Protecting spacecraft from atomic oxygen

Findings are reported from Space Shuttle flights STS-3, 4, 5, and 8 regarding the degradation of materials exposed to atomic oxygen in low earth orbit. Atomic oxygen, a strong oxidizing agent, is present in low concentrations at such altitudes, and a spacecraft can sweep up considerable volumes of it at orbital velocities of 5 miles/s, especially if mission lifetimes are measured in years or decades. Material loss has been found to be a function of the fluence (the number of oxygen atoms striking a unit area of surface over a given period); fluence is proportional to atmospheric density, orbital velocity, surface attitude relative to velocity vector, and duration of exposure. Atmospheric density depends first on altitude and second on the phase of the 11-year solar activity cycle. Metals, in the experiments, reacted less than nonmetals. Graphs and a table are included, permitting calculation of how far a surface of various organic materials will recede on a spacecraft in low earth orbit. The limited data base on atomic oxygen interactions with materials, using both Shuttle flight experiments and ground-based facilities must be augmented. Space Station design is imminent, and the understanding of such interactions is critical to the success of that design.