"Sir, the explosion successfully ripped the asteroid to shreds! The world is safe!"
This could be the victorious statement to conclude the next Armageddon-style blockbuster movie after the world's nations clubbed together to construct an awesome nuclear missile designed to wipe out the threat of an incoming asteroid. The hero gets the girl, mission controllers give each other high-fives as we cut to a cheesy montage of international celebrations, firework displays and teary-eyed world leaders pledging a new era of world peace.
Unfortunately, in research presented at the Lunar and Planetary Science Conference in Texas earlier this month, this "happy ending" storyline could have a nasty twist. Scientists have found that if a nuclear weapon did blow an asteroid apart, it could reassemble itself in a very short period of time, continuing its path to death and destruction.
"Um, sir, the pieces of asteroid have re-formed. We have incoming! Again!" Cut to another montage of screaming people on the streets, babies crying and the hero suggesting the cast should get hammered on a 200 year-old bottle of whisky he'd been saving for a "special occasion" (or doomsday).
Reforming asteroids
Don Korycansky of the University of California, Santa Cruz, and Catherine Plesko of the Los Alamos National Laboratory in New Mexico have simulated the nuke versus asteroid scenario and demonstrated that if the explosion of an interceptor nuke was too small, the asteroid will reform under its mutual gravity much faster than expected.
(This is assuming the asteroid was made of rock, acting like a "rubble pile" rather than a solid lump of iron ore. It's debatable whether any explosion could do anything about an asteroid that's mainly metal, apart from heating it up a little.)
Trying to destroy asteroids with nuclear explosions is a risky business at the best of times, but this research has found that a 1 kilometer-wide asteroid could reassemble itself in a matter of hours.
"The high-speed stuff goes away but the low-speed stuff reassembles [in] 2 to 18 hours," said Korycansky at the meeting.
Keep Your Nukes In Their Silos
But it's okay, another study from 2009 has set a lower limit on the size of bomb to dispense with an asteroid, preventing it from reassembling. All we'd need is a 900-kiloton weapon (50 times the size of the bomb dropped on Hiroshima in 1945) to do the job properly.
I'd argue that attempting to blow up an incoming asteroid should only be considered as a last ditch attempt at neutralizing the threat. So long as we have enough lead time, there are other (less destructive) ways of changing an asteroid trajectory.
We could change the color of the asteroid to absorb or reflect more sunlight, thus changing the amount of radiation pressure on the chuck of rock. Over time, this will alter the asteroid speed and/or direction.
We could send up a spaceship of sufficient mass to hover close to the asteroid, its own gravity steering the asteroid off course (this is known as a "gravity tractor").
How about landing rocket engines on the asteroid surface? Or detonating a series of miniature nuclear bombs near the asteroid, small enough to not damage it, but large enough to give it a kick in the right direction?
Asteroid Politics
But all these methods need time, and although it is likely that we'd spot a large doomsday asteroid many years before it poses a threat to Earth, one could blindside us.
If that was the case (as Hollywood would have us believe), we might have to go all Armageddon-style as a last minute desperate measure.
However, there is another rather unpalatable solution: let the asteroid hit us.
The ramifications of blowing up a deadly asteroid would be a highly contentious issue, especially if the asteroid is big enough to wipe out a city, say, but small enough to keep the majority of damage localized to a country or continent.
For example, if it was discovered that an asteroid was heading straight for Los Angeles and the U.S. government decided to fire a nuclear missile at it, only for the resulting explosion to cause chunks of rock to rain down on Moscow, Beijing, Tokyo and London, although LA would be saved, the U.S. wouldn't be very popular. World wars have been triggered over much less.
In the grand scheme of things, planning to evacuate and then sacrifice a city where an asteroid has been predicted to hit might be a better option than letting the nuke decide where to scatter the debris.
It would be like trying to decide whether to get shot by a single bullet from a pistol (and knowing where that bullet will hit you, so you can prepare yourself) or many pieces of shot from a shotgun (and not knowing which major organs will get hit). It's a tough decision, but logic (and international politics) may dictate that the predictable single bullet might be a better option.
But if we are faced with a civilization-ending asteroid appearing out of nowhere, like the one that wiped out the dinosaurs, I'd be the first to suggest assembling the world's nuclear arsenal.
Leading image: The Japanese Hayabusa spacecraft reached near-Earth asteroid 25143 Itokawa in 2005 to collect a sample of dust from its surface. Such missions are important to understand these potentially dangerous Earth-crossing space rocks (JAXA).
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