Janus [JAY-nus] is the sixth satellite of Saturn. It was discovered by Audouin Dollfus in 1966 and was named after the god of gates and doorways. It is depicted with two faces looking in opposite directions. Janus has an irregular shape with a size of 196x192x150 kilometers (122x119x93 miles) in diameter. It is heavily cratered with several craters 30 kilometers (19 miles) in diameter. The pervasive cratering indicates that its surface must be several billion years old. Prometheus appears to have fewer craters indicating a younger surface while Pandora appears to have an older surface. Janus has few linear features.
Janus and Epimetheus share the same orbit of 151,472 kilometers (94,125 miles) from Saturn's center or 91,000 kilometers (56,547 miles) above the cloud tops. They are only separated by about 50 kilometers (31 miles). As these two satellites approach each other they exchange a little momentum and trade orbits; the inner satellite becomes the outer and the outer moves to the inner position. This exchange happens about once every four years. Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites. If this is the case, the disruption must have happened early in the history of the satellite system.
| Janus Statistics | |
|---|---|
| Discovered by | Audouin Dollfus |
| Date of discovery | 1966 |
| Mass (kg) | 2.01e+18* |
| Mass (Earth = 1) | 3.3635e-07 |
| Radius (km) | 98x96x75 |
| Radius (Earth = 1) | 1.5365e-02 |
| Mean density (gm/cm^3) | 0.67* |
| Mean distance from Saturn (km) | 151,472 |
| Rotational period (days) | 0.6945 |
| Orbital period (days) | 0.6945 |
| Mean orbital velocity (km/sec) | 15.87 |
| Orbital eccentricity | 0.007 |
| Orbital inclination (degrees) | 0.14 |
| Escape velocity (km/sec) | 0.0523 |
| Visual geometric albedo | 0.8 |
| Magnitude (Vo) | 14.5 |
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