The Effect of 1.5 Years of Space Exposure on Optical Properties of Spacecraft Polymers

The temperature of spacecraft directly relates to the ratio of solar absorptance to thermal emittance. Certain polymers are used externally on spacecraft for passive thermal control because of their low solar absorptance and high thermal emittance. Determining the durability of optical properties is important when considering spacecraft design. One of the objectives of the Zenith Polymers Experiment was to determine the effect of low Earth orbit (LEO) zenith exposure on optical properties of various spacecraft polymers. The Zenith Polymers Experiment was flown as part of the Materials International Space Station Experiment 7 (MISSE 7) on the exterior of the International Space Station (ISS) for 1.5 years. The experiment flew 25 polymers in a zenith orientation and exposed them to solar radiation, charged particle radiation, thermal cycling, and grazing atomic oxygen. Thirteen of the 25 samples flown were tested for optical property and atomic oxygen durability. Total and diffuse reflectance and transmittance of 13 flight and 13 non-exposed samples were obtained post-flight. The total reflectance and transmittance were used to compute the integrated air mass zero solar absorptance. Increases in solar absorptance for flight samples ranged from 0.001 (fluorinated ethylene propylene) to 0.234 (ethylene-chlorotrifluoroethylene, for which the appearance changed from clear to dark golden brown). Solar absorptance is crucial to controlling the thermal characteristics within the spacecraft. Results show that prolonged space exposure increases the solar absorptance of some materials. Therefore, knowing which polymers have high increases in solar absorptance, such as polyvinyl fluoride, will benefit future spacecraft design.