A study published in the journal Icarus finds that the angle at which a nuclear X-ray burst strikes an asteroid can sharply change how far the blast pushes it off course. The research, led by physicist Nathan Moore of Sandia National Laboratories, used a new computer model that simulates deflection on realistic, irregularly shaped asteroids rather than the smooth spheres common in earlier work. Science Daily reported the findings.
In one simulation, an asteroid's forward speed changed by 3.9 millimeters per second when irradiated from one direction, compared with 0.8 millimeters per second from another — close to a fivefold gap. For a second simulated asteroid, the change in spin rate differed by a factor of 17 depending on where the X-rays came from.
"We expected aim to matter," Moore said, according to Science Daily. "What surprised us was how much."
Why it matters
The work bears on planetary defense: the question of whether humanity could nudge a threatening asteroid onto a safer path before impact. A nuclear burst in space would not push an asteroid the way a physical shove would. Instead, its X-rays would superheat a patch of the surface, vaporizing rock, and the escaping material would drive the asteroid in the opposite direction. How much material leaves, and from where, depends on the terrain hit.
That makes the shape of a real asteroid — ridged, pitted, lumpy — a central variable rather than a rounding error. A deflection plan built on a spherical stand-in could misjudge the outcome, the study suggests.
The computing problem the team solved
Earlier models forced researchers into a trade-off. They could represent an asteroid's complex geometry, or they could run highly detailed calculations of the physical forces at play — but not both, because combining the two demanded impractical computing power.
Moore's team combined two simulation codes already in use at Sandia for unrelated problems. ALEGRA, normally applied to fast, high-energy experiments such as magnetically imploding wires on the lab's Z machine, handles the burst and its immediate effects. SPARTA, a direct simulation Monte Carlo code typically used for comparatively weak, slowly evolving forces like solar wind drag on a satellite, models the slower movement of vaporized material. The innovation was linking them.
The resulting algorithm runs more than 1,600 times faster than its predecessors, Moore said. "With the old models, computers just couldn't handle it."
The project grew out of an earlier Z machine experiment that provided the first demonstration that X-rays could deflect a small piece of asteroid material. Moore said he then began asking how to scale the calculation up to a full-sized asteroid. The obstacle was the habit of treating asteroids as spheres.
"You can make estimates that way, but asteroids come in all kinds of odd, lumpy shapes," he said. Inspiration came from a colleague's work on how objects in space release gas. "No more spheres," Moore said. "We would be able to model deflection for any asteroid."
What the model can and cannot do
The software can rebuild an asteroid from detailed 3D images taken during spacecraft flybys. Researchers can then simulate an X-ray burst while varying its strength, its distance from the target, and the direction the radiation arrives from. The model calculates how much material would vaporize across the surface, displays the results as a heat map, and estimates the resulting push, changes in rotation, and surface damage.
Those outputs matter because a successful mission would ideally shift an asteroid's path without shattering it into several pieces that still threaten Earth. Redirecting one large body is generally preferable to creating a cluster of hazards.
The simulations have a known limit. "Right now, our computer model only considers breakup of rocks on the surface. We can't predict breakup of the entire asteroid body accurately yet," Moore said. Reaching that point will take further research, funding, and more capable computing tools. An intense X-ray burst sets off many physical effects, some of which feed into one another, and accounting for all of those interactions is what makes a real deflection attempt so hard to forecast.
The research was funded through Sandia's Laboratory Directed Research and Development program.
Moore's summary of the practical lesson was blunt: "Our study shows how important it is to aim carefully. Pick the right side and you win. Guess wrong and it could be a bad day."
Watch for follow-up work on modeling whole-body breakup, the gap the team has flagged as the next major hurdle before any such mission could be planned with confidence.
