<font color="yellow">"wouldn't a follow up nuke blast vaporise the smaller peices?"</font><br /><br />'Vaporize' means 'turn to vapor'. This essentially means that you're asking if the nuke would not only melt many many tons of solid rock, but also boil it. While I realize (or at least assume) that you didn't mean to be taken literally -- I bring this up as an example of where your thought processes are going off on this.<br /><br />Unless you've worked with explosives -- there's no real feel for what they do. I was a combat engineer and worked with TNT, C4, Ammonium Nitrate, and dynamite (the military grade -- not the nitro version). If you pile a sh*tload of explosives on top of the ground and light it off, what you get is a really big bang, and a very small crater. I mean ***very** small, depending on how hard the ground is, wed get craters an inch or less deep in the center and a foot or two in diameter even when using 20-30 pounds of TNT. This is because conventional explosions essentially create a very rapid expansion of gas. If something isn't directing or containing the expansion, it travels in the path of least resistance -- namely away from whatever solid object it is placed against. Explosions in space are even *less* destructive than ones on the ground, as there is no atmosphere to be expanded, and energy is transmittted only as radiation.<br /><br />In the case of a missile-delivered nuke hitting an asteroid, what would essentially happen (assuming it detonated on impact at ground-level) is that the nuke would go off. The conversion of matter to energy for the hydrogen would generate a huge amount of energy in the form of electromagnetic radiation (everything from radio to Gamma-Rays). The vast majority of this radiated energy would be directed *away* from the asteroid into open space. That portion which is directed toward the asteroid would impact the surface material -- much of which would melt and some of which would vaporize in an a