Encouraging results from a computer simulation point to the use of nuclear devices as a viable defense against Earth-threatening asteroids that suddenly appear out of the blue.
The Center for Near Earth Object Studies at NASA’s Jet Propulsion Laboratory runs an asteroid impact simulation once every two years. The exercise from earlier this year was unique in that the fictional threatening asteroid, dubbed “2021 PDC,” was detected just six months before its scheduled meeting with Earth’s surface. It was deemed an insufficient amount of time to deploy a mitigation strategy, so the participants focused primarily on disaster response. It served as a rude reminder of our vulnerability to these undiscovered asteroids.
Ideally, we’d have a few years or even decades to mount a response, such as using the gravitational influence of bulky spacecraft to gently nudge an asteroid from its Earth-bound trajectory. Alternatively, we could use kinetic impactors to change an object’s path or a nuclear device to smash it into thousands of pieces. This latter strategy, known as disruption, is the kind of thing we’ve come to expect in mindless Hollywood films, but it could work if done decades in advance; over the years, the ensuing fragments would likely go on their own orbital journeys and no longer threaten Earth.
But as promising new research published in Acta Astronautica shows, the disruption strategy might even work on asteroids that are just months away from striking Earth. This is very good news, as it suggests we have a fighting chance against incoming asteroids that suddenly and unexpectedly appear on our radar.
That a nuclear bomb or some other powerful device could be used to break up an asteroid is hardly a revelation. The overarching question going into the new study was the fate of the resulting fragments. It seemed possible that the ensuing debris would continue their journey toward Earth, potentially making a bad situation even worse. The scientists behind the new paper, led by physicist Patrick King from Johns Hopkins University Applied Physics Laboratory, ran simulations to study exactly this—the orbital tendencies of fragments pouring out from a nuked asteroid.
The study served as King’s PhD thesis while a student at Lawrence Livermore National Laboratory (LLNL). King’s colleague, Michael Owens, also at LLNL, developed a software program called Spheral, which modeled the lingering effects of a nuclear disruption on the fictitious asteroid. Spheral tracked the fragments from the moment of detonation and as the resulting fragmentary cloud orbited the Sun. The simulation even took the gravitational effects of other planetary bodies into account.
The simulation demonstrated the effects of a one megaton bomb that ignited near the surface of a 328-foot-long (100-meter) asteroid. The scientists ran the simulation multiple times, with asteroids traveling along five distinct orbits.
The results were very encouraging. For all asteroids tested, nuclear strikes performed months in advance of an impact served to significantly reduce the volume of incoming material.
“Simply stated, for a 100-meter object that’s projected to hit Earth, if we employ a robust nuclear disruption technique by at least one month before impact, we can prevent 99% or more of the impacting mass from hitting the Earth,” King explained in an email.
Some simulated outcomes were particularly good, including scenarios in which over 99.9% of an asteroid’s mass missed Earth when nuked two months in advance. Results for larger asteroids were not as impressive but still exceptionally good, with 99% of their total mass missing Earth if nuked six months prior to impact.
Of course, these results were derived from simulations, so the new findings should be viewed with a smidge of caution.
“We employed several approximations in order to make the study feasible, but we believe that we have captured the essential physics necessary to make general observations about nuclear disruption as a technique,” noted King when addressing the study’s limitations. “Much of our uncertainty is driven by our inherent uncertainty in the properties of the asteroids themselves; we definitely need to support more space missions to investigate their properties in more detail.”
When asked if breaking up an asteroid into many smaller pieces could actually make things worse, King said it’s one of the tradeoffs that must be made when deciding whether to employ the disruption technique.
“By making several fragments instead of one single impactor, it is possible that we multiply the threat,” he wrote. “On the other hand, disruption has its own advantages, and a disruption could succeed where a deflection might not. This is all part of the process of evaluating what technique to use.” Encouragingly, King said that, for scenarios similar to the ones proposed in the new study, it may even be possible—but not guaranteed—that the disruption technique could “significantly reduce the scale of the disaster if we disrupt the object by as little as two weeks before impact.”
Ultimately, as the scientists wrote in their paper: “We find that disruption can be a very effective planetary defense strategy even for very late … interventions, and should be considered an effective backup strategy should preferred methods, which require long warning times, fail.”
We’re not quite ready to nuke any asteroids, but we are taking the required steps to mount a planetary defense. NASA is on the verge of deploying its DART mission, set to launch on November 24. The spacecraft will smash into Dimorphos on October 2, 2022, in hopes of altering the asteroid’s speed and trajectory. The proof-of-concept mission could set the stage for more meaningful—and potentially life saving—actions, particularly in the event of having to launch a kinetic impactor in an emergency.
