Celestia is a free 3D astronomy program for Windows, Mac OS X, and GNU/Linux. Created by Chris Laurel, it is licensed under the GNU General Public License.
The program, based on the Hipparcos Catalogue, allows user to display objects ranging in scale from artificial satellites to entire galaxies in three dimensions using OpenGL. Unlike most planetarium software, the user is free to travel about the Universe.
NASA and ESA have used Celestia, but it should not be confused with ESA's similar program, Celestia 2000.