Hate to break it to you all, but the chances are that we won't see the asteroid that kills us until a few months before impact, when it is much too late to do anything about it. This is something to do with the way light is refracted in space, we know there are things out there big enough to destroy the earth, but because they're in shadows we can't see them. Scientists also reckon though that even if we spotted something within 10 years of it hitting us, we'd be unable to do anything about it because at present there are no rockets capable of carrying men (let alone weapons) that are ready to take off, and all those ideas we've got from films about putting nukes on asteriods and letting them do their thing are impractical - they would turn one dangerous object into thousands, and would make them radioactive to boot. And there's not even any point in hiding on the other side of the world because any catastrophic impact would result in a dust cloud encircling the earth for generations, blocking sunlight and leading to the death of any plants and animals that remained, lowering the earth's temperature by several degrees and probably throwing the planet into another ice age. The best you can hope for is that a) it never happens in your lifetime, or b) you're in the impact zone and die instantly. Hope that's reassured everyone :-)