Asteroid/comet impact is a real threat! A threat which unlike the terrorist issue could actually wipe us all out and possibly even all life on the whole planet! Nuclear weapons are one of a few methods we could use to destroy/alter the course of an approaching object with. There are other methods but many of these would take years or decades to work. If we were to discover an obect at short notice then the nuke option would possibly be our best and only option available. Or we could sit back and let it hit!