It's the thrilling climax to many asteroid apocalypse movies, but it doesn't stand up to science. Apparently, blowing up a large object in space comes with its own problems... not the least of which is, it probably won't work.
New Scientist reports on research by Don Korycansky of UC Santa Cruz and Catherine Plesko of the Los Alamos National Laboratory in New Mexico that simulated explosions on asteroids moving through space, and discovered that the problem may be gravity:
[T]oo small a bomb would cause the fragments to fly apart only slowly, allowing them to clump together under their mutual gravity... When the speed of dispersal was relatively low, it took only hours for the fragments to coalesce into a new rock. "The high-speed stuff goes away but the low-speed stuff reassembles [in] 2 to 18 hours," Korycansky says.
According to Korycansky and Plesko, it would take a 900-kiloton nuclear device to properly disperse a 1 kilometer asteroid. Suddenly, there seems a point to the nuclear arms race after all.
'Terminator' asteroids could re-form after nuke [New Scientist]