Orbiter

Orbiter is a free advanced space flight simulator. There are many scenarios to choose from, including launching the Space Shuttle from Kennedy Space Center to deploy a satellite, meeting with the International Space Station or taking the futuristic Delta-glider for a tour through the solar system. This game is firmly based on realism and has a steep learning curve.

download: http://orbit.medphys.ucl.ac.uk/download.html#download