So let’s say a decently big — oh, call it 4 kilometers wide — asteroid is heading right for Earth. We have a few years advance warning, thanks to sky surveys always on the lookout for such threats, but we know it’ll hit Earth, causing global devastation. What can we do?

We have a few options.

We know for a fact that slamming a spacecraft into the rock can work; NASA’s DART mission was designed to test exactly this, and it worked very well. However, that’s most effective on small asteroids, and one as big as our pretend impactor might need dozens or even hundreds of DART-like missions to move it enough. What are the alternatives? 

One idea that’s been around a while is, um, somewhat brute force: nuke it.

Not to destroy the asteroid — that won’t work, since for one thing asteroids are big and a nuke might not do the job, but also when you do this you get thousands of smaller problems instead of one big one. And, I’ll note, thousands of problems that may now also be loaded with radioactive material.

Instead of obliterating it, the nuke is meant to explode near but not on the surface, so that the blast heats it up. Fast. High-energy X-rays from the nuclear explosion vaporize the surface material on the asteroid, and that gas expands incredibly rapidly, pushing on the asteroid like a rocket thrust. This can change its speed and velocity, changing the trajectory hopefully enough to miss Earth.

Theoretical analysis shows this would work on a Bennu-sized (~500 meter) asteroid. But we need real-world experiments to show if this really will work.

 New research just published by a team led at Sandia National Labs — where they zap and explode things with very high energies indeed — concludes it will work [link to journal paper]. They mimicked the explosion of a nuke by using an X-ray pulse to hit a couple of small samples, one of quartz and the other fused silica, to see what would happen.

The design is clever. A blast of X-rays generates a shock wave inside the sample, and older experiments measured how much momentum that push gave the sample. But that’s not all that happens; the expanding vapor from the high-energy pulse also pushes on the sample, and those older setups don’t measure that additional force. So this new experiment suspends the sample in the center of a ring of foil. When the X-rays hit the foil it vaporizes so quickly that the sample is left hanging in mid-air (actually, a vacuum inside a chamber) like Wile E. Coyote when a cliff drops out from under him. The X-rays generate the energy that vaporizes the rock before it can fall by any real amount, so it gets pushed backward while essentially floating. Behind the sample are infrared lasers that can precisely measure the velocity at which the sample moves to see how well the pulse did.

In both cases, the samples were blasted backwards at high speed, about 70 meters per second, or 250 kph. The samples were small, just 12 mm wide and very thin, but the vapor expanding away from them was traveling at over 20 kilometers per second — 72,000 kph! That gave the samples a helluva kick. The good news is these numbers agreed with theory, so that means the physics is pretty well understood^*.

The scientists then used math and physics to theoretically scale up to a real asteroid like our imaginary one above. They found that a single one-megaton blast can create enough vapor expansion to change a four-km asteroid’s velocity by about one centimeter per second. That’s the target range proposed by scientists to prevent impacts on Earth.

I know that doesn’t sound like much, but the critical factor is lead time. Imagine you’re driving a car and see someone crossing the street right in front of you. You have to hit the brakes hard, decelerating hugely to stop in time to miss the person. Now imagine you’re a kilometer away, and you determine that the person will be crossing the street right as you arrive there in a few minutes. If you just tap the brakes lightly, slow your speed a little bit, by the time you get there the person will be long gone (or, conversely, you can accelerate a bit and pass that spot before the person gets there).

Because of this the point isn’t to change the direction of the asteroid so much as the speed. That’s what DART tested, in fact, and was so successful at. It looks like a nuke could do the same thing on a much larger scale, and more efficiently.

Mind you, there are ramifications here.

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