Orbit Epimetheus (moon)

epimetheus (lower left) , janus (right) seen on 20 march 2006, 2 months after swapping orbits. 2 moons appear close because of foreshortening; in reality, janus 40,000 km farther cassini epimetheus.



rotating-frame depiction of horseshoe orbits of janus , epimetheus

epimetheus s orbit co-orbital of janus. janus s mean orbital radius saturn is, of 2006 (as shown green color in adjacent picture), 50 km less of epimetheus, distance smaller either moon s mean radius. in accordance kepler s laws of planetary motion, closer orbit completed more quickly. because of small difference completed in 30 seconds less. each day, inner moon additional 0.25° farther around saturn outer moon. inner moon catches outer moon, mutual gravitational attraction increases inner moon s momentum , decreases of outer moon. added momentum means inner moon s distance saturn , orbital period increased, , outer moon s decreased. timing , magnitude of momentum exchange such moons swap orbits, never approaching closer 10,000 km. @ each encounter janus s orbital radius changes ~20 km , epimetheus s ~80 km: janus s orbit less affected because 4 times more massive epimetheus. exchange takes place close every 4 years; last close approaches occurred in january 2006, 2010 , 2014, , next in 2018. such orbital configuration known in solar system.

the orbital relationship between janus , epimetheus can understood in terms of circular restricted three-body problem, case in 2 moons (the third body being saturn) similar in size each other.