Celestia Forums

I downloaded the 'Minor Moons of the Giant Planets' hoping to get a view of Prometheus and Pandora (Sheparding moons of Saturn's rings) similar to the way they appear in this photo:

http://apod.gsfc.nasa.gov/apod/ap951223.html

However, when I rewind the Celestia clock to 12/23/95, Promethus and Pandora appear much farther apart.

http://jade.bensoft.com/saturn_1223951152.jpg

Am I expecting too much accuracy?

I can't see the problem...

These two moons don't have the same revolution period so it is normal !

I tried cycling through the whole day of 12/23/95 and they never come close. Their orbital periods are different so I'm sure that if I observed it long enough they would pass. My real question is why doesn't this happen on 12/23/95?

I did notice that the orbital period of Pandora is listed as 0.6285 on this JPL page:

http://sse.jpl.nasa.gov/features/planet ... ndora.html

And the script (quoted below) lists it as 0.628804 which might be the problem.

"Prometheus" "Sol/Saturn"
{
Texture "asteroid.jpg"
Mesh "asteroid.cms"
Radius 50

EllipticalOrbit
{
Period 0.612986
SemiMajorAxis 139350
Eccentricity 0.0024
Inclination 0
MeanAnomaly 97
}

Albedo 0.1
}

"Pandora" "Sol/Saturn"
{
Texture "asteroid.jpg"
Mesh "asteroid.cms"
Radius 42

EllipticalOrbit
{
Period 0.628804
SemiMajorAxis 141700
Eccentricity 0.0042
Inclination 0
MeanAnomaly 308
}

Albedo 0.1
}

Actually, Pandora and Prometheus are very strange moons with orbits that no-one, as yet, fully understands. Apparently they seem drift from their expected positions, but quite significant amounts, for no obvious reason. So I suppose you'll have to wait until the boffins figure out what's going on - orbital data for these moons is currently very shaky. Not that it was innacurately measured - it just doesn't seem to work !

(Mad Boris)

Guess I opened Pandora's Box (I couldn't resist). I actually wondered if somehow the proximity of the rings were a factor in making the shepards difficult to plot, weird gravitational forces and other ring factors causing chaos that would make their movement unpredictable.

From the Nineplanets site:

The 1995/6 Saturn Ring Plane Crossing observations found that Prometheus was lagging by 20 degrees from where it should have been based on Voyager 1981 data. This is much more than can be explained by observational error. It is possible that Prometheus's orbit was changed by a recent encounter with the F ring, or it may have a small companion moon sharing its orbit.

/Alexis

RTFWP :)