Atomic oxygen is constantly generated on the topside of the atmosphere by ionizing radiation. The ionizing solar radiation, UV and particles, will on impact dissociate molecular oxygen to atomic oxygen. However, due to the ratio between the UV and the particle flux from the sun, and due to comparable cross sections for the reactions, the UV generation largely dominates. Atomic oxygen is very reactive, and will enter into chemical reactions with most materials. This is especially true for earth orbiting spacecraft structures, as the orbital velocity corresponds to a kinetic energy released at collision which is above the activation energy for many chemical processes.