The kinetic energy works of ejected mass, not just the mass of the bomb. The bomb provides energy for acceleration of said mass - you vaporize and eject a big c

The kinetic energy works of ejected mass, not just the mass of the bomb. The bomb provides energy for acceleration of said mass - you vaporize and eject a big chunk of rock with the bomb if correctly placed. Which is why placing the bomb underground a'la Armageddon is much more effective at diverting it.