"If that thing was going to hit earth, the US alone could get about 4000 nukes up their that are 150 times more powerful than the ones dropped on Hiroshima and Nagasaki. Also, since it would be in space where there naturally is radiation(i think), they could even use a hydrogen bomb or two."
First off, a hydrogen bomb is an atomic bomb, and second, i wouldn't be so confident, the momentum of one these doomsday type asteriods is trully gargantuan, simply hurling bombs at it is not neccesarily a great idea. The best bet would probably be to attempt to divert its course, i'm far from certain we would be able to do it though.... perhaps, human ingenuity cannot be underestemated, lets hope we don't ever have to find out. (though it seems governments are taking more seriously the threat of asteroid collisions and research being done into methods of aversion.)