Kerbal Space Program - Its educational and written in Mono too!

My son is really taken with Kerbal Space Program. This great games allows you to design your own  space craft and so run your own on-going space program, all with a realistic physics engine.

What is particularly nice is that this cross platform Mono based application is being built in a very agile manner with a new release most weeks, each adding features as well as bug fixes. There also seems to be an active community of people building plug-ins for extra space craft components and rovers.

I am not sure how much orbital mechanics will appear in his school exams this year, but it is certainly educational in the longer term.