Epimetheus is a moon of Saturn that was first observed by Walker in 1966, but the situation was confused since Janus is in a very similar orbit and so Walker officially shares the discovery of Epimetheus with Fountain and Larson who showed in 1977 that there were two satellites involved. The situation was clarified in 1980 by Voyager 1.
Epimetheus and Janus are "co-orbital". Janus' orbital radius from Saturn is 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 actually approaching each other closely. The exchange takes place about once every four years. This arrangement is unique in the solar system, as far as is currently known.
There are several 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.
- Orbital radius: 151,422 km at the time Voyager 1 visited, alternately 151,472 km
- Diameter: 115 km (144 x 108 x 98)
- Mass: 5.6*1017 kg
- Orbital period: 0.6942 days
- Orbital inclination: 0.34°