Vega Strike is an free and Open Source 3D Action-Space-Sim (works on Windows, Linux and Mac OSX) that lets you trade, fight, and explore in a vast universe . You should definitely try this if you are a fan of movies such as Star Trek, Star Wars, or anything else involving spaceships of all sizes in space fighting, merchandising, and exploring. The physics is very realistic, and it feels a lot like flying an actual spaceship. Just one slight crash into a mothership causes damage, and just one hit from a space pirate sends sparks flying all over the place.
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