What Can Stop a Speeding Bullet? A Whipple Shield, Of Course

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(Credit: Fraunhofer Institute for High-Speed Dynamics)

What happens when you’re hit by something going 15,000 miles per hour? Total obliteration, more or less.

That’s a very real scenario that spacecraft engineers must keep in mind every time they put something in space. Collisions with objects in orbit are rare, but they do happen. In the past, paint chips have left craters in the space shuttle and a French satellite was disabled in 1996 after its gravity-gradient boom was severed by a chunk from an exploded rocket.

Shields Up!

To protect expensive spacecraft, shields are in order. And not just any hunk of metal will do. Objects in space are moving fast — craft in low-Earth orbit whizz by at around 17,000 miles per hour — and we can’t make a single shield thick enough to protect against that kind of speed. So, engineers have turned to something called a Whipple shield, named for its creator, Fred Whipple. Instead of a single layer of material, a Whipple shield relies on several, each separated by empty space.

The outermost layer is designed to break apart when hit, shattering the projectile at the same time. The resulting cloud spreads out the force of the impact across a greater surface area, decreasing the force at any one point and increasing the chances that the inner layer will hold. Many spacecraft today, the International Space Station included, utilize Whipple shields.

You can see a Whipple shield in action in the video from the European Space Agency. It shows a 2.8 millimeter aluminum bullet shot from a gas gun impacting the shield. The bullet is traveling around 15,000 miles per hour, but it doesn’t manage to penetrate the thin secondary shield. The reason is clear — upon impact with the first layer the bullet gets totally vaporized, robbing most of its destructive power. The shield is a fiber metal laminate, or thin layers of metal held together by a composite.

Updated versions of Whipple shields use fillings of Kevlar or Nextel ceramic fibers between layers to add even more protective power. And as an added benefit, the shields are far lighter than conventional armor would be, though they do make the spacecraft a bit bigger.

The one downside, of course, is that Whipple shields are pretty much a single-use product. Once the first layer has been breached, the shield won’t work anymore. But, because impacts in orbit are still so rare, the chances of being hit twice in the same place are exceedingly low. And, for now, that’s enough.