“If we have enough warning time, we could potentially launch a nuclear device, sending it millions of miles away to an asteroid that is headed toward Earth,” said Mary Burkey, an LLNL physicist and leader of a team that published their research in the Planetary Science Journal on December 19. “We would then detonate the device and either deflect the asteroid, keeping it intact but providing a controlled push away from Earth, or we could disrupt the asteroid, breaking it up into small, fast-moving fragments that would also miss the planet.”
Burkey worked with coauthors Robert Managan, Nicholas Gentile, Megan Bruck Syal, Kirsten Howley, and Joseph Wasem on a new approach to simulate the energy deposited on an asteroid’s surface by a nuclear device. According to LLNL, “this new tool improves our understanding of the nuclear deflection's radiation interactions on the asteroid's surface while opening the door to new research on the shockwave dynamics affecting the inner asteroid.”
Energy density delivers: NASA’s Double Asteroid Redirection Test (DART) took a 10-month, one-way trip to confirm that NASA can successfully navigate a spacecraft into an asteroid.
DART struck an “asteroid moonlet” named Dimorphous at roughly 14,000 miles per hour, altering its trajectory. But DART—at 1,260 pounds—was heavy, as a kinetic impactor must be to change the course of a much more massive target. And that’s where nuclear devices could help. Just as nuclear thermal rockets would be more efficient and quicker than massive conventional rockets for long-distance space propulsion, a relatively lightweight nuclear device could be delivered to an asteroid target more quickly than a kinetic impactor and then would rely on detonation—not collision—to throw an asteroid off course.
A smarter alternative? According to the LLNL team’s paper, unlike a kinetic impactor, which relies on the object’s mass and velocity to knock an asteroid off course, the impact of a nuclear device can be fine-tuned by adjusting the height above the asteroid at which the device is detonated.
If the asteroid should be kept intact and simply pushed onto a different trajectory, a nuclear device’s detonation can be timed so that its “height of burst” is relatively high above the surface of the asteroid, compared to the asteroid’s diameter. Radiation from such a detonation would “illuminate a large swath of the object's surface area, causing the material to heat up, vaporize, and expand rapidly.” That expansion would provide “blowoff momentum” that would alter the asteroid’s total velocity. If an asteroid is deemed too small for controlled deflection or is at risk of fragmenting into several smaller objects that could pose new threats, the device could be detonated close to the asteroid instead. Such an “intentional disruption mission” would produce blowoff momentum but also would “cause a shock wave to propagate through the asteroid, breaking it into many small, harmless, fast-moving fragments.”
Modeling a mission: In the event of a planetary defense emergency, high-fidelity multiphysics simulation modeling such as that developed by the LLNL team will give decision-makers risk-informed information that could prevent an asteroid impact, protect essential infrastructure, and save lives, explained team member Megan Bruck Syal, LLNL’s planetary defense project lead. “While the probability of a large asteroid impact during our lifetime is low, the potential consequences could be devastating,” she said.
According to the published paper, the model covers a variety of potential mission initial conditions: four different asteroid-like materials at a range of porosities, two different source spectra, and a broad range of radiation fluences, source durations, and angles of incidence.
“Every detailed, high-fidelity simulation and every broad sensitivity sweep brings the field closer to understanding how effective nuclear mitigation would be,” the researchers concluded. “In the event that an Earth-impacting asteroid is located, an operational capability to quickly and credibly assess mission options and uncertainties for planetary defense may potentially save lives and essential infrastructure.”
