NASA NIAC project has determined a conceptual solution to asteroid impacts.
Use an interceptor which separates into two parts.
First part 10 meter ahead makes a crater.
The nuclear bomb part follows but is 20 times more effective by blowing up in the crater.
The fragmentation would reduce the impact to about 0.1% of non-fragmented asteroid. Fragmentation is 1000 times better than letting the whole asteroid hit.
Earth would pass through the fragment cloud.
We have to have the assembled interceptor rocket(s) so that it would be ready to launch.
– Superconducting active radiation shielding