Low-Drag, Atomic-Oxygen Resistant Materials for Satellites in Very Low Earth Orbit

The environment encountered by satellites in very low Earth orbit (VLEO, 180 – 350 km altitude) contains mostly atomic oxygen (AO) and molecular nitrogen (N₂), which collide with ram surfaces at relative velocities of ~7.5 km s^-1. Structural, thermal-control, and coating materials containing organic polymers are particularly susceptible to AO attack at these high velocities, resulting in erosion, roughening, and degradation of function. Copolymerization or blending of a polymer with polyhedral oligomeric silsesquioxane (POSS) yields a material that can resist AO attack through the formation of a passivating silicon-oxide layer. Still, these hybrid organic/inorganic polymers become rough through AO reactions as the passivating layer is forming. Surface roughness may enhance satellite drag because it promotes energy transfer and scattering angle randomization during gas-surface collisions. As potential low-drag and AO-resistant materials, we have investigated POSS-containing films of clear and Kapton-like polyimides that have an atomically smooth AO-resistant coating of Al₂O₃ that is grown by atomic layer deposition (ALD). Coated and uncoated films were exposed to hyperthermal molecular beams containing atomic and molecular oxygen to investigate their AO resistance, and molecular beam-surface scattering studies were conducted to characterize the gas-surface scattering dynamics on pristine and AO-exposed surfaces to inform drag predictions. The AO erosion yield of Al₂O₃ ALD-coated films is essentially zero. Direct simulation Monte Carlo (DSMC) simulatios of drag on a representative satellite structure, which are based on the observed scattering dynamics, suggest that the use of the Al₂O₃ ALD-coated POSS-polyimides on external satellite surfaces have the potential to reduce drag to less than half that predicted for diffuse scattering surfaces. These smooth and AO-resistant polymer films thus show promise for use in the extreme oxidizing and high-drag environment in VLEO.