I found this a few days ago while looking for a planesphere program:

Celestria.  A freeware space "simulation" that allows you to explore the galaxy.  One of the nice features is expandibility, with additional star systems, and other objects, easily added.  Here's some screenies:

Polyphemus, with Pandora off to the left, for Avatar fans (There's three versions of the Avatar system availible)



Saturn (For SiFi fans, they have the Valley Forge, Berkshire and Sequoia form the movie "Silent Running".  Though, for some reason, the textures do not show up on my rig.  I think it has to do with the version of Celestia I'm running)



Of the addon objects, several are ships from all the popular SF franchises, such as Star Trek, and Babylon 5, along with a few movies, like 2001: A Space Odyssey.

Some of the add-on that have Jupiter-like planets "cheat" on their moon textures by "recycling the textures of various solar system objects, but coming up with new texture maps is easy.  They are pure JPG, no tedious mucking about with DXTBmp, or the like, just create a 1024X512 JPG, and save it to the "extra/starsystem/texture, and boom, new texture.

In a nut-shell, "Geek Heaven".  No pun intended.

http://www.shatters.net/celestia/