Low-Earth-orbit atomic oxygen erosion of polymer surfaces

Spacecraft in low Earth orbit (250-700 km altitude) encounter a residual atmosphere that is 90% atomic oxygen in its ground electronic state produced by the solar uv photodissociation of molecular oxygen. At a relative velocity of 8 km s", an observer on the spacecraft at an altitude of 300 km experiences an average flux of 10 oxygen atoms cm~ s" approaching in the ram direction with a relative kinetic energy of 5 eV. When exposed to this flux, vitreous carbon, polymers, and many polymer-based composite materials suffer surface erosion. Upon microscopic examination, the roughened surface is seen to consist of troughs and points that have a shag-carpet appearance. We show that, when a target atom has weak bonds tangential to the surface, the impact energy of the incident O atom rapidly dissipates into the bulk of the solid, and the target atom cannot react. This produces a point. By contrast, when tangential bonds are strong, the reverse occurs, and a trough results. The anisotropic bonding on which this model depends can be found in both polymers and vitreous materials.