I'd really like to see it open sourced, but I guess that isn't happening. Setting it up properly is a bit of a mess right now, you need a 3rd party graphics engine called D3D9Client for that (or stick to using the old D3D7 engine). A lot of useful functionality is also 3rd party plugins you have to install from a variety of sites. You can play it right out of the box, but it's much better with 3rd party additions. If it was open source, perhaps it would be a bit easier to manage.
... now to toot my own horn for a bit:
I've also been working on space simulation and orbital mechanics projects for fun (and some university classes earlier).
If there's anyone here who likes Orbiter and/or Kerbal Space Program and is interested in related programming projects, get in touch (in HN comments or IRC/freenode for my HN username).
Here's a quick screenshot of what I'm working on right now: http://ctrlv.in/884711 (just started doing some 3d visualization earlier this week)
And here's a two body orbital mechanics library I have been working on for a few years: https://github.com/rikusalminen/twobody (it should have everything you need to write your own KSP clone - look around in the different branches)