Good point, but the chances of a failed launch are fairly small, and the damage from accidentally detonating a large nuke on the Earth's surface, while bad, would be small compared to human extinction.
If we are sufficiently uncertain about the comet's path, we might accidentally deflect it towards us. Obviously, if this is just as likely as deflecting it away from us, it would not be worthwhile to try. But most likely we would be able to reduce the probability of a catastrophic impact even after taking into account the possibility of such an error.
If we are sufficiently uncertain about the comet's path, we might accidentally deflect it towards us. Obviously, if this is just as likely as deflecting it away from us, it would not be worthwhile to try. But most likely we would be able to reduce the probability of a catastrophic impact even after taking into account the possibility of such an error.