Oxidation Caused by Water Outgassed from the Thermal Blanket on the SDO Spacecraft

The Solar Dynamics Observatory (SDO) is a Sun-observing spacecraft that includes two spectrometers that use aluminum membranes to filter solar radiation. The transmission of those filters degraded by a factor of 5 during the first five years after launch. Previously, we showed that degradation was comparable to that induced in the laboratory by UV synchrotron radiation on similar aluminum filters. Here, we show that a physics-based model fit to the results of our synchrotron exposures can quantitatively describe the SDO degradation if the water vapor pressure p H2O on the SDO is allowed to be a free parameter. The fitted value of p H2O for both spectrometers, approximately 10 −8 mbar (10 ^-6 Pa ), is consistent with the flux of outgassed water estimated for the thermal blankets on SDO.