I actually just bought Kerbal Space Program, because it's available on Linux, and also supports a vibrant modding community -- include a massive cluster of mods

I actually just bought Kerbal Space Program, because it's available on Linux, and also supports a vibrant modding community -- include a massive cluster of mods (Realism Overhaul) that turns the Kerbal solar system into a realistic simulation of ours. Add in the Principia mod, which fixes minor glitches in the KSP physics engine, and you're good to go.