Could nuclear weapons (H-bombs) be used in the near future, assuming advanced warning of an impending direct hit, in some way as a defense against incomming large astroids?
The danger would be that of changing a rifle firing a single bullet into a shotgun firing innumerable pellets! Ouch!!!
It has been suggested that a blast to the side might change the trajectory sufficiently to cause a miss. Alternatively, if your H-Bomb were sufficiently powerful to completely vaporize the asteroid... but how many megatons would that take?
I believe there is little doubt we could save ourselves (even with current technology) if the asteroid was spotted early enough that we had sufficient lead time, but what if we had only weeks, or days, or even hours?