Celestia – The free space simulation
The free space simulation that lets you explore our universe in three dimensions. Celestia runs on Windows, Linux, and Mac OS X. 
Unlike most planetarium software, Celestia doesn’t confine you to the surface of the Earth. You can travel throughout the solar system, to any of over 100,000 stars, or even beyond the galaxy.
All movement in Celestia is seamless; the exponential zoom feature lets you explore space across a huge range of scales, from galaxy clusters down to spacecraft only a few meters across. A ‘point-and-goto’ interface makes it simple to navigate through the universe to the object you want to visit.
Celestia is expandable. Celestia comes with a large catalog of stars, galaxies, planets, moons, asteroids, comets, and spacecraft. If that’s not enough, you can download dozens of easy to install add-ons with more objects.
- Jack’s Celestia Add-ons
Jack is a prolific creator of add-ons for Celestia. He has produced models and trajectories for many famous (and not-so-famous) space missions. - Selden’s List of Resources for Celestia
Selden is one of the most active members of the Celestia community and compiled a large catalog of notes, documents, and add-ons for Celestia. His collection is of particular interest to more technical and scientific-minded users.