Celestia is a 3D astronomy program created by Chris Laurel. The program is based on the Hipparcos Catalogue (HIP) and allows users to travel through an extensive universe, modeled after reality, at any speed, in any direction and at any time in history. Celestia displays and interacts with objects ranging in scale from small spacecraft to entire galaxies in three dimensions using OpenGL, from perspectives which would not be possible from a classic planetarium or other ground-based display.
NASA and ESA have used Celestia in their educational and outreach programs, as well as for interfacing to trajectory analysis software.
Celestia is available for Linux, Mac OS X, and Microsoft Windows. Released under the GNU General Public License, Celestia is free software.