I don't see it mentioned in the comments but those interested in realistic space flight simulation should check Orbiter [1]. Orbiter tries to be as physically accurate as possible by using accurate drag models, elliptical gravity (no circular orbits) and many other interesting features. It has a large community behind it and is scriptable with Lua. I have only played with it for a couple of days and I am impressed by it. It is definitely not a game per se, but lots of fun can be had with it. Oh and is freeware, so no need to pay for it.