Low Earth Orbital Atomic Oxygen Interactions with Spacecraft Materials
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Low Earth Orbital Atomic Oxygen Interactions with Spacecraft Materials Bruce A. Banks, Kim K. de Groh and Sharon K. Miller Electro-Physics Branch, NASA Glenn Research Center Cleveland, OH 44135, U.S.A. ABSTRACT Atomic oxygen, formed in Earth’s thermosphere, interacts readily with many materials on spacecraft flying in low Earth orbit (LEO). All hydrocarbon based polymers and graphite are easily oxidized upon the impact of ~4.5 eV atomic oxygen as the spacecraft ram into the residual atmosphere. The resulting interactions can change the morphology and reduce the thickness of these materials. Directed atomic oxygen erosion will result in the development of textured surfaces on all materials with volatile oxidation products. Examples from space flight samples are provided. As a result of the erosive properties of atomic oxygen on polymers and composites, protective coatings have been developed and are used to increase the functional life of polymer films and composites that are exposed to the LEO environment. The atomic oxygen erosion yields for actual and predicted LEO exposure of numerous materials are presented. Results of in-space exposure of vacuum deposited aluminum protective coatings on polyimide Kapton indicate high rates of degradation are associated with aluminum coatings on both surfaces of the Kapton. Computational modeling predictions indicate that less trapping of the atomic oxygen occurs, with less resulting damage, if only the space-exposed surface is coated with vapor deposited aluminum rather than having both surfaces coated. INTRODUCTION Although knowledge of atomic oxygen existed in the early days of space exploration, an awareness of the damaging effects on spacecraft materials was not well known until the Space Shuttle began flying missions at much lower altitudes in low Earth orbit (LEO) [1]. Early in-space observation that the residual atmosphere was interacting with spacecraft surfaces came in part as a result of comparison of day and night pictures of the space shuttle, as shown in Figure 1, where the glow from de-excitation atoms and molecules leaving shuttle surfaces oriented in the ram (forward facing) direction are shown [2 and 3]. A second indication of LEO atomic oxygen interactions came from observations of increases in the diffuse reflectance of polymers such as polyimide Kapton H due to surface texturing. Such observations lead to further tests which documented the rate of atomic oxygen erosion of commonly used spacecraft polymers, and resulting modifications to LEO spacecraft design in efforts to enable spacecraft to be durable to the LEO atomic oxygen environment. This paper provides an overview of the LEO atomic oxygen environment, its interaction with spacecraft materials, approaches for protection from atomic oxygen, and on-orbit and computational results of protection methods.
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a. In sunlight
b. At night
Figure 1. Space shuttle with the bay oriented in the direction of travel (ram direction). THE LEO ATOMIC OXYGEN ENVIRONMENT Atomic oxygen, in LEO, is forme
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