[X] My Assistant

[Jason]

The orbits of these two moonlets vary in distance from Saturn by only 50km - less than the radius of either, so they nominally share an orbit.
The inner moon (Epimetheus?) is slowly catching up with the outer (Janus?) and I understand that in January/February next year they are due to pass one another, and because of their gravitational interaction, to swap positions.
(Epimetheus speeds up relative to Janus and moves to a higher orbit, and Janus does the reverse)
Does anyone know if Cassini will be in a position to take some images which could be made into a movie of the phenomenon? - would be awesome to see the process actually happening.......

[Phil Stooke]

No, it's not really the same situation.

First, because Earth and Moon have very different masses (ratio 80 to 1), the Earth moves much less than the Moon, so basically it has an almost elliptical orbit and the Moon wiggles around it, in a wave pattern superimposed on its solar orbit nominal ellipse. Janus and Epimetheus have masses differing by maybe 4 to 1 or something (don't have the numbers). So they behave differently. From Saturn, each has a fairly elliptical orbit, but they do the odd orbit swap when they approach each other.

More importantly, though, if you work in a reference frame attached to the larger of the bodies, in the Earth-Moon system the Moon simply orbits Earth (or the barycenter, strictly). But for Janus, it's not orbited by Epimetheus, instead Epimetheus is seen to follow a giant horseshoe path - approaching Janus from one direction, then reversing its path and receding... all the way around Saturn until it approaches from the other direction. The position of Janus acts as a forbidden zone which Epimetheus can't approach too closely.

Phil

[Mark6]

More importantly, though, if you work in a reference frame attached to the larger of the bodies, in the Earth-Moon system the Moon simply orbits Earth (or the barycenter, strictly).  But for Janus, it's not orbited by Epimetheus, instead Epimetheus is seen to follow a giant horseshoe path - approaching Janus from one direction, then reversing its path and receding... all the way around Saturn until it approaches from the other direction.  The position of Janus acts as a forbidden zone which Epimetheus can't approach too closely. 

I see. That sounds similar to Earth and asteroid Cruithne.