If a truly huge asteroid were to be discovered on a collision course with Earth, and if it were clearly impossible to divert it with a nuclear bomb because of its great mass, there might just be another way to do it, provided that we had enough time. I call this "Asteroid Billiards". The idea is that we find some modest sized asteroid that we CAN nudge around with a few nukes, and nudge it into a collision course with the potential Earth impactor. A typical impact speed would be tens of kilometers per second, putting into the diversion effort thousands or even millions of times as much energy and momentum as the original nuke had. We are, in effect, using the second asteroid as an amplifier to increase the effect of our piddling 10 megaton nukes. Almost any collision that you could imagine would divert the impactor sufficiently to cause it to miss Earth. There might well be sufficient energy released as heat to vaporize both asteroids nearly completely, producing rock and metal vapors that would be dispersed harmlessly by the solar wind.

David F Mayer