Radiation should be able to deflect asteroids as large as 4 km across

The old joke about the dinosaurs going extinct because they didn't have a space program may be overselling the need for one. It turns out you can probably divert some of the more threatening asteroids with nothing more than the products of a nuclear weapons program. But it doesn't work the way you probably think it does.

Obviously, nuclear weapons are great at destroying things, so why not asteroids? That won't work because a lot of the damage that nukes generate comes from the blast wave as it propagates through the atmosphere. And the environment around asteroids is notably short on atmosphere, so blast waves won't happen. But you can still use a nuclear weapon's radiation to vaporize part of the asteroid's surface, creating a very temporary, very hot atmosphere on one side of the asteroid. This should create enough pressure to deflect the asteroid's orbit, potentially causing it to fly safely past Earth.

But will it work? Some scientists at Sandia National Lab have decided to tackle a very cool question with one of the cooler bits of hardware on Earth: the Z machine, which can create a pulse of X-rays bright enough to vaporize rock. They estimate that a nuclear weapon can probably impart enough force to deflect asteroids as large as 4 kilometers across.

No nukes! (Just a nuclear simulation)

The Z machine is at the heart of Sandia's Z Pulsed Power Facility. It's basically a mechanism for storing a whole lot of electrical energy—up to 22 megajoules—and releasing it nearly instantaneously. Anything in the immediate vicinity experiences extremely intense electromagnetic fields. Among other things, this can be used to heavily ionize materials, like the argon gas used here, generating intense X-rays. These served as a stand-in for the radiation generated by a nuclear weapon.

For an asteroid, the researcher used disks of rock, either quartz or fused silica. (Notably, they only did one sample of each but got reasonably consistent results from them.) Mere mortals might have stuck the disk on a device that could register the force it experienced and left it at that. But these scientists were made of sterner stuff and decided that this wouldn't really replicate the asteroid experience of floating freely in space.

To mimic that, the researchers held the rock disks in place using thin pieces of foil. These would vaporize almost instantly as the X-ray burst arrives, leaving the rock briefly suspended in the air. While gravity would have its way, the events triggered by the radiation evaporating away a bunch of the rock would be over before the sample experienced any significant downward acceleration. Its movement during this time, and thus the force imparted to it by the evaporation of its surface, was tracked by a laser interferometer placed on the far side of the disk from the X-ray source.

With all that set, all that was left was to fire up the Z machine and vaporize some rock.

I’m melting!

The researchers divide the ensuing events into three phases. The first, starting after the radiation reaches the target, involves streams of superheated liquid flowing out into the vacuum and forming a gas somewhat away from the rock's surface. This process erodes a bit under 25 micrometers of the rock's surface before ending at 0.05 microseconds when the radiation from the Z machine fades out. As events continue, however, the gas's expansion starts imparting momentum to the sample, with a peak acceleration of over 10⁷ meters per second².

That ends at three microseconds after the radiation burst arrives. By now, the gas is expanding away at over 20 kilometers a second, but no new material is being liberated from the rock sample. Over the next 20 microseconds or so, the amount of momentum transferred to the sample drops steadily until the process is essentially complete.

All the data gathered from the two real-world tests were then used to build a simulation of the behavior of this system. These simulations were able to determine details like how quickly the energy of the X-rays was deposited into the sample (90 percent of the energy in just 14 nanoseconds) and the pressures generated by the rapidly expanding gas.

Once the simulations were accurate enough, they were scaled up to an actual asteroid-sized object, taking into account things like the surface curvature, which will influence the amount of radiation that reaches a given point on the surface, and how the ensuing force will influence the asteroid's trajectory.

With a radiation exposure that delivers roughly 1,000 joules per square centimeter, superheated regions developed on the asteroid, producing pressures of over 100 gigapascals (roughly a million times the atmospheric pressure at sea level). That's enough to shock-melt quartz, even if said quartz weren't already being heated by radiation.

Taking an estimated value of the amount of force needed to shift an object's orbit sufficiently to miss the planet, the researchers calculate that a radiation burst of this magnitude would be enough to deflect asteroids with a diameter of as much as 4 kilometers. That's a bit less than half the size of the impactor that did in the dinosaurs, but this is assuming we had limited warning of the impactor's approach to Earth. The earlier warning we have, the more time we have to deflect it and the less momentum we need to impart.

This is a big step forward in understanding a process that we're unlikely to be able to test at full scale any time soon. But, as the researchers involved acknowledge, it's still pretty limited. The asteroids we've visited so far have complicated surfaces composed of various materials. And some of those, like water, other ices, and dust, are likely to be easier to vaporize, altering the amount of pressure that builds up near the asteroid. But, as long as Sandia is occasionally willing to devote an experiment chamber to these sorts of experiments, we could eventually build out a more realistic understanding of how materials respond when bathed with this sort of radiation.

Nature Physics, 2024. DOI: 10.1038/s41567-024-02633-7 (About DOIs).