Nuking asteroids, saving the Earth

Andy G wrote:
>... I was naively thinking that any Earth-impacting asteroids
>could be drilled into and have nuclear detonations set off
>deep within their cores, to break them apart. This would
>change the vectors of much of the material of these asteroids,
>ensuring more "misses"; and both reduce the size and widen
>the spread of that shrapnel which would reach the Earth's
>atmosphere.

This technique seems to have been abandoned by everybody except
hollywood. Do the math on the level of destruction afflicted to a locale under
an impactor, versus the destruction done to the entire asteroid-facing
hemisphere when it's peppered with a shotgun blast of mountain sized
chunks.
More smaller detonations spreads the energy available more efficiently -in
terms of kilotons available related to damage inflicted to targets on the
ground, than a single large blast. It's why MIRVs have several smaller
warheads for a city area instead of a 100mt single warhead.
What's impossible to calculate is how a bomb placed at the core will affect an
asteroid which has been only poorly characterised to know all of its interior.
You won't save the planet by landing Bruce Willis and a few heroes and
quickly drilling and placing bombs, because they'll need to have a lot of time
to explore and study the rock, and then, maybe, place the bombs at the exact
precise place to shatter the rock to do the most help to the Earth (after
factoring spin and such).

It makes a much less exciting movie, but the best way would be to thoroughly
survey the asteroid while you're de-spinning it (many techniques available,
none requiring magic), then attach a solar or nuke thermal rocket to it, fed
from asteroidal ices. You only need to make it miss this one pass that it's
threatening, then it's gone out into the solar system. For effectively no
additional cost, keep the rocket firing, and use the near-miss you've created to
warp the course to swing past Venus a few turns around the Sun later. Use
that to swing it back past the Earth-Moon system again, for the final
maneuvers to place it into HEEO, where the mining facilities have been
placed to do the real work of turning it into useful things (as opposed to the
bootstrapping operation landed on it which just produces ices for reaction
masses and life support for the rotating crews riding it).