Well, given our current level of technology, that would be a species-killer.
Movies like 'Armegeddon' and 'Deep Impact' aside, nukes won't help a bit against an asteroid. Really - their biggest effects are the fireball and shockwaves generated by their interaction with an atmosphere. No atmosphere in space! The actual vaporization a nuke does is very, very little.
Best bet to survive an asteroid is to find it a LONG way ahead of time (say, 850 years), then send some ships out to mount giant rockets to the side of it and changes its course. Over a few million mile elliptical path, you only have to change its course by a half degree or so to make it completely miss Earth.
So, yeah, it's a problem. And, we'd better have a small fleet of ships heading towards that rock within a hundred years or so if we want to survive it.
If it had only been found 50 years out, we'd have no chance whatsoever to survive it (space travel takes a LONG time, and we'd have to develop the technology to beat it in the first place)
A fanatic is one who, upon losing sight of his goals, redoubles his efforts.
--George Santayana