Imagine you're on a spaceship returning from a mission on a foreign planet, suddenly, the propulsor begins to fail. Occasionally, you have no propulsor whatsoever, that would let you in a very vulnerable position in case of an alien attack wouldn't it?(which of course, is going to happen). The [unpropulsed]ship starts to orbit around the earth, you don't have a lot of time until the ship breaks into earth's atmosphere, in which case, aliens will reach earth. You cannot let that happen to your planet, so your mission now is to get to the power generator in the ship, and blow it up. Yes, yes, you'll die, but at least aliens on the ship won't get to earth ;)
Yep, that's basically my game