AZ93 with a fluoropolymer overcoat is an option to simplify ground handling of space hardware. The overcoat applied on some on-orbit International Space Station (ISS) hardware provides contamination protection for optically sensitive ceramic thermal control coatings. However, if the fluoropolymer is not eroded on-orbit by atomic oxygen (AO), then it will darken. This will increase the solar absorptance resulting in possible thermal performance degradation. If the fluoropolymer overcoat was not present, optical performance would be significantly improved. To characterize the optical performance of the AZ93 with the fluoropolymer overcoat for modeling the UV degradation, laboratory testing of the coating was performed at Marshall Space Flight Center (MSFC). Sample coupons prepared by AZ Technology were exposed under vacuum to ultraviolet radiation. At periodic intervals, the samples were removed from the testing chamber to acquire images and to measure the solar absorptance. The images showed visible differences between AZ93 with the overcoat and without the overcoat as vacuum ultraviolet (VUV) exposure increased. Darkening is more pronounced in the samples with the fluoropolymer overcoat. This was also evident in the solar absorptance measurements. Optical properties of AZ93 with the fluoropolymer overcoat significantly degraded in comparison to those without the overcoat. A short period of little change followed by an exponential rise in solar absorptance was observed. The optical degradation of the fluoropolymer overcoat is described in terms of surface reaction chemistry and kinetics and is found to follow a pseudo first order reaction rate.
The International Space Station (ISS) solar arrays provide power that is needed for on-orbit experiments and operations.
The ISS solar arrays are exposed to space environment effects that include contamination, atomic oxygen, ultraviolet
radiation and thermal cycling. The contamination effects include exposure to thruster plume contamination and erosion.
This study was performed to better understand potential solar cell optical performance degradation due to increased
scatter caused by plume particle pitting. A ground test was performed using a light gas gun to shoot glass beads at a solar
cell with a shotgun approach. The increase in scatter was then measured and correlated with the surface damage.
This paper presents an overview of International Space Station (ISS) on-orbit environments exposure flight experiments. International teams are flying, or preparing to fly, externally mounted materials exposure trays and sensor packages. The samples in these trays are exposed to a combination of induced molecular contamination, ultraviolet radiation, atomic oxygen, ionizing radiation, micrometeoroids and orbital debris. Exposed materials samples are analyzed upon return. Typical analyses performed on these samples include optical property measurements, X-ray photo spectroscopy (XPS) depth profiles, scanning electron microscope (SEM) surface morphology and materials properties measurements. The objective of these studies is to characterize the long-term effects of the natural and induced environments on spacecraft materials. Ongoing flight experiments include the U.S. Materials International Space Station Experiment (MISSE) program, the Japanese Micro-Particles Capturer and Space Environment Exposure Device (SM/MPAC&SEED) experiment, the Russian SKK and Kromka experiments from RSC-Energia, and the Komplast flight experiment. Flight experiments being prepared for flight, or in development stage, include the Japanese Space Environment Data Acquisition Attached Payload (SEDA-AP), the Russian BKDO monitoring package from RSC-Energia, and the European Materials Exposure and Degradation Experiment (MEDET). Results from these ISS flight experiments will be crucial to extending the performance and life of long-duration space systems such as Space Station, Space Transportation System, and other missions for Moon and Mars exploration.
This paper documents the development and validation of a new return flux model for the International Space Station (ISS). This model has been developed to augment current ISS external contamination modeling tool capabilities. The model is capable of characterizing return flux from ISS molecular emission sources. These sources include materials outgassing, vacuum venting, propellant purging and thruster firings. The BGK method (named after its authors: Bhatnagar, Gross and Krook<sup>1</sup>) was selected for modeling ambient scatter. This method was used to reduce computational times, as the ISS geometric models used for external contamination modeling may contain up to 40,000 surface elements. The model has been validated by comparison with analytical results and with results from the ESA COMOVA software. Validation with on-orbit flight experiment data will be conducted when adequate experimental data is available. Previously flown experiments (i.e., REFLEX) have not produced data with high enough fidelity to validate this model. The model has been applied to the ISS to characterize return flux from the European Columbus module onto its own payload locations. Analysis results indicate the return flux contribution to ESA payload surfaces will be small, but not negligible.