Planetary safetyWorry: gravitational force would cause nuked asteroids to reform

We have written before about how, even if the vision of a nuclear weapons-free world which President Obama outlined in his speech in Prague last year, ere to be realized, mankind would still need a few nuclear weapons for the purpose of destroying menacing asteroid hurtling toward Earth. The question is how large should the nuclear warheads retained for hits purpose be.

David Shiga writes that if a sizeable asteroid is found heading toward Earth, blasting it with a nuclear bomb which is too small 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 3,281-foot-in-diameter (1-kilometer 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) at the Lunar and Planetary Science Conference in Houston, Texas.

Shiga notes that, reassuringly, a 2009 study led by David Dearborn of the Lawrence Livermore National Laboratory in California showed that a 900-kiloton nuclear device would permanently disperse a 1-kilometre asteroid.

Stuart Fox writes that the asteroid that killed the dinosaurs was ten times larger than the 1-kilometer across asteroids in the simulation, meaning that if a similar asteroid were to approach Earth and nuked, it would reform much faster after being nuked. Rather than blowing up the asteroid directly, many scientists advocate blowing the nuke up next to the asteroid. That explosion could nudge the rock out of the way of Earth, without creating dangerous smaller meteorites or allowing it to come back together.