"Don't criticize what you can't understand..."
I think we don't need to do that in an asteroid attack. We can just send two or three rockets to go to the asteroid and use their gravity (no matter how small a rocket is, it always has a gravity and that is enough) to change the path of the asteroid.One point I don't think has been mentioned. Given an emergency (asteroid strike) type situation, how much fuel -- how many ships -- are available on Earth to ferry humans to Mars (or wherever)? I think you are looking at a "two by two" into the Ark situation with many not very happy about being left behind.