Flight Experiments on the Effects of Contamination on Electron Emission of Materials

Abstract

We report on a study of the effects of prolonged exposure to the space environment and of chargeenhanced contamination on the electron emission and resistivity of spacecraft materials. The State of Utah Space Environment & Contamination Study (SUSpECS) was deployed on the International Space Station (ISS) in March 2008 onboard the MISSE-6 payload during STS-123. The Materials International Space Station Experiment (MISSE-6) program is designed to characterize the performance of candidate new space materials over the course of its ~17 month exposure to the LEO environment, with a target return date of August 2009 on STS-127. Approximately 165 samples are mounted on three separate SUSpECS panels in the ram and wake sides on the ISS. They have been carefully chosen to provide needed information for different ongoing studies and a broad cross-section of prototypical materials used on the exteriors of spacecrafts. Design of the sample panels are detailed, including a three tiered configuration intended to provide variable atomic oxygen and ultraviolet radiation exposure. The methods used to simulate charge enhanced contamination by actively biasing samples to low positive and negative voltages are also described. A primary emphasis of SUSpECS is the study of modifications to the electron emission resulting from exposure to the space plasma environment and to environmental contamination. There is presently little available data related to the effects of sample deterioration and contamination on emission properties for materials actually flown in space. Electron emission and transport properties of materials are key in determining the amount of charge build-up and the time for the charge to dissipate, as well as the likelihood of deleterious spacecraft charging effects. Such materials properties are essential parameters in modeling spacecraft charging with engineering tools like NASCAP-2K code. SUSpECS studies will test the validity of our predictions from ground-based studies that very thin layers of contamination can lead to severe charging effects under certain circumstance. Electron-, ion-, and photon-induced electron emission yield curves, crossover energies and emission spectra, as well as resistivity and dielectric strength, have been tested for most SUSpECS samples in their pristine conditions before flight. These measurements will be compared with post-flight measurements. Additional pre-and post-flight characterization measurements include optical and electron microscopy, reflection spectroscopy, emissivity and Auger electron spectroscopy.

Plasma Conditions During the Galaxy 15 Anomaly and the Possibility of ESD from Subsurface Charging

“…We conclude from the above that OSC's efforts to cover exposed surfaces of the Galaxy 15 spacecraft with conductive Kapton ® make it more unlikely that surface charging, although significant, would have led to ESD as the root cause of the anomaly. Of course, these results are invalid if aging, radiation and/or contamination effects 19 changed the material properties of exposed spacecraft surfaces in the 4½ years Galaxy 15 was on-orbit prior to the anomaly. For this reason, surface charging on Galaxy 15 remains a viable consideration for the anomaly root cause.…”

Charge-Enhanced Contamination and Environmental Degradation of MISSE-6 SUSpECS Materials

“…Additional studies of critical thermal control and optical coating materials for the USU SDL Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) composites, mechanical and thermal properties of ATK Thermal Protection Systems and Lightweight Structure Systems materials, and NASA Solar Probe Mission composite and heat shield materials have been described elsewhere [2].…”

“…A grounded sample guard was positioned above the three sub-panels to minimize possible contact with biased sub-panels by astronauts during EVAs. The beveled edges of the sample clamp and guard shield were designed to minimize fringing fields to provide nearly parallel voltage contours typical of larger biased samples, as shown by field simulations (see Figure 3 in Ref [2]).…”

“…is to "characterize the performance of new prospective spacecraft materials when subjected to the synergistic effects of the space environment" [1]. The SUSpECS sample panels include pertinent materials and coatings selected and characterized by each group member for a comprehensive study of the effects of the low Earth orbit (LEO) space environment and contamination on electrical, mechanical, and optical properties of materials related to several on-going projects of high relevance to manned space exploration and other long duration space missions [2].…”