it is obviously impossible to reconcile the 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. Shortly before, Jean Texereau had photographed Janus on October 29 1966 without realising it (IAUC 1995). On December 18, Richard L. 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.
Janus was also 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.
Although the name "Janus" 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.
For more information about Janus and Epimetheus' unusual shared orbit, see Epimetheus.
Janus is extensively cratered with several craters larger than 30 km but few linear features. Its 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: List of geological features on Saturn's smaller moons.
| Saturn |
|---|
| Janus' group | Mimas | Enceladus | Tethys | Dione | Rhea |
| Titan | Hyperion | Iapetus | Siarnaq's group | Phoebe's group |
| (For other moons, see: Saturn's natural satellites) |
| see also: Cassini-Huygens |