Which is why you would use waves of consecutive detonations consisting of more and more (smaller) nukes. Think of a cone pointed towards the asteroid. The tip would be the first initial large nuke. Because of the mass and velocity of the asteroid, it is unlikely that the fragments would spread out all that much. You are right that there would likely be fragments that still have an orbit that would lead to collision. After the first explosion you would detonate 5 more nukes spread out evenly to further perturb and break down the asteroid remnants. You would repeat this many times. Each time the nukes could be smaller as the mass of the asteroid remnants would be getting smaller and smaller making the force of the nukes more effective against them. This would probably only be suitable for ruble pile asteroids, but I would imagine those are the hardest to use ablation with, so it may still be appropriate to use more destructive methods.
There are two goals with this: (1) break down or deflect any large chunks to prevent damage related to ground impacts. (2) cause enough change to the orbits of the asteroid remnants such that any subsequent collision with earth would be spread out over time to prevent overheating of the atmosphere via clouds of debris.
The best solution is always to have the asteroid remain as intact as possible, but for certain asteroid types and scenarios, it may just have to be good enough especially as a backup.
There are two goals with this: (1) break down or deflect any large chunks to prevent damage related to ground impacts. (2) cause enough change to the orbits of the asteroid remnants such that any subsequent collision with earth would be spread out over time to prevent overheating of the atmosphere via clouds of debris.
The best solution is always to have the asteroid remain as intact as possible, but for certain asteroid types and scenarios, it may just have to be good enough especially as a backup.