Imagine this: an asteroid is on a collision course with Earth. Not good. Now imagine that we’re able to blow it up, but it re-forms in, say, a few hours. Not good at all.
The latter is what Don Korycansky of the University of California, Santa Cruz, and Catherine Plesko of the Los Alamos National Laboratory in New Mexico discuss in their paper, “Re-aggregation times of potentially hazardous object fragments after a hazard mitigation impulse.”
According to the paper, many of the proposed techniques for changing the orbit of a potentially hazardous object (PHO), if one were to head our way, involve short-duration impulses such as stand-off blasts, surface detonations, and kinetic impacts. However, “such methods have the potential to knock fragments off the parent body… 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,” states the paper.
How long it would take the fragments to re-aggregate depends on the “physical properties of the original object and the impulse applied,” the paper continues. Korycansky and Plesko simulated blowing up asteroids one kilometer across with varying impulses to see how long it takes for the asteroids to put themselves back together. They presented the simulations at the Lunar and Planetary Science Conference in Houston, Texas earlier this month.
The results are a little alarming: when the fragments dispersed at a relatively low speed, it took only hours for the fragments to coalesce into a new rock. In other words, hitting an asteroid with a bomb that’s too small would cause the fragments to fly apart slowly, allowing them to re-clump under their mutual gravity – in two to 18 hours.
Not to fear, though. Other studies have shown that a 900 kiloton nuclear weapon (which is within our capability) is enough to blow up a one kilometer asteroid – and keep it blown up.