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.
We present the results of a laboratory test to determine the effects of bulk-deposited DC-704 silicone-oil contaminant film on the transmittance properties of an anti-reflective (AR) coated fused-silica optical substrate. Testing and optical measurements were performed in vacuum in the Boeing Contamination Effects Test Facility (CETF). The test and measurement procedures are described herein. Measurement results are presented showing the change in transmittance characteristics as a function of contaminant deposit thickness and vacuum-ultraviolet (VUV) exposure levels. The results show an initial degradation in the transmittance of the contaminated sample. This is followed by a partial recovery in transmittance as the sample is exposed to additional VUV radiation. The results also show a loss of transmittance in the ultraviolet portion of the spectrum and an increase in transmittance in the infrared portion of the spectrum. Thin-film interference analysis indicates that some of the observed transmittance results can be successfully modeled, but only if the contaminant film is assumed to have the complex index of refraction of SiO<sub>2</sub> rather than DC-704 silicone oil. Post-test Scanning Electron Microscope (SEM) scans of the test sample indicate the formation of contaminant islands and the presence of a thin uniform contaminant film on the sample.
The physical processes involved in the permanent deposition of contaminant molecules on sunlit spacecraft surfaces have been explored in greater depth by a series of measurements in a ground test facility. Solar simulation was provided at narrow line wavelengths of 0.12, 0.18, and 0.25 (mu) by the use of low-pressure hydrogen and mercury lamps instead of the broader-spectrum krypton and xenon lamps used in previous studies. Surface coverage was varied from sub- monolayer to bulk both by surface temperature change and by changes in the incident molecular flux. Photofixing of contaminants deposited on cooler surfaces in the dark was measured by warming the surfaces after periods of ultraviolet irradiation. For the model contaminant DC-704, photodeposition rates at 0.12 (mu) were about 6 times larger than 0.18 (mu) . This calls into question the practice of comparing ultraviolet simulators to the solar ultraviolet in terms of total photon incidence below 0.2 (mu) . Proper ground simulation of contaminant photodeposition may require the use of Lyman-alpha lamps for certain materials. The results of this work also indicate that even at constant surface coverage of unreacted contaminants, the photodeposition rate is strongly dependent on surface temperature.
A 50-cm cryogenic mirror at one end of an aluminum telescope was successfully cleaned by nitrogen snow in a series of demonstration tests. The mirror was maintained below 70 Kelvin under vacuum during the cleaning, with a 15 Kelvin cold cap pumping the nitrogen gas to maintain a realistic space environment. The snow was produced by an assembly of 6 nozzles and valves attached to the exterior of the telescope. The nozzles protruded less than 1 cm into the telescope and were well outside the mirror diameter. Contamination of the mirror was produced by silica and alumina dusts propelled into the telescope by special velocity-moderating sources. Cleaning effectiveness was measured by scatter of 10.6-mu laser light at 2 degrees from three spots on the mirror surface. All scatter system components were exterior to the telescope, with only small holes for the passage of laser radiation. The clean mirror BRDF of 5 multiplied by 10<SUP>-4</SUP> sr<SUP>-1</SUP> was raised as high as 3 multiplied by 10<SUP>-2</SUP> sr<SUP>-1</SUP> by the contamination process and subsequently reduced to the original level by one or two seconds of nitrogen snow spraying. Nitrogen snow cleaning under vacuum proved much more effective than carbon-dioxide snow cleaning of the same mirror in air.
A system for detecting and removing contaminants from a half-meter diameter primary mirror in a simulated space telescope has been designed, fabricated, and tested under vacuum conditions. Various molecular and particulate contaminants are deposited on the mirror by special sources in the open end of the telescope, causing an increase in optical scatter which is detected at infrared wavelengths by special sensors on the wall of the telescope. Depending on the type of contaminant, either of two systems of contamination-removal hardware is activated to clean the mirror surface and restore the original scattering level.
A laboratory study has been performed by Boeing Aerospace Electronics under the sponsorship of the Air Force to investigate the effects of contamination on the electrostatic charging phenomena of spacecraft thermal blankets for the Ulysses mission. Induced contamination by the upperstage PAM-S Star-48B motor nozzle post-burn outgassing was studied for three types of electrically conductive thermal blankets: indium-tin-oxide-coated electrodag-coated and aluminized Kapton. Blanket samples were irradiated with 50 eV electrons at temperatures ranging from 25C to -100C and contaminant deposition thicknesses ranging from 200 to 3000 A (assuming specific gravity of 1. 0). The experimental results show that the charging characteristics of all three contaminated blankets are similar. Charge accumulation was observed to be a strong function of contaminant deposition temperature and a non-linear function of primary electron flux. 1.
Experimental investigations are described for ion-beam sputtering and RF-plasma sputtering to determine the effectiveness of the methods for removing contaminants from an optical surface. The effects of ion-beam sputtering are tested with an ion gun and measured by mounting a 5-MHz quartz-crystal microbalance on a sample holder and simulating spacecraft contamination. RF-plasma sputtering involves the application of an alternating electric field to opposing electrodes immersed in a low density gas, and is tested with the same setup. The energy dependence of the sputtering yields is measured to determine whether the different contaminants are removed and whether the mirror surface is affected. Ion-beam sputtering removes all contaminants tested, but also affects the mirror surface at high energies. When the correct DC bias is applied, RF sputtering can remove the contaminants without removing the metal-mirror surface.