Kerbal Space Program is a multi-genre game where the players create their own space program.

In KSP, you must build a space-worthy craft, capable of flying its crew out into space, without killing them. At your disposal is a collection of parts, which must be assembled to create a functional ship. Each part has its own function and will affect the way a ship flies (or doesn’t). So strap yourself in, and get ready to try some Rocket Science!

Main Features:

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 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 staging sequence allows for complex ships and advanced functionality.