Celestia  is probably one of the coolest apps (ever) that I have come across and it really gets the geek-meter high up there, not to mention it is so educational as well, so if you are a geek, or hungry for some knowledge and exploring or got kids in school who might benefit then read on. You can see the giant hurricane on Jupiter, take a quick spin to the moon Io, set off for icy Europa and zoom back to Earth. All these computer simulations are possible using Celestia which is a free real-time space simulation that lets you experience our universe in three dimensions. 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 travel 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.

You control your position and velocity using a series of simple keystrokes and you can have shortcuts that transports you to say Mars, Saturn or any location in the universe that you define. In addition to space, you can also travel through time. For example you can position yourself outside the solar system and watch the planets zip around the sun as if the universe was running on fast-forward. You can also visit the Russian space station Mir starting on Feb. 20th 1986 (when it was launched) to March 23 2001 (when it fell back to Earth). If you try and find it on say March 24th 2001, then you won’t find it.

The coolest factor is that this is all open source which allows everyone to improve it and contribute to it. There are many people who had created add-ons . Celestia can also take you to places that never existed, because many people created add-ons that re-create fictional words from scifi such as the planet Tatooine (from Star Wars) or the space station from 2001: A Space Odyssey.

This allows you to create your own customised tours through the galaxies and also allows you to share it with the others on the net. The author, Chris Laurel is a Microsoftee, who took a year and half off to explore his 3D graphics programming skills.

If you are interested in creating your own add-ons then check out the Introduction to Celestia Addons .