A Consideration of Atomic Oxygen Interactions with the Space Station

Significant surface interactions which are very likely due to exposure to atomic oxygen, the major component of the low Earth orbital environment, have been observed on recent space flights. These interactions are manifested as surface recession and, therefore, mass loss and appear to arise from oxidation of the materials involved. A computer model has been developed to compute atomic oxygen fluence (total integrated flux) for a generalized spacecraft in orbital flight based on Mass Spectrometer and Incoherent Scatter (MSIS) ambient density predictions. Calculations for Space Station surfaces using this model have been made. Assuming a constant altitude flight strategy, total fluence on ram-facing surfaces during a complete solar cycle is 1.2 xlO22 atoms/cm2 and 6.7 x 1021 atoms/cm2 for solar-facing surfaces with both sides exposed. Using material reactivities for composite materials and thin films developed from flight experiments, total recession for ram and solar inertial surfaces will be 0.036 cm (14 mils) and 0.020 cm (8 mils), respectively, per solar cycle. For a station life of 30 years, approximately three cycles will be experienced; the resulting recession appears unacceptable for certain surfaces. These effects must be accounted for if long-lived operation is to be expected.