Epimetheus (moon) - Orbital Relationship Between Epimetheus and Janus

Orbital Relationship Between Epimetheus and Janus

Epimetheus and Janus are co-orbital: Janus's mean orbital radius from Saturn is as of 2006 (as shown by green color in the picture) only 50 km less than that of Epimetheus, a distance smaller than either moon's mean diameter. In accordance with Kepler's laws of planetary motion, the closer orbit is completed more quickly, but only by about 30 seconds. Each day the inner moon is an additional ¼° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction boosts the inner moon's momentum and saps the outer moon's momentum. With this added momentum, the inner moon's distance from Saturn and orbital period are increased, and the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that the moons "trade" orbits, never approaching closer than about 10,000 km. The exchange takes place about once every four years; the last close approaches occurred on January 21, 2006, and in 2010, when Janus's orbital radius increased by ~20 km, while Epimetheus's decreased by ~80 km; Janus's orbit is less affected because it is 4 times more massive than Epimetheus. As far as it 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.