Low-Energy Electron Exposure of Space Materials

M ATERIALS that reside on the exterior of a spacecraft, and are therefore unshielded, are exposed to the entire space environment. In addition to receiving a substantial dose of ionizing radiation from exposure to solar ultraviolet radiation, external spacecraft materials or surfaces can also be subjected to extremely large fluxes of ionizing radiation from the charged-particle environment. In particular, the electron and proton fluxes impart an extremely large dose of ionizing radiation into the surface layers of exposed materials, which generally produces both optical and mechanical degradation through a variety of mechanisms. These effects have been studied for years on many materials, particularly thermal control coatings [1–19]. Any effective simulation of the space environment for the purpose of material evaluation requires accurate modeling of these charged-particle environments. This requires environment models that describe the electron and proton fluxes as a function of particle energy for all particle energies (within reason). The models AE8 [20] and AP8 [21] have been the standards for describing the trapped-particle environments for electrons and protons, respectively, for all orbits. Unfortunately, corresponding and comprehensive engineeringmodels for the plasma environments do not currently exist for all orbits. Generally speaking, the models AE8 andAP8 are referred to as trapped-particle environment models because their intent is to describe the particle fluxes that are essentially trapped in the Van Allen radiation belts. Historically, this has been defined as particle fluxes with energies greater than 40 keV for electrons and 100 keV for protons. Particle populations with energies lower than these limits are referred to as plasma particles, which are not confined to the idealized radiation belts. The AE8 and AP8 models neglect the lower energy ranges of both electron and proton populations for largely historical reasons: Theflight data upon which they are based spanned only limited energy ranges, which resulted in the development of models that started at relatively high energy levels relative to the plasma environments. Particle populations below these energy levels (40 keV for electrons and 100 keV for protons) were either ignored or approximated through crude extrapolation of the model’s flux-energy curves. Although the Advanced Technology Satellite 6 (ATS-6) model [22] has often served to describe plasma electron and proton environments, primarily at geosynchronous Earth orbit (GEO), the model is based on only 45 days of flight data. An excellent review of the temporal and energy coverage of numerous satellite data and space environment models, including AE8, AP8, and ATS-6, is provided in [23]. Use of the plasma-population-deficient models AE8 and AP8 can easily lead to a two-order-of-magnitude underprediction of the energy deposited into the shallow depths of an external spacecraft material. It is important to note that the fluxes of particle populations increase exponentially with decreasing energy; consequently, there are orders-of-magnitude-more particles below the cutoff range of these trapped-particle models. Since these low-energy particles deposit energy and stop in very shallow depths of materials, their large combined fluence results in a tremendous deposition of energy in the surface of materials. Without consideration of the proper plasma environment, the energy deposition in exterior spacecraft materials is incorrectly predicted by energy-deposition codes. Recently,more comprehensive and accuratemodels of the plasmaparticle environments at GEO have been published. These models use a large data set of orbital measurements as their basis and provide themost accurate description of the lower-energy particle population currently available. The Los Alamos National Laboratory Magnetospheric PlasmaAnalyzer (LANL-MPA)model [24] is based onmore than 16 years of orbital measurements and characterizes GEO electron and proton fluxes from about 45 keV down to approximately Received 29 September 2010; revision received 28 April 2011; accepted for publication 23 May 2011. Copyright © 2011 by The Aerospace Corporation. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0022-4650/11 and $10.00 in correspondence with the CCC. Research Scientist, Materials Science Department, Space Materials Laboratory, 2310 East El Segundo Boulevard, M2-248. Associate of the Technical Staff, Materials Science Department, Space Materials Laboratory, 2310 East El Segundo Boulevard, M2-248. Associate Member of the Technical Staff, Materials Science Department, Space Materials Laboratory, 2310 East El Segundo Boulevard, M2-248. §Distinguished Scientist, Space Materials Laboratory, 2310 East El Segundo Boulevard, M2-248. JOURNAL OF SPACECRAFT AND ROCKETS Vol. 48, No. 6, November–December 2011