Nice idea :-). Actually, I guess this should be quite easy: In principle, the JSBSim flight dynamics model used in Outerra also works for space. It's rather limited, though as it only employs the simplest gravity model (just spherical, no Sun/Moon/Jupiter influence) and doesn't cover some other aspects of orbital dynamics. But hey - for a game it should do. NORAD publishes the orbital elements of every known object regularly, so if this is updated on the fly errors should not be huge. Alternatively, there is a number of open source orbit propagation libraries around that will be more accurate.
Probably cameni and angrypig are drowing in the flood of good ideas - if you simulate the whole world, people will want you to do everything there is in the world ;-).
