Effects of Radiation Induced Optical Degradation on Earth Bound Solar Sail Manoeuvring Capabilities

Metallised thin films represent the state of the art for current solar-sail missions operating around Earth. Previous testing has demonstrated that when exposed to high fluences ( 2.2 x 10 17 particles/cm 2 ) of low energy (<12 keV) protons, hydrogen blistering can occur in the metallised films. This blistering leads to degradation of the optical performance of the metallised surface as the reflectivity becomes less specular and more diffuse. In this paper, experimental degradation data from ground-based radiation testing is used to evaluate the effect of degradation on the solar-sail manoeuvring capabilities. The testing was previously conducted using The German Aerospace Centre (DLR) Institute of Space Systems Complex Irradiation Facility (CIF) and links proton fluence to reductions in specular reflectivity for metallised thin films. The impact on the manoeuvring capabilities of a solar sail due to the effects of proton induced degradation is demonstrated through numerical modelling of the degraded sail in Earth orbit. There was found to be a strong temperature dependence on the time to the onset of radiation degradation and the rate of degradation after onset. Warmer sail temperatures were found to cause a faster onset of degradation but a lower overall degradation across the mission. This warm degradation progression compares to the colder sail temperatures that were found to have a more significant level of degradation but a later onset of damage. The worst case trapped radiation environment for sail degradation was found to be around 21,000 km altitude but remains at levels sufficient to cause degradation in a feasible mission lifetime at geostationary orbit.