Celestia Forums

Hi there. I am new to these forums, but have been a Celestia user for several months. My 4 small kids LOVE this program. It is a great teaching tool.

I noticed that the two Saturn moons Dione and Helene have very similar orbits, but not exactly the same. They also seem to circle Saturn at the same rate. I tracked them a few thousand years back to year -3484 where it appears Helene used to orbit Dione.

My guess is that some near miss or collision with a 3rd object knocked Helene loose from Dione, and Dione has been falling behind ever since.

Dean

Dean:
I think what you've found is just an artefact of Celestia's simulation of these bodies - their positions aren't going to be accurately portrayed that long ago. In particular, the position of Helene is going to drift markedly relatively to Dione, because Dione's orbit is defined by complicated calculations inside Celestia, whereas Helene moves in a simple ellipse, accurate only for dates around 2000.
In fact, Helene is oscillating gently around a stable position in Dione's orbit called a Trojan point, 60 degrees away from Dione's position.

Grant

Saturn also has two other satellites, Janus and Epimetheus, which are in horseshoe orbits with each other. Saturn definately has an interesting collections of moons. I can't wait till July when Cassini arrives and sends back hi-res pictures of them.

Hmm . . . perhaps we should should add a custom orbit for Helene that's Dione's position plus 60 degrees.

--Chris

chris wrote:Hmm . . . perhaps we should should add a custom orbit for Helene that's Dione's position plus 60 degrees.

If you do that, you could do the same job for Telesto and Calypso relative to Tethys.
Wouldn't be as good as a true ephemeris-driven orbit, of course, since these bodies stray quite widely along tadpole orbits around the Trojan points, rather than sitting precisely on the spot. But at least they wouldn't stray over time.

Grant