For the past couple of months I've been working on a physically accurate and realistic space simulator, in my home-made engine with C++ and OpenGL, and I just released the first public tech demo!
The long-term goal of the game is to be able to design your own crafts, satellites, rovers and the like, positions them in orbit and the surface of various celestial bodies, and optionally write code for them to achieve truly autonomous operation – as well as just free-roaming and exploring the solar system!