Hey
I did the EPQ last year and it is the most horrific thing haha! Although is does get you in to a nice habit for referencing and citing and generally writing longer things from research.
I don't know a great deal about astronomy but the topic sounds really cool - Mine was on something about the Israel/Palestine conflict and the creation of a Palestinian state (which I somehow, and God knows how, got an A in). I set mine out as an argument of for and against points, but of course they have to be very detailed because of the several thousand word count needed. This could take up a nice section of the kind of thing you're wanting to write about assuming you know a lot about it - ie. the different options humans could have and the drawbacks these could bring! And a section on how humans could sustain themselves when on Mars could make up a big section. Add in a nice introduction and conclusion (both of which should be way bigger than you will ever have written for exams and things), that should bring you a nicely sized project.
Don't forget to save references and links as you find them, take it from me you do not want to lose them!
Hope this helps!
James