If a sizeable 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.
Link: NewScientist Article ("'Terminator' asteroids could re-form after nuke"
From LPI Abstract:
Asteroids and comets range in composition from rubble piles to delicate conglomerations of ice and rock to solid objects. They are occasionally found on trajectories that pose an impact hazard to the Earth. There is an ongoing scientific debate about how to best mitigate the risk posed by these potentially hazardous objects (PHOs). Several of the techniques proposed involve applying shortduration impulses to a PHO in order to change its orbit, by means of standoff blasts, surface detonations, or kinetic impacts. However, such methods have the potential to knock fragments off the parent body or disrupt it completely. The resulting fragments may continue to threaten ground or orbital assets if they have not been dispersed far enough on diverging trajectories, or collectively deflected away from the original Earth intercepting trajectory to a sufficient degree. Here we explore the question of the time required after an impulse for fragments to reaggregate or disperse to large radii.
Link: LPI Abstract (REAGGREGATION TIMESOF POTENTIALLYHAZARDOUS OBJECT FRAGMENTSAFTER A HAZARD MITIGATION IMPULSE. D. G. Korycansky, CODEP, Department of Earth and Planetary Sciences, University of California, Santa Cruz CA 95064 (firstname.lastname@example.org), C. S. Plesko, Los Alamos National Laboratory Applied Physics Division.)