Oooo. I just had an idea, assuming this all works. Anybody see the episode of Star Trek with the doomsday machine? Have a really tough ship that moves slow, maybe 2 or 3 squares per turn. And has a QR so it doesn't run out of supply. Put it out on the edge of the quadrant and give it orders to head for the home planet, going over all teh colonies along the way. Set it up so it takes like ten years to get to the home planet and the player has to find a way to stop it before then.
Geoschmo