Kerbal Space Program (PC Download)

The Vehicle Assembly Building and the Space Plane Hangar allows players to build spacecraft out of any imaginable combination of parts.
Fully-fledged, Physics-based Space Flight Simulation ensures everything will fly (and crash) as it should.
Take your Kerbal crew out of the ship and do Extra Vehicular Activities.
Fly out to Moons and other Planets.
Procedural Terrain delivers detailed terrain at a vast scale. The Kerbal Planet is 600km in radius!
Mod-enabled, allows players to create new content and modify the game.
Ship systems. Keep an eye out for engine temperatures and fuel levels, and try not to explode.
Build ships with multiple stages, and jettison parts away as they burn out.
Full control over the ship’s setup allows for complex ships and advanced functionality.
Dock spacecraft together to build Space Stations, Massive Starships, or even surface bases on other worlds.
Manage your crews.
Research and Develop new technologies to advance your Space Program.