Where a Nuclear Blast Hits a Lumpy Asteroid Changes Its Deflection Nearly Fivefold
A new model that drops the spherical-asteroid shortcut found deflection speed varied nearly fivefold, and spin by a factor of 17, depending on which face gets hit.
If a dangerous asteroid were ever headed for Earth, one defense would be a nuclear detonation near it, blasting one side with X-rays to nudge it off course. New simulations from Sandia National Laboratories suggest that for a lumpy, irregular asteroid, where those X-rays land can change the outcome dramatically.
In a study published in the journal Icarus, physicist Nathan Moore and colleagues simulated irradiating different sides of the same asteroid. The momentum from the blast and the spin it caused changed sharply with the angle. In one case the deflection velocity, meaning the change in the asteroid's linear speed, was 3.9 millimeters per second from one direction but only 0.8 millimeters per second from another, a nearly fivefold gap. For a different asteroid, the change in rotation speed varied by a factor of 17.
"We expected aim to matter," Moore said. "What surprised us was how much."
Earlier models had to pick between realistic asteroid shapes and high-fidelity physics. Most researchers treated asteroids as spheres because it makes the math easier. "The trouble is, spherical asteroids don't exist," Moore said. His team built what it calls the most accurate deflection model to date by combining two existing Sandia codes. ALEGRA, normally used for fast, high-energy experiments such as magnetically imploding wires on Sandia's Z machine, handles the X-ray energy deposition. SPARTA, a direct simulation Monte Carlo code usually applied to weaker, slower forces such as satellite drag, tracks the vaporized material that flies off. "The key innovation here was figuring out how to combine the two simulation approaches," Moore said.
The new algorithm is more than 1,600 times faster than the old approach, which Moore said was critical because older models overwhelmed available computers. Moore has worked on the problem since a Z machine experiment showed for the first time that X-rays could deflect a small piece of asteroid material.
The simulations start from high-resolution 3D shape data of the kind gathered in spacecraft flybys, then model an X-ray burst with adjustable size, distance and target side. The program produces a heat map of how much of each surface region vaporizes, then calculates how hard the asteroid is pushed, how fast it spins, and how much surface rock breaks up. Breakup matters because the goal is to push the asteroid, not shatter it into smaller pieces that could still be deadly.
There are real limits. The model only handles breakup of rock on the surface, not the failure of the whole body, which Moore said will take more research and better tools. The simulation also needs accurate shape data from all sides, and humans might not have time to collect it. Placing a device at the ideal distance from the right side of a fast-moving object would also be difficult.
Still, the work gives planners a tool. In theory, one could enter an asteroid's shape and compute the best blast strength, distance and direction. "Our study shows how important it is to aim carefully," Moore said. "Pick the right side and you win. Guess wrong and it could be a bad day."
