Binary asteroids & tiny moons

[JackHiggins]

Hi all

Here are 2 ssc files i've created, one has all known binary asteroids for which the companion body's orbit has been calculated (or at least estimated). Here's Eugenia & Petit-Prince (Eugenia was the first after Ida found to have a moon):


And the other ssc has all currently known moons of Jupiter, Saturn, Uranus & Neptune (not including the ones in the original minormoons.ssc file). It has the 12 moons of Jupiter discovered in the last few months, and 3 new moons of neptune discovered last november. Here's what those planets look like with this ssc installed:
Jupiter


Saturn


Uranus


Neptune


Download both files from my website at http://homepage.eircom.net/~jackhiggins/celestia/
(they're down at the bottom of the page)

Tell me what you think!!

[granthutchison]

Jack:
Looking through your tiny moons file, I see a couple of problems, both of them arising because the data you've taken from JPL are in a different format from that used by Celestia:

1) Many of the inclinations quoted are relative to the Laplacian plane (the invariant plane in which the satellite precesses) rather than the equatorial plane of the parent planet. For inner satellites, the difference is trivial, but for outer satellites of planets with high obliquity, your error can be large - in the case of Uranus, your satellite inclination can by out by more than 90 degrees.

2) The satellite ascending node is usually given relative to the planet's equatorial ascending node on the plane of the Earth's equator, not its ascending node on the ecliptic, which is what Celestia uses. So the node of a satellite will be displayed wrongly in Celestia, creating quite dramatic positional errors for steeply inclined satellites, and a systematic longitude error in satellites with low inclinations.

I recently put together a similar compilation, but used Horizons data and some arithmetical tweaking to convert the orbital elements to Celestia-friendly coordinates - all the named minor moons have now been added to the latest release of minormoons.ssc, but the numbered moons file is still languishing in the Celestia CVS tree. I think Chris is right not to add it to the standard package until there's some facility to turn orbits on and off within Celestia - Jupiter space is getting pretty cluttered!

But you can download the number moons file from http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/celestia/celestia/extras/numberedmoons.ssc (there's a "download" hyperlink on the fourth line below the page title).

Grant

[JackHiggins]

Oh ok I didnt realise they were in a different format to the one used by Celestia! In what way did you change the elements to make them useable by Celestia?

If there was an option to turn on/off different types of orbits, like you can do with labels it would be good- e.g. choices would be planets/moons/asteroids/comets/moons/retrograde moons- or something like that. Actually if retrograde moons could just be made to have a different label colour that'd be good too.

I think I better check my ssc's more carefully in future before I post them!!

Thanks!

[granthutchison]

In what way did you change the elements to make them useable by Celestia?

The nodes are easily corrected, because there's a simple offset between JPL and Celestia:

Mars:
Celestia = JPL - 40.863031

Jupiter:
Celestia = JPL + 20.453422

Saturn:
Celestia = JPL - 39.977646

Uranus:
Celestia = 174.986608 - JPL

Neptune:
Celestia = JPL - 24.527368

Pluto
Celestia = 197.515689 - JPL

Inclination relative to the planet's equator changes with time, as the satellite twists around in its Laplacian plane. This is most dramatic for Uranus, the outer satellites of which precess more or less in the plane of its orbit, rather than its equator - so satellites can switch between retrograde and direct orbits relative to Uranus' equator! (If you watched this from Uranus, you'd see the retrograde orbit tilt up slowly and pass over the pole, so that it became a direct orbit.) Prospero, with a listed inclination of ~152 degrees, has done this at present - its J2000 inclination to Uranus' equator is just 68 degrees.
So you really need to get the JPL Horizons system to tell you a body-centred inclination at epoch. In my update to minormoons.ssc, I've used an epoch-specific inclination, node and pericentre for all of the outer satellites.

Grant