Unlike the smooth, round objects often used in computer simulations, real asteroids have uneven surfaces and irregular shapes. Those differences can dramatically influence how an asteroid moves when exposed to intense radiation.

A new study published in Icarus found that changing the direction of an X-ray burst could produce vastly different results, even when targeting the same asteroid. In one simulation, the asteroid's speed changed by 3.9 millimeters per second when irradiated from one direction, compared with only 0.8 millimeters per second from another. That represents a nearly fivefold difference in deflection velocity, the change in the asteroid's forward speed caused by the burst.

"We expected aim to matter," said Nathan Moore, a physicist at Sandia National Laboratories who led the research. "What surprised us was how much."

The effects on an asteroid's rotation could be even more dramatic. For another simulated asteroid, the change in how quickly it spun varied by a factor of 17 depending on the direction of the X-rays.

These findings suggest that understanding an asteroid's shape and carefully selecting where to direct a nuclear blast could be essential to protecting Earth from a future impact.

The research received funding through Sandia's Laboratory Directed Research and Development program.

A New Way to Simulate Asteroid Deflection

To investigate how different asteroid shapes respond to nuclear blasts, Moore and his colleagues developed what they describe as the most accurate asteroid deflection model yet.

Earlier computer models faced a major limitation. Researchers typically had to choose between representing an asteroid's complex shape and performing highly detailed calculations of the physical forces involved. Combining both approaches demanded more computing power than was practical.

The new system overcomes that obstacle, allowing scientists to study realistic asteroid shapes while maintaining the precision needed to calculate their response to an X-ray burst. The result is the most detailed simulation of asteroid deflection produced so far.

"Our new algorithm is over 1,600 times faster, which was critical to this breakthrough," Moore said. "With the old models, computers just couldn't handle it."

Why Lumpy Asteroids Change Everything

Moore's interest in improving these calculations grew out of an earlier experiment using Sandia's Z machine. That experiment provided the first demonstration that X-rays could deflect a small piece of asteroid material.

The next challenge was determining how the same physical process might affect an entire asteroid.

"I started wondering how to scale up the calculation to more accurately predict deflection of a full-scale asteroid."

A major problem was the widespread practice of treating asteroids as spheres. Although this simplifies the mathematics, it overlooks the irregular shapes of real asteroids, which can have ridges, depressions, and uneven surfaces.

"You can make estimates that way, but asteroids come in all kinds of odd, lumpy shapes," he said. "That's when I latched onto some work one of my colleagues was doing related to outgassing of objects in space. I think we had an aha moment when we realized we could do this. No more spheres. We would be able to model deflection for any asteroid."

The solution involved bringing together two sophisticated simulation systems already used at Sandia for very different types of physical problems.

"The ALEGRA code is normally used to model fast, high-energy experiments, like magnetically imploding wires on the Z machine, while SPARTA, a direct simulation Monte Carlo code, is usually used to model comparatively weak, slowly evolving forces, like the drag on a satellite from the solar wind," Moore explained.

"The key innovation here was figuring out how to combine the two simulation approaches."

Predicting What Happens After a Nuclear Blast

The new computer model allows researchers to reconstruct an asteroid using detailed 3D images captured during spacecraft flybys. They can then simulate a nuclear X-ray burst while adjusting the explosion's strength, its distance from the asteroid, and the direction from which the radiation arrives.

The software calculates how much material would vaporize across different parts of the asteroid's surface and displays the results as a heat map. Using that information, it estimates the resulting push, changes in the asteroid's rotation, and the extent of surface damage.

These calculations are important because a successful planetary defense mission would ideally change an asteroid's trajectory without breaking it into multiple dangerous fragments. Redirecting one large object could be preferable to creating several pieces that might still threaten Earth.

However, the simulations cannot yet predict every possible consequence of a nuclear blast.

"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.

Developing that capability will require additional research, investment, and more advanced computational tools. An intense X-ray burst can trigger numerous physical effects, some of which influence one another. Accounting for all those interactions makes it exceptionally difficult to predict exactly how an asteroid would behave during a real deflection attempt.

Choosing the Right Blast Could Save Earth

Despite these remaining challenges, the findings offer valuable guidance for future planetary defense efforts. The research shows that the direction of a nuclear blast is not simply a minor detail. It can substantially change how effectively an asteroid is redirected.

"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."

The new simulation tools could eventually help mission planners identify the most effective way to deflect a threatening asteroid. In principle, researchers could enter detailed information about its shape, then calculate the ideal explosive strength, distance, and direction needed to move it away from Earth.

Turning those calculations into a real mission would still be extremely difficult.

For one thing, the software requires reasonably accurate information about the asteroid's shape from every direction. If an asteroid were discovered on a collision course with Earth, there might not be enough time to gather the necessary observations.

Another challenge would be delivering and positioning a nuclear device at precisely the right location relative to an asteroid traveling through space. Even if scientists could identify the ideal blast configuration, executing it would require extraordinary precision.

Nevertheless, the improved simulations could eliminate much of the uncertainty surrounding one possible method of asteroid defense. By better understanding how nuclear X-rays interact with the complex surfaces of real asteroids, scientists are moving closer to predicting which strategies would work best.

For Moore, that progress is an encouraging step toward ensuring humanity has a better chance of preventing a catastrophic asteroid impact.