The Art of
Hypervelocity Impact


September 2013

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Space junk litters our skies. Tracking it and avoiding it heaps extra costs on already expensive ventures. And the debris of yesterday’s satellites and aircraft may have already reached critical levels, according to a report from the National Research Council in 2011. Dream all you want of heading to Mars. If we don’t clean up the trash in orbit, or find a way to effectively shield spacecraft, you’re going nowhere.

But just how much damage does a single piece of orbiting rubbish do? Can the skin of our spacecraft possibly nullify its threat? One way to answer such questions is to take a space junk-sized piece of junk, fire it out of a gun at seven kilometers per second, and see what kind of mess you’ve made of the target.

That’s just what Jonathan Mihaly does at the Small Particle Hypervelocity Impact Range facility at the California Institute of Technology. The machine, SPHIR for short, “gun” for shorter, is designed to smash projectiles, usually nylon or aluminum, into a target and record the results.

Quantifying Space Junk

Space junk comes in three size categories. The biggest pieces can be seen with radar from down here on earth (pretty much anything longer than a centimeter). We can count them and subsequently avoid them. We can get a bead on the tiniest pieces—smaller than a millimeter—by doing a before/after with the surface of things that have been to space and back. But the detritus that falls between those two sizes can’t be seen from down here.

“Our facility is right there,” says Mihaly. “For better or worse, we’re smaller than other impactors. We’re shooting things that are two millimeters long.”

Caltech's SPHIR facility, with two spectrograph systems mounted on top of the target chamber. Image: Caltech.edu

 

 

 

$500 Shots

The SPHIR’s top speed to date is 10 kilometers per second. But it’s not the speed that makes it unique. In addition to the “invisible” size of the pieces of junk it fires, its recording system allows it to collect multiple types of data from each impact. And, perhaps best of all, it’s far cheaper than its competitors, costing a mere $500 per shot. “We’ve tried to maximize the amount of scientific information we get per experiment, maximize that and reduce the cost of the experiment,” says Mihaly.

The SPHIR is a two-stage gun. In the first, hydrogen, or sometimes helium, is used to create a pressured shock pulse. That pulse is fired down to a nozzle, which converges the force to a point. That bursts a diaphragm and rockets a shock wave toward the target. 

Where other facilities use X-rays to take a snapshot of an impact, the SPHIR uses a beam of collimated laser light. This helps keep costs down as there’s no need for additional equipment—and procedures—to prevent researchers from irradiating themselves. Since it’s monochromatic and doesn’t require a flash, the laser allows other recording equipment to simultaneously take a peek at the impact.

Marksmanship is a human affair, and the gun’s precision is a matter for its operator, namely Mihaly. Aligning the gun is a tedious affair, he says, a “painful process of shims, referencing surfaces, alignment rods. Do it once and go back to the beginning. At the end of the day it’s trial and error.” But, like any gun enthusiast, Mihaly is proud of his accurizing skills. “We’ve improved it by a kilometer a second since we got the gun.”

Improving Shields

Right now Mihaly is using all that speed to examine the effect of target thickness on debris clouds. When a projectile hits a metal plate at hypervelocity it creates two debris clouds, one of the projectile material and a second of the plate material. “So now you have these two populations of debris moving down stream at high speed,” says Mihaly. A typical shield in space has two layers. The first is used as, essentially, a sacrificial layer to protect the second. Needless to say, you don’t want the debris cloud from the first layer of the shield to exacerbate potential damage to the second. Making the layers thicker is the answer, but with weight a fairly crucial parameter in space—or in getting there—knowing exactly how thick a wall needs to be is fundamental.

The debris clouds from an impact fly into a “capture pack.” “Just foam you can buy at Home Depot,” says Mihlay. The debris makes tiny holes in the foam. So to find out just where it all went, the layers of the pack are placed on a light table. The debris path is mapped by looking where the specs of light shine through: a bit of tedium Mihaly has been willing to delegate. “We have a very efficient and hard working high school student working for us,” he says.

Hard work, high speed, and low cost. The recipe might not solve the space junk problem, but it just may get us through it in one piece.

Michael Abrams is an independent writer.

We've tried to maximize the amount of scientific information we get per experiment, maximize that and reduce the cost of the experiment.

Jonathan Mihaly, California Institute of Technology

 
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