Celestia is a software which explore the universe in 3D, & it is free just like Stellarium. It takes you a step further than a planetarium like software, by not confining you to Earth. For Astronomy enthusiasts or for anyone for that matter, this can provide an out of this world experience. Like, for example, viewing the entire solar system, & watching the planets go round the sun, or following Halley’s comet as it orbits the sun.
Celestia is basically a space simulation software, with lots of addons making it expandable as well. You can comfortably explore the different stars, planets, satellites & other heavenly bodies in the universe, by moving around. It also lets you scale the passage of time to watch the changes in the universe. It gives you the option to select a body, & follow it or sync your orbit with it, thus allowing you to observe it more closely. You can also zoom in on different bodies to take a closer look.
The right click menu gives you some useful options, like getting information about the body from the internet, follow/sync orbit, goto the selected body & select associated bodies (satellites, planets etc). There are keyboard navigation shortcuts, in additon to the mouse & menu based navigation.
Celestia is around 12 MB (smaller than Stellarium), with most of the size being contributed by the images & textures. The program is quite processor intensive & you’ll need to have a good display card to view its full potential. It may also have trouble running with some display cards.
One of the first things you should probably do after installing Celestia is try out the demo. This gives you a pretty good coverage of the power of the software. Once you are familiar with the controls, you’ll be all set to explore the universe as you like. There are also add-ons, which let you add more objects, including space crafts (even fictional ones like the Death Star from Star Wars & the Star Trek ships) to the Celestia universe.