[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.......

[abalone]

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.

You are correct when you say that they dont actually pass each other but the one that moves to a higher orbit is the one that is doing the catching up. When it moves to a higher orbit it slows down and starts to drop behind again, but how close do they get to each other

[abalone]

but how close do they get to each other

I have done a little search on the net and the only mention of distance is about " as close as 100 km". That would still be very impressive if you were on the surface of one of these to see the other with a diameter over 100km that close. It would fill one quarter of the sky.

[Phil Stooke]

I don't have a number but 100 km can't be right.

Phil

[abalone]

I don't have a number but 100 km can't be right.

Phil

I though it was too small as well but it was the only figure I could find on any web page that looked half reasonable

[Decepticon]

New Images Up... http://saturn1.jpl.nasa.gov/multimedia/ima...2/N00037089.jpg

[Mark6]

If you look at these two moons in a reference frame where one of them (or their center of mass) is stationary, then don't they simply orbit each other? I think saying that they "swap orbits around Saturn" is akin to Earth and Moon "swapping orbits around the Sun"; the path Moon traces around Sun is always convex and is very close to ellipse, it just speeds up and slows down periodically

[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

[abalone]

I don't have a number but 100 km can't be right.

Phil

Any progress on establishing a minimum separation distance?

[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.