Orbiter

It's an awesome free space flight simulator. It's pretty realistic, and therefore, kind of hard at the beginning - right now I can hardly go to a stable orbit and Im trying to figure out how to reach ISS, and going to the moon or other celestial body seems impossible to me, haha. If you're into space flight in any way, you have to try it.

http://orbit.medphys.ucl.ac.uk/

Be sure to download some addons for it, like the high definition planet surface meshes, the sound addon and the DeltaGliderIV spaceship, which is the most realistic thing for this game (not in a technical way, but it has an awesome onboard computer, live crew, life support systems, autopilots etc).
 
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