Michael J. Meshishnek

Root cause determination of on-orbit degradation of the VIIRS rotating telescope assembly

The Visible Infrared Imaging Radiometer Suite (VIIRS) is a sensor onboard the recently launched Suomi NPP spacecraft. Shortly after launch, VIIRS was found to exhibit a pronounced decrease in the optical throughput of several bands, with the near-infrared bands being more affected than those in the visible. The anomaly investigation team performed several experiments that concluded the primary source of degradation was throughput loss in the VIIRS rotating telescope assembly, likely caused by ultraviolet light illumination. This paper will discuss the laboratory investigation that determined the root cause of the telescope degradation to be UV photo-darkening of a tungsten oxide contaminant film that had been inadvertently deposited during the mirror manufacturing process. We will present data from experiments conducted on witness mirrors manufactured along with the telescope, as well as other mirrors of the same type that were not contaminated.

Simulated space environmental exposure of optical coatings for spacecraft solar rejection

Dielectric multilayers composed of niobium pentoxide and silicon dioxide, designed for broadband solar rejection, were exposed to a simulated space environment of ultraviolet light and low-energy (10-20-keV) electron radiation. Samples exhibited various degrees of exposure-induced absorption extending from the ultraviolet to the infrared. Processing variations were correlated to damage susceptibility, and methods were identified that produced parts that exhibited no degradation even though the same materials and coating design were used. Coatings prepared under energetic deposition conditions that provided the densest and most moisture-stable coatings exhibited the best stability to the exposure conditions used.

Advancements in the development of thin film amorphous silicon space solar cell for the PowerSphere concept

The authors describe how the development of the PowerSphere concept over the last year focused on the design and fabrication of amorphous silicon solar cells that would meet the space environmental requirements. This is a cooperative effort between The Aerospace Corporation and Iowa Thin Film Technologies, Inc. Modifications to the terrestrial product line necessary to produce a thin film amorphous silicon solar cell suitable for space applications have been identified. A number of experiments have been performed in The Aerospace Corporations Laboratories to develop a more robust top contact to collect cell current for the otherwise terrestrial cell produced by Iowa Thin Film Technologies production line. The up-to-date results of this effort are presented in this paper.

Air-Induced Recovery of Proton-Exposed Space Materials

T HE fact that materials degrade in space-radiation environments has been studied for decades [1–19] and continues to be an issue of significant importance for space-based systems. Consequently, ground-based simulation of space environmental effects remains an invaluable method for ensuring the performance of any orbital asset, particularly as newmaterials, coatings, and processes are introduced. High-fidelity simulation of space environmental effects requires accurate definition and modeling of the orbital mission exposure, appropriate calculation of the absorbed radiation dose as a function of material thickness, proper simulation of this dose-depth profile in the laboratory, and precise characterization of test-induced changes in material properties. When performing material characterization, in vacuomeasurements are absolutely essential to avoid atmospheric effects on tested materials. Failure to do this can lead to results that are skewed by the too-often-overlooked dynamic of air-induced recovery of radiation-induced degradation, also known as bleaching. For some materials, ignoring the effects of bleaching can easily lead to a significant under estimation of radiation-induced losses and an inaccurate prediction of on-orbit performance. Bleaching is known to occur primarily for thermal control materials [20], and recent work by The Aerospace Corporation has added to the evidence that somematerials, which have been degraded from exposure to simulated space environments, display air-induced recovery behavior. In particular, samples of silverized Teflon, aluminized Kapton, and Z-93P ceramic white paint that were exposed to simultaneous ultraviolet (UV) and electron radiation in a test designed to simulate the conditions present at geosynchronous Earth orbit (GEO) exhibited noticeable bleaching upon subsequent exposure to air. In each case, a portion of the test-induced optical losses was recovered after a brief period ( 2 min) of air exposure. The behavior is material-specific, exhibiting differences in affected spectral region and magnitude of change. The recovery can occur rapidly or over long timescales, with the rate of recovery appearing to depend on material porosity and/or ambient temperature. The spectral response of the three materials is exemplified in Fig. 1. Although air-induced bleaching of optical degradation in some thermal control materials is relatively common, and the effect is not typically observed for materials that are more resistant to the effects of space-radiation exposure, such as solar-cell coverglass.Additional data generated by The Aerospace Corporation, however, indicated that certain coated coverglass materials, namely, those with thin-film coatings of magnesium fluoride (MgF2) and indium tin oxide (ITO), might also be susceptible to bleaching. In another recent GEO simulation, samples of MgF2/ITO-coated 0214 coverglass (0214 refers to the manufacturer-specific type of coverglass substrate) exhibited slight, yet discernible, air-induced recovery of spectral transmittance following simultaneous exposure to UV and electron radiation. Figure 2 shows the material’s spectral response to a short ( 2 min) postirradiation air exposure. Primarily, because of facility limitations, proton exposures at The Aerospace Corporation historically have been performed, usually precedingUV/electron irradiation, in a separate exposure facility: the low-energy accelerator facility (LEAF). The LEAF has a vacuum chamber attached to a linear ion accelerator; the accelerator’s beam is passed through a 30 deg magnetic turn to select the appropriate ion species, and the beam is rastered across the exposure target. The LEAF lacks in vacuo optical measurement capability; thus, air exposure is unavoidable when characterizing proton-induced material changes. Because of this, it is unknown whether, or to what degree, bleaching occurs for proton-irradiated coverglass materials, although it seems reasonable to suspect that somematerials degraded by proton exposure would also exhibit air-induced recovery. Given the apparent bleaching ofUV/electron-induced degradation in MgF2/ITO-coated coverglass and the lack of in vacuo optical measurement capability of the LEAF, this research effort was undertaken to investigate the air-induced response of protonirradiated space materials. Making this possible is The Aerospace Corporation’s newest space environmental effects exposure facility, which had already possessed in vacuo optical measurement capability and now features a variable-energy proton flood gun. This recently enhanced facility makes possible measurements of protonirradiated materials that are free from atmospheric effects and allows Received 21 September 2011; revision received 5 December 2011; accepted for publication 9 January 2012. Copyright

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

Space radiation environmental testing on POSS coated solar cell coverglass

Light weight, flexible, radiation hardened solar cells coatings are of interest for applications on satellite power generation owing to the potential advantages in terms of having higher specific power (lightweight), lower specific volume (flexible), and higher end-of-life power (superior radiation resistance), as compared to current state-of-the-art Ce-doped micro sheet solar cell coverglass. The space radiation environment causes gradual optical performance degradation of coverglass and coatings, thus limiting the lifetime of the solar array. The objective of this project is to assess the POSS (polyhedral oligomeric silsesquioxane) coating in simulated space proton radiation environments. Due to the unique molecular structure, POSS may be equipped with suitable optical property along with superior radiation hardness, thus better protecting the solar cells.

Degradation of a multilayer dielectric filter as a result of simulated space environmental exposure

The exterior optical surfaces of satellites are directly exposed to the harsh space environment. Here, a multilayer dielectric solar rejection filter was deposited on a silicon substrate and then subjected to electron and proton irradiation, simulating an orbital environment. Following the exposure, damage was observed that was attributed to dielectric breakdown. Optical and scanning electron microscopy revealed extensive pitting as a result of this exposure. The typical size of dischrage pits was 50 - 100 microns at the surface, extending to the substrate material, where a 10 micron diameter melt region was found. Pit damage occurred at pre-existing coating defects and was accelerated by pre-exposure to proton radiation. Pitting was not observed on similar samples that had also been overcoated with a conductive thin-film.

Simulated Low-Earth-Orbital Exposure of Thermal Control Materials

A variety of common spacecraft thermal control materials were exposed to a laboratory test designed to simulate the effects of the low-Earth-orbital space-radiation environment. The goal of this test program was to garner performance data on the subject materials, which included two thickness types of old and new rear-surface-silverized fluorinated ethylene propylene Teflon® tape manufactured more than a decade apart, an embossed version of rear-surface-silverized fluorinated ethylene propylene Teflon tape, perforated rear-surface-aluminized Kapton, and a zinc oxide/methyl silicone white paint. In addition to providing high-fidelity data on the end-of-life performance of the materials, the testing revealed that older samples of the rear-surface-silverized fluorinated ethylene propylene Teflon tape showed widely disparate responses and greater solar absorptance increases compared to samples of the same, newly manufactured materials. Samples of the new rear-surface-silverized fluorinated ethylene propylene ...

On-Orbit Degradation of Silver Mirrors Exposed to Ultraviolet Radiation

Silver mirrors on the Suomi-NPP spacecraft exhibited significant degradation during early orbit operations. This paper describes the investigation that identified the source of the loss as a UV-sensitive contaminant deposited during manufacturing.

Optical Reflector Materials Experiment‐I (ORMatE‐I) and ORMatE‐II on Board MISSE

The first Optical Reflector Materials Experiment (ORMatE‐I) is on‐board MISSE‐6. The follow‐on experiment, ORMatE‐II, is part of MISSE‐7. Both these projects are a collaborative effort among The Aerospace Corporation, the US Naval Research Laboratory (NRL), and the Air Force Research Laboratory Materials Directorate (AFRL/ML). ORMatE‐I is a study of optically reflective materials focused on SiC for use as a lightweight mirror substrate. Several types of SiC material grown by different methods and vendors are included as well as diverse coating materials and deposition techniques. Advanced glass substrate technologies, like ULE and corrugated borosilicate, are also on‐board. Additional SiC and composite materials will be evaluated on ORMatE‐II along with silver mirrors deposited by various means. A description of both experiment suites and a summary of the pre‐flight optical characterization will be presented.