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.