Oh, and about asteroids.
It's a good idea, yeah... But it's very hollywoodic in its contemporary status. When a new asteroid heading to Earth detected, many words about "anti-space defence" being spoken. But NOTHING is done.
First, there are no need to possess such many WHs to strike asteroids only. Few of WHs are enough. Much more can be produced relatively quickly if needed - asteroid can be detected months and years before it strikes the Earth.
Second - is guidance system. Ballistic missiles are desighned to fly relatively low. I don't believe they can precisely hit an asteroid in far space. In near space, the hit can be fatal for both - asteroid and Earth itself. Earth possess space missiles, which can hit asteroid, comet or anything you like. So, we need those civilian general-purpose rockets and few warheads, which can be quicly mounted to a rocket.
Now, the question: for what the hell we have all those ICBMs, SLBMs and so on - if the only reason to have this arsenal (as you say) - is to shot down asteroids?