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You'll need a big bomb to keep us apart
The regenerating liquid-metal robots in the Terminator movies have a cosmic relation: incoming asteroids that quickly reassemble if blasted by a nuclear bomb.

If a sizable asteroid is found heading towards Earth, one option is to nuke it. But too small a bomb would cause the fragments to fly apart only slowly, allowing them to clump together under their mutual gravity. Simulations now show this can happen in an alarmingly short time.

Don Korycansky of the University of California, Santa Cruz, and Catherine Plesko of the Los Alamos National Laboratory in New Mexico simulated blowing up asteroids 1 kilometre across. 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. The simulations were presented (pdf) last week at the Lunar and Planetary Science Conference in Houston, Texas.

Reassuringly, a 2009 study led by David Dearborn of the Lawrence Livermore National Laboratory in California showed that a 900-kiloton nuclear device - which is within our capability - would permanently disperse a 1-kilometre asteroid.