Nuking Collision-Course Asteroids Could Throw Earth A Last-Minute Lifeline

Launching a nuclear bomb at an asteroid on collision course with Earth may sound like the plot of a sci-fi movie, but new research suggests even a terrifyingly late intervention could help avoid an extinction-level crash. The potential for a so-called asteroid disruption has been well argued over recent decades, as astronomers and defense agencies try to figure out just what Earth could do in the fact of an incipient asteroid strike.

Such an event would, of course, be rare: the universe is large and Earth is, comparatively speaking, tiny. Nonetheless, just as the dinosaurs are believed to have discovered, a rock colliding with the planet can put a serious downer on a species' future.

Efforts for long-distance detection of potentially dangerous astroids that might one day present a risk to Earth is still underway. Unfortunately, there's always the possibility of a rogue rock slipping through the net. The Lawrence Livermore National Laboratory – part of the US Department of Energy – looked at just what might be involved for a late-stage intervention through either kinetic impactor or nuclear explosion means.

The results, published in Acta Astronautica by lead author Patrick King, saw the LLNL's Planetary Defense Group calculate the varying impact of different sizes of intervention carried out at different timescales. "While scientists would prefer to have more warning time," the LLNL points out, "they need to be prepared for any possible scenario, as many near-Earth asteroids remain undiscovered."

Turns out, we may not need to be quite as worried as previously thought. By using hydrodynamics calculations based on different astroid orbits and fragment velocity distributions, the researchers explored how a 1 Megaton nuclear warhead would affect a Bennu-shaped, 100 meter diameter asteroid if it exploded a few meters from the surface.

Five orbits were considered, and in all five cases the nuke was able to reduce the fraction of impacting mass on Earth by a factor of 1,000 or more, even if that explosion was just two months before the asteroid was due to strike. That means 99.9-percent of the asteroid mass would in fact miss the Earth, post-explosion.

Situations where larger space rocks are likely would remain effective if they were broken up earlier, the researchers concluded. "For a larger asteroid, the dispersal would be less robust," the LLNL explains, "but even dispersal velocities reduced by an order of magnitude would result in 99 percent of the mass missing Earth, if disruption is staged at least six months ahead of the impact date."

Nuclear explosions aren't, in fact, the preferred way to deal with such a threat to the planet. Kinetic impactors – such as used on the DART mission – have more real-world evidence to their effectiveness, for instance. However they also require more lead-time, leaving nukes a good alternative in cases where an incipient strike could be far sooner.

One challenge is that, though the asteroid may be broken up regardless of the method to achieve that, you still have to factor the potential affect of all the remaining shards and pieces. "This is a complicated orbital question though," study co-author Michael Owen explains, "if you break up an asteroid into pieces, the resulting cloud of fragments will each pursue their own path around the sun, interacting with each other and the planets gravitationally. That cloud will tend to stretch out into a curved stream of fragments around the original path the asteroid was on. How quickly those pieces spread out (combined with how long until the cloud crosses Earth's path) tells us how many will strike the Earth."

To make that easier, Owen and the rest of the team modified a piece of software called Spheral, which has been used before to model nuclear disruption of an asteroid. Building on that, they added gravitational evolution of the resulting fragment cloud into the mix, including both the Sun's affect and the impact of other nearby fragments.

The White House currently has a project underway to explore how Earth can be better-equipped to deal with upcoming astroid strikes, based on the assumption that future impacts are a matter more of "when" rather than "if" that takes place. The National Near-Earth Object (NEO) Preparedness Strategy and Action Plan aims to use modeling of this sort to improve warnings of future hazards, in addition to coming up with ways to minimize the impact on the planet.