Oxygen? In space?

Most of us think of space as a void. If Sigourney Weaver has taught us anything, it's that in space no one can hear you scream. But the truth—and this is truth of the scientific sort, not the horror movie alien brand—is that there is oxygen out there. "The atmosphere gets less and less dense. In LEO, it's tenuous. But it's not so tenuous that there aren't still 100 million oxygen atoms per cubic centimeter," Massa says.

Transition state structure for a common polymer fragment reacting with atomic oxygen.

The density of oxygen in LEO is, of course, worthy of mention because of the damage that it can do. But oxygen's state at that altitude, about 180 to 650 kilometers above the earth's surface, is even more significant. Oxygen is at its most stable and is least reactive as a molecule, its common state in the earth's atmosphere. The two atoms are bound together and share two electrons between them. In LEO, however, oxygen molecules are split by ultraviolet light from the sun and endure as atomic oxygen.

Atomic oxygen is ravenous, having two unpaired electrons and wanting nothing more than to make itself stable again by forming chemical bonds. Thus, it is highly reactive. As hungry as it is, though, the effects of atomic oxygen on a given material depend upon the reaction's activation energy. The activation energy is the minimum amount of energy required to place molecules or atoms in a state in which they can undergo a chemical transition. In other words, it's the ooomph oxygen atoms must have in order to break the molecular chains that make up the spacecraft polymers. When these chains are broken, new molecules form and float off into space at the expense of the polymers.