Re: Hit the US?
Assuming the best case of a head on impact the energy released is the kinetic energy of the impactor plus a equal amount of kinetic energy of the target. So as you say mV^2 of the impactor. The impactor is launched with a rocket and according to the rocket equation only a very small part of the rockets energy ends up in the payload so we are in the realms of very very much less than a small nuke. Also when the impactor hits sufficient mass to release the energy in the large relatively empty volume of the target that piece of the target is going to go from orbital velocity to zero and get very hot, however the bulk of the target is not going to get hit so will be moving away from the impact zone unscathed at orbital velocity. The solution is to vaporize the impactor just before the impact so that its cross sectional area matches that of the target so all the energy is released. The chances of getting this exactly right at opposing orbital velocities are slim and a partial impact would be a good result with the most likely scenario being that big bits of the target would survive. Whilst an impactor would kill the target it's not going to stop it coming down.