Remember, Remember, the Meteor Fender

November 5, 2008
Not like this.

Not like this.

Imagine that you’re a spacecraft flying through space at up to many dozens of kilometers per second.

Now imagine that a tiny piece of space dust lies directly in your path; it too may be traveling at hypervelocity.  Even if you could see it, it’s difficult to swerve in space.

Even an extremely small piece of space detritus (micrometeoroids, a tiny chunk of comet, heck, even possibly human-made objects) can cause a catastrophic impact to a vehicle with very delicate components, due to very high relative speeds.

Impacts with space junk are a very real danger for space vehicles and astronauts.  The kneejerk reaction is to increase shielding: space suits are built with an outer layer of armor called Thermal Micrometeoroid Garment (TMG) which is designed to protect the wearer from solar radiation, heat loss, and impacts.

But with increased shielding comes increased mass, and obviously, an object’s weight is an important factor in being able to get it into space in the first place.  So what is a spacecraft designer to do?

Fred Lawrence Whipple, the astronomer with the coolest name in an industry with tons of pretty cool names.

Fred Lawrence Whipple, the astronomer with the coolest name in an industry with tons of pretty cool names.

Enter American astronomer Fred Whipple.  Whipple was a clever guy, and has a long list of astronomical accomplishments including hypothesizing the composition of comets, tracking the orbit of Pluto (then freshly discovered), studying the trajectory of meteoroids, discovering a handful of comets and a main-belt asteroid, as well as a bunch of other amazingly cool stuff.

One of his lasting contributions to aerospace is the fantastically-named Whipple shield, a kind of mass-conserving armor specifically designed to protect spacecraft against micrometeoroid and space debris impacts.

The design is simple: thin plates of armor (typically aluminum) are layered with small spaces between them.  The idea is that a small object traveling at high speed will impact the outer layer and break into smaller pieces and significantly slow.  The space between plates allows the object’s pieces to spread out, lowering their impact pressure.  If the object has sufficient velocity to punch through the next plate, the process repeats, and (hopefully) by the time the pieces of debris reach the actual hull of the vehicle, they simply bounce off harmlessly.

Stardust's Whipple shielding.

Stardust's Whipple shielding. I look so good!

The famous Stardust spacecraft has an excellent example of clearly visible Whipple shielding, which to me is deliciously ironic: Stardust was specifically made to impact and collect stellar and cometary dust, and return them to earth so they can be studied.  (It was successful, by the way–it returned its material sample capsule to Earth in early 2006 in an exciting and recording setting reentry sequence, but that’s another story.)

Whipple shielding, as with all forms of aerospace technology, continues to be studied and improved.  Newer designs use Kevlar or ceramic cloth to increase the baffling-power of the shielding plates.  And its usefulness is readily apparent: vulnerable parts of the International Space Station (including the human staff!) are protected by Whipple shielding, which is necessary due to its prolonged exposure to space debris, both natural and human-made.

The Whipple shield is a great example of good science and procedure: it’s elegant, it’s clever, it’s relatively cheap, and it gets the job done.  Plus, it’s got a cool name.

Fred Whipple was born November 5th, 1906.  He died in the summer of 2004.


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