Epimetheus occupies essentially the same orbit as the moon Janus. 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.
Audouin Dollfus observed a moon on December 15, 1966, which he proposed to be named "Janus". On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus. However, at the time, it was believed that there was only one moon, unofficially known as "Janus", in the given orbit.
Twelve years later, on 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. This was confirmed in 1980 by Voyager 1, and so Walker officially shares the discovery of Epimetheus with Larson and Fountain.
The 1980 Voyager discovery was designated S/1980 S 3, and it was officially named "Epimetheus" in 1983. The name Janus was officially approved by the IAU at the same time, although the name had been used informally since Dollfus proposed it a few days after the 1966 discovery.
Orbital relationship between Epimetheus and Janus
Epimetheus and Janus 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 were inevitable. But as the inner moon catches up with the outer moon their gravitational attraction boosts the inner moon's momentum and raises its orbit, while the outer moon equally loses momentum and drops into a lower orbit. The moons thus "trade" orbits and begin to move apart again, since the forward moon is now in the lower, faster orbit. The nearest they ever approach is some 10,000 km. The exchange takes place about once every four years; the next closest approach is in 2010. 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. As far as is currently known, this arrangement is unique 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. 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 .
There are several Epimethean craters larger than 30 km in diameter, as well as both large and small ridges and grooves. The extensive cratering indicates that Epimetheus must be quite old. Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites, but if this is the case the disruption must have happened early in the history of the satellite system. From its very low density and relatively high albedo, it seems likely that Epimetheus is a very porous icy body. There is a lot of uncertainty in these values, however, and so this remains to be confirmed.
- The Planetary Society: Epimetheus
- 'Solar System Dynamics' by Murray and Dermott The standard text on the subject, describes the orbits in detail.
- Quicktime illustration of co-orbital motion from Murray and Dermott
|edit Saturn's natural satellites|
|Pan | Daphnis | Atlas | Prometheus | S/2004 S 6 | S/2004 S 4 | S/2004 S 3 | Pandora | Epimetheus and Janus|
|Mimas | Methone | Pallene | Enceladus | Telesto, Tethys, and Calypso | Polydeuces, Dione, and Helene | Rhea|
|Titan | Hyperion | Iapetus | Kiviuq | Ijiraq | Phoebe | Paaliaq | Skathi | Albiorix | S/2004 S 11 | Erriapo | Siarnaq|
|S/2004 S 13 | Tarvos | Mundilfari | S/2004 S 17 | Narvi | S/2004 S 15 | S/2004 S 10 | Suttungr | S/2004 S 12|
|S/2004 S 18 | S/2004 S 9 | S/2004 S 14 | S/2004 S 7 | Thrymr | S/2004 S 16 | Ymir | S/2004 S 8|
|See also: Pronunciation key | Rings of Saturn | Cassini-Huygens | Themis|