Re: What is the effect of a nuke in space?
Use multiple nukes as your mass driver. Just make sure the mass ejected leaves the "orbit" of the asteroid and make sure it leaves in a useful direction creating consistant (non offseting) thrust; otherwise, I agree: You will accomplish nothing.
If it ever happens, I hope we have a 15 year window in which to plan.
5 years to draw up a plan of attack. 10 years to execute it. Any shorter period, and we are not likely to mount an effective response. Any longer and we are likely to procrastinate ourselves out of existence.