Engines of Our Ingenuity

No. 345:
SHUTTLE GLOW

by John H. Lienhard

Click here for audio of Episode 345.

Today, we expect the unexpected. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

"How high is the sky?" asks Donald Hunton, an Air Force physicist. Airplanes can still fly 12 miles up. But the air is only a 20th as thick as it is on the ground. The space shuttle orbits at altitudes 15 times higher than that -- 170 miles up. Strange things happen to the air that's left at those heights.

For one thing, the air is one ten-billionth as thick as it is down here. It's so thin that molecules can easily travel miles without hitting another molecule. And those molecules are hotter than the ones around us -- over 1000x F -- but there aren't enough of them to heat anything. The air's too thin. It's composition is odd because lightweight molecules and hot molecules float to the top of the atmosphere. There's a lot more hydrogen and helium up there.

That high-altitude air is giving space engineers fits. Down on earth's surface we're protected from heavy ultraviolet radiation. At high altitudes that radiation breaks diatomic oxygen into atoms. And high-temperature atomic oxygen is viciously corrosive stuff.

So unexpected problems arise. The first symptom astronauts saw was a glowing corona around surfaces facing into what we might call "the wind." It's not a kind of wind you can feel. The air's too thin for that. But it reveals itself in a glowing shock wave around the shuttle. The glow is caused by complicated chemical reactions, and those reactions are made worse by the high speed. The shuttle moves so fast that it breaks certain molecules apart when it hits them.

Worse than that, atomic oxygen attacks many surfaces. The shuttle returns to earth with camera lenses fogged, with clear plastic covers turned yellow, and with paint dulled.

Research is now aimed at inventing materials that won't be affected by atomic oxygen. Space engineers are especially worried about the Hubble space telescope. It's scheduled to fly in 1990, and they've found the coating designed to protect its lens will trigger a bright corona. Any corona will blind the telescope as it squints into the inky reaches of space.

But don't worry. The Hubble telescope will fly. Engineering designers expect the unexpected. They're at their best when they have to slip the clutches of problems from their blind side. A new material, an altered configuration, a new concept -- it's fun, I tell you. It's so much fun to beat nature at her own game.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Hunton, D.E., Shuttle Glow. Scientific American, November, 1989, pp. 92-98.

The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.


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