3D Universe Simulator

 OpenUniverse (OU for short) is a fun, fast and free OpenGL space simulator. It
 currently focuses on the Solar System and lets you visit all of its planets,
 most major moons and a vast collection of smaller bodies in colorful, glorious
 and realtime 3D. If you've ever had a chance to visit Mercury or asteroid
 Geographos, here you'll find them looking exactly the same way, following
 exactly the same path as when you've left them.
 .
 Notice that OpenUniverse is not actively being maintained anymore and that
 users are recommended to use Celestia instead.