Henrik, wouldn't they have needed a larger explosive in the case of your altered plan A? If less mass is uses as propellant, wouldn't it need more energy to generate the same amount of thrust?

RE, the OP, placing an explosive in a container allows it to do more work as the energy is absorbed by more of the material. If you explode a nuke near the surface of an asteroid, only a small fraction of the energy released would fall upon the surface since most would radiate in other directions from the omnidirectional/spherical explosion. Moreover, Reflectance and emission of that energy can further reduce the amount of propulsive power.

Putting the nuke inside the asteroid not only allows all of the energy to be captured, it acts as a tamper, holding together long enough to allow more of that energy to be converted into mechanical energy. This is what happens when you pop a nuke in the atmosphere. You don't just get one release of energy, you get two, hence the characteristic double-flash. Actually, saying it's two releases is probably over-simplifying, but I think it exemplifies the concept. The first release is seen, but then the surrounding air heats up so much that it becomes opaque. Then it absorbs more energy, then radiates more energy itself at lower temperatures/freqencies. The fireball expands, and as it does so it cools by multiple methods, adiabatic, conduction and radiation. A lot is released as thermal radiation due to the clarity of the atmosphere at certain wavelengths. Inside an asteroid, however, much of this thermal energy would continue to be absorbed, producing vapors from various materials, the phase-change expansion and thermal expansions of which would continue to provide mechanical energy to propel the asteroid explosion.

All this really means is, it's not how big it is, but how you use it.

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"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau