I'm thinking to model a really BIG asteroid, about 2 km of radius, and make a scenario where the asteroid falls to Earth. Now, build a spaceship to rendezvous it, dock with it (the asteroid will be a spacecraft.dll type ship with docking ports to attach a mass driver) and burn to change his orbit, like in Arthur c. Clarke book (God hammer, in spanish is "El martillo de Dios"), where a ship does it with an asteroid called Kali who is going to hit the Earth...
Any idea about it?