Comparison of Flight and Ground Tests of Environmental Degradation of MISSE-6 SUSpECS Materials

The effects of prolonged exposure to the LEO space environment and charge-enhanced contamination on optical, thermal, and electron emission and transport properties of common spacecraft materials has been investigated by comparing pre- and post-flight characterization measurements. The State of Utah Space Environment & Contamination Study (SUSpECS) deployed in March 2008 on board the Materials International Space Station Experiment (MISSE-6) payload, was exposed for ~18 months on the exterior of the International Space Station (ISS), before retrieval in September 2009. A total of 165 samples were mounted on three separate SUSpECS panels on the ram and wake sides on the ISS. Electron-, ion-, and photon-induced electron emission yield curves, crossover energies and emission spectra, resistivity, dielectric strength, optical and electron microscopy, UV/VIS/NIR reflection spectroscopy, and emissivity were tested for pre-flight SUSpECS samples in their pristine conditions. Results reported here focus on the comparison of two specific sets of materials samples. The first comparison focuses on six sets of four identical samples [Au, Al, carbon-loaded polyimide (Dupont Black Kapton 100XC), and carbon-loaded polyester (Sheldahl Thick Film Black)]. Two sample sets were located on the top and bottom tiers of a three-tiered sample panel designed to provide variable atomic oxygen and UV exposure. The four other sample sets were located on the wake side sample panel, with sets biased for the duration of the flight at 0 VDC, +5 VDC, -5 VDC, and -15 VDC, respectively. The biased sample configuration was designed to approximate typical conditions of materials subject to spacecraft charging. Positively charged components will typically charge to only a few volts positive. By contrast, negatively charged materials can charge to large voltages. Biases of -5 V and -15 V were chosen as representative of modest and more extreme negative charging. The second comparison reported here focused on four materials [carbon-loaded polyimide, polyester (Dupont Mylar), Al2O3 (sapphire), and SiO2 (quartz)] that showed some of the most pronounced environmentally–induced changes in optical properties. Samples of each material on the wake and three-tiered sample panels were exposed to a complex environment during the flight. Identical witness samples were also exposed to a simulated subset of the environment in the Characterization of Combined Orbital Surface Effects (CCOSE) space environment test chamber at the USAF Arnold Engineering Development Center to mimic the space exposure profile. The primary optical characterization methods employed for the comparison were UV/VIS/NIR and FTIR transmission of the sapphire and quartz and UV/VIS/NIR reflectance of the polyimide and polyester. Comparison of pre-flight, post-flight, and simulated exposure samples served two primary purposes: to investigate the validity of simulated environmental testing methods and to help distinguish the effects of specific components of the complex space environment that samples were simultaneously exposed to during the flight