During the space shuttle's third flight, back in 1982, astronauts Jack Lousma and C. Gordon Fullerton saw something remarkable. Above the tail of the shuttle, where there should have been empty space, they saw a faint orange glow instead, flickering with ghostly light. Was the space shuttle haunted? What was going on?

To understand this phenomenon, known as "shuttle glow," you should know something about the region of space where the shuttle orbits. NASA shuttles stay in low Earth orbit, which is generally about a hundred and seventy miles above the Earth's surface. At that height, the atmosphere has tapered away to almost nothing-less than a billionth of the pressure is at ground level-but there are still gasses there. Indeed, low Earth orbit is well within the ionosphere, a major part of our atmosphere.

The thin gasses of the ionosphere rush past the space shuttle at about five miles per second, forming a very fast, very thin wind. This wind is so thin you'd never be able to feel it, even if you stuck your head out the space shuttle window the way a dog sticks it's head out a moving car.

The wind causes two things to happen. First, it erodes the surfaces of some materials. The erosion is slight, but it can be significant for camera lenses or other sensitive equipment. Second, the shuttle's surface can scoop up atoms from this wind, and these atoms can undergo chemical reactions. One reaction, involving nitrogen and oxygen, can produce a luminous orange glow.

And that's what the astronauts saw. It wasn't a ghost-it was shuttle glow.