Nuclear Blast vs. Asteroid: Which Side You Hit Changes the Push Nearly Fivefold

Where a nuclear X-ray burst strikes an asteroid may matter as much as the blast itself. A new computer study from Sandia National Laboratories found that the push given to the same asteroid changed nearly fivefold depending on the direction of the radiation, according to a summary published by ScienceDaily.

The work, published in the journal Icarus, is a simulation, not a real-world test. It suggests that the lumpy shape of a real asteroid has to be taken into account in any serious plan to deflect one.

The central numbers from the study

• 3.9 millimeters per second: the change in one simulated asteroid's speed when the X-ray burst came from one direction.
• 0.8 millimeters per second: the speed change for the same asteroid when the burst came from another direction.
• Nearly fivefold: the gap between those two results in deflection velocity, which is the change in the asteroid's forward speed caused by the burst.
• A factor of 17: how much the change in spin rate varied with direction for a second simulated asteroid.
• Over 1,600 times faster: how much quicker the team's new algorithm is than older approaches, according to physicist Nathan Moore, who led the research.

How the idea works and why shape complicates it

The concept is that a nuclear explosion releases X-rays that heat an asteroid's surface. Material on that surface vaporizes, and the effect pushes the object onto a safer path.

Many earlier simulations treated asteroids as smooth spheres, which simplifies the math. Real asteroids have ridges, depressions and uneven surfaces, and those features change how they react to intense radiation. Moore said his team expected aim to matter, but the size of the effect surprised them.

Earlier models forced researchers to choose between capturing an asteroid's complex shape and doing highly detailed physics calculations. Doing both took more computing power than was practical, according to the summary.

What the new model does

The team merged two simulation programs already in use at Sandia. One, called ALEGRA, normally models fast, high-energy experiments, such as wires imploded by magnetic fields on Sandia's Z machine. The other, called SPARTA, usually handles weak, slowly changing forces, such as solar wind drag on a satellite. Moore called combining them the key innovation.

Researchers can rebuild an asteroid from detailed 3D images taken during spacecraft flybys. They then simulate an X-ray burst while changing its strength, its distance from the asteroid and the direction it arrives from. The software maps how much material would vaporize across the surface as a heat map. From that it estimates the resulting push, any change in rotation and the extent of surface damage.

Moore said the work grew out of an earlier experiment on the Z machine. That experiment gave the first demonstration that X-rays could deflect a small piece of asteroid material. The new study asks how the same process would play out on a whole asteroid.

What this means in practice

For now, this is research on how to plan a possible defense, not an operation or a response to a known threat. The findings suggest that knowing an asteroid's shape, and choosing where to aim, could be essential to a successful mission. A poorly aimed blast could deliver far less push than expected.

The aim also affects spin, and a good mission would ideally change an asteroid's path without breaking it into several dangerous pieces. Redirecting one large object could be preferable to creating fragments that might still threaten Earth.

Limits of the simulations

The model has a significant gap. It currently considers only the breakup of rocks on the surface, and Moore said it cannot yet accurately predict whether the entire asteroid would break apart. Closing that gap will take more research, investment and more advanced computational tools, according to the summary.

The results come from simulated asteroids rather than from a test on a real one. The research was funded through Sandia's Laboratory Directed Research and Development program.

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Around Prague, based on reporting by ScienceDaily