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NASA NTRSDate not supplied

Ground Testing of the MISSE-16 Materials

The harsh space environment imposes very stringent requirements upon spacecraft materials, especially those located on exterior surfaces of space objects in low Earth orbit (LEO). As humankind moves from space exploration to space commercialization, these materials may have to last for 15–20 years without considerable degradation of their optical, electrical, mechanical, and thermal properties. Operational requirements dictate that these materials continue to function according to expectations. Hence, we must understand the effects of the space environment on materials currently in use as well as on untested materials. For the years, the Materials International Space Station Experiment Flight Facility (MISSE-FF) have flown many different materials to investigate the effect of LEO space weather exposure on the performance and durability of materials and devices. The MISSE-16 scientific team members plan to launch fifteen different novel and well-characterized spacecraft-relevant materials to the LEO environment for a duration of six months during the MISSE-16 mission. Changes in spectral reflectivity will be measured throughout the mission as the samples are exposed to the space environment. The same chemical damage the produces changes in optical reflectance also causes changes in numerous other physical properties such as electrical conductivity, mechanical strength, and chemical reactivity. Correlation of the changes in each of these properties as a function of radiation type, flux, and fluence allows various material properties to be inferred from one experimentally tractable measurement: color change. Flying identical test fixtures on the ram, zenith, and wake positions of the MISSE-FF, collecting spectrally resolved images of the materials, and concomitantly measuring the ambient space environment allows deconvolution of the material effects which occur as a result of exposure to neutral atomic oxygen (AO), unfiltered solar ultraviolet (UV) radiation, and electrons. The flight experiment will function as a ground truth reference for our team’s ongoing laboratory-based space weather-material interactions experiments. Comparison of the MISSE-16 data with extensive testing of “flight-duplicate” samples under simulated space weather conditions will enable development of chemical models for prediction of material degradation. This paper discusses preliminary results from the ground test campaign including measurements of UV/VIS/IR reflectance spectra, bidirectional reflectance distribution function (BRDF), surface potential decay, and surface morphology under electron and AO exposure.

Elena A. Plis↗
NASA NTRS1980

Titanium dioxide antireflection coating for silicon solar cells by spray deposition

A high-speed production process is described for depositing a single-layer, quarter-wavelength thick antireflection coating of titanium dioxide on metal-patterned single-crystal silicon solar cells for terrestrial applications. Controlled atomization spraying of an organotitanium solution was selected as the most cost-effective method of film deposition using commercial automated equipment. The optimal composition consists of titanium isopropoxide as the titanium source, n-butyl acetate as the diluent solvent, sec-butanol as the leveling agent, and 2-ethyl-1-hexanol to render the material uniformly depositable. Application of the process to the coating of circular, large-diameter solar cells with either screen-printed silver metallization or with vacuum-evaporated Ti/Pd/Ag metallization showed increases of over 40% in the electrical conversion efficiency. Optical characteristics, corrosion resistance, and several other important properties of the spray-deposited film are reported. Experimental evidence indicates a wide tolerance in the coating thickness upon the overall efficiency of the cell. Considerations pertaining to the optimization of AR coatings in general are discussed, and a comprehensive critical survey of the literature is presented.

Kern, W.↗
NASA NTRS1993

Photoionization of the hydrogen atom in strong magnetic fields

The photoionization of the hydrogen atom in magnetic fields B about 10 exp 11 - 10 exp 13 G typical of the surface layers of neutron stars is investigated analytically and numerically. We consider the photoionization from various tightly bound and hydrogen-like states of the atom for photons with arbitrary polarizations and wave-vector directions. It is shown that the length form of the interaction matrix elements is more appropriate in the adiabatic approximation than the velocity form, at least in the most important frequency range omega much less than omega(B), where omega(B) is the electron cyclotron frequency. Use of the length form yields nonzero cross sections for photon polarizations perpendicular to the magnetic field at omega less than omega(B); these cross sections are the ones that most strongly affect the properties of the radiation escaping from an optically thick medium, e.g., from the atmosphere of a neutron star. The results of the numerical calculations are fitted by simple analytical formulas.

Potekhin, Aleksandr IU.↗
NASA NTRS2010

Spectroscopic Observation of the Stardust Re-Entry in the Near UV with SLIT: Deduction of Surface Temperatures and Plasma Radiation

Thermal radiation of the heat-shield and the emission of the post-shock layer around the Stardust capsule, during its re-entry, were detected by a NASA-led observation campaign aboard NASA's DC-8 airborne observatory involving teams from several nations. The German SLIT experiment used a conventional spectrometer, in a Czerny-Turner configuration (300 mm focal length and a 600 lines/mm grating), fed by fiber optics, to cover a wavelength range from 324 nm to 456 nm with a pixel resolution of 0.08 nm. The reentering spacecraft was tracked m uansuinaglly a camera with a view angle of 20 degrees, and light from the capsule was collected using a small mirror telescope with a view angle of only 0.45 degrees. Data were gathered with a measurement frequency of 5 Hz in a 30-second time interval around the point of maximum heating until the capsule left the field of view. The emission of CN (as a major ablation product), N2(+) and different atoms were monitored successfully during that time. Due to the nature of the experimental set up, spatial resolution of the radiation field was not possible. Therefore, all measured values represent an integration of radiation from the visible part of the glowing heat shield, and from the plasma in the post-shock region. Further, due to challenges in tracking not every spectrum gathered contained data. The measured spectra can be split up into two parts: (i) continuum spectra which represent a superposition of the heat shield radiation and the continuum radiation of potential dust particles in the plasma, and (ii) line spectra from the plasma in the shock layer. Planck temperatures (interpreted as the surface temperatures of the Stardust heat shield) were determined assuming either a constant surface temperature, or a temperature distribution deduced from numerical simulation. The constant surface temperatures are in good agreement with numerical simulations, but the peak values at the stagnation point are significantly lower than those in the numerical simulation if a temperature distribution over the surface is assumed. Emission bands of CN and N2(+) were tracked along the visible trajectory and compared to a spectral simulation with satisfying agreement. Values for the integrated radiation of the transitions of interest for these species were extracted from this comparison.

Winter, Michael W.↗
NASA NTRS1994

A laser-based sizing/velocimetry technique to investigate the secondary atomization of aluminum gel propellants

A laser-based, forward-scatter diagnostic technique, employing a single laser sheet, has been developed to simultaneously measure the size and velocity of individual 10-150 micron droplets in a dilute polydisperse droplet stream (less than 1000 particles/cc) and to detect the presence of burning aluminum in these same droplets. Spectral emission from aluminum vapor in the 390-400 nm wavelength region is used as an indication of burning aluminum. The technique utilizes a 4-mm uniformly illuminated probe volume, eliminating trajectory-dependent particle sizing and size-dependent system detection bias. Particle sizing is based on a correlation of particle size with near-forward scattered light intensity. Calculations show average particle sizing variation to be within 3.5% over the expected range of refractive indices. Calibrations using a range of optical pinholes (10-100 micron) were used to verify the above sizing correlation.

Mueller, D. C.↗
NASA NTRS1982

Ellipsometric studies of sputtered silicon nitride on n-GaAs

Ellipsometry, which is the study of optical systems through their effect on polarized light, is a nondestructive technique offering extremely high sensitivity and in situ operation. One of the intriguing possibilities is to make use of the high sensitivity of ellipsometry for studying the very thin interface between an insulating film and a semiconductor substrate. Researchers have been especially interested in native or anodic oxides. In a recent study, Aspnes and Theeten used ellipsometry to study the interface between silicon and its thermally grown oxide, SiO2. These authors were able to conclude that the interface consisted of a 7 +/- 2 A graded region of atomically mixed silicon and oxygen. In this paper we report both analysis and experimental results for the system: sputtered silicon nitride on (100) n-type gallium arsenide.

Peng, Y. K.↗
NASA NTRS2018

Degradation of Intercalated Graphite Epoxy Composites Exposed to Low Earth Orbit

Four different graphite fiber/epoxy composites were exposed to the wake-side low Earth orbit (LEO) environment on the Optical Reflector Materials Experiment III (ORMatE-III) platform mounted on the exterior of the ISS for two years in order to determine their long term durability in the space environment. Three of the composite samples used bromine intercalated P100 fibers and one used pristine P100 fibers. One of the P100-Br samples was coated with a protective SiO2 layer, and half of another was coated with SiO2. Results were compared with the EOIM-III experiment which exposed the same materials in LEO in the ram direction for 42 h on the Space Shuttle. Although the atomic oxygen (AO) fluence of the ORMatE-III samples was one-third of the EOIM-III exposure, the resulting effects were qualitatively the same. Both found that SiO2 coated intercalated graphite composites showed no AO erosion, verifying that conventional protection strategies are applicable to bromine intercalated composites. Both found that bromine intercalation does not alter the AO erosion rates of graphite fiber/epoxy composites. Both found no bromine was detected to have migrated into the surrounding epoxy, even for highly eroded fibers and epoxy, allaying fears that bromination could compromise the properties of the epoxy. And both found no corrosive bromine was detected to have been released by the fibers allaying fears that outgassing could degrade sensitive electronics. The uncoated EOIM-III samples suffered slightly greater erosion resulting in a slightly higher solar absorptance. The general conclusion is that ram AO exposure had very similar effects on graphite fiber epoxy composites in both the EOIM-III and ORMatE-III experiments, and in neither case did bromine intercalation of the fibers substantially influence that degradation.

Intercalated graphite↗
NASA NTRS1990

Theoretical investigation of non-equilibrium chemistry and optical radiation in hypersonic flow fields

Future space vehicles returning from distant missions or high earth orbits may enter the upper regions of the atmosphere and use aerodynamic drag to reduce their velocity before they skip out of the atmosphere and enter low earth orbit. The Aeroassist Flight Experiment (AFE) is designed to explore the special problems encountered in such entries. A computer code was developed to calculate the radiative transport along line-or-sight in the general 3-D flow field about an arbitrary entry vehicle, if the temperatures and species concentrations along the line-of-sight are known. The radiative heating calculation at the stagnation point of the AFE vehicle along the entry trajectory was performed, including a detailed line-by-line accounting of the radiative transport in the vacuum ultraviolet (below 200 nm) by the atomic N and O lines. A method was developed for making measurements of the haze particles in the Titan atmosphere above 200 km altitude. Several other tasks of a continuing nature, to improve the technical ability to calculate the nonequilibrium gas dynamic flow field and radiative heating of entry vehicles, were completed or advanced.

Whiting, Ellis E.↗
NASA NTRS1986

A tunable, solid, Fabry-Perot etalon for solar seismology

A solid etalon has been designed and fabricated from a 50-mm diameter wafer of optical-quality lithium niobate. The finished etalon has a free spectral range of 0.325 nm at 588 nm. The parallel faces are coated with silver, and the central 15-mm aperture of the etalon has a finesse of 18.6. The reflective faces double as electrodes, and application of voltage will shift the passband. This feature was used in a servo circuit to stabilize the passband against temperature and tilt-induced drifts to better than three parts in one billion. Operated in the stabilized mode for day-long sessions, this filter alternately samples the wings of a narrow atomic absorption line in the solar spectrum and produces a signal proportional to velocity on the solar disk. The Fourier transform of this signal yields information on acoustic waves in the solar interior.

Rust, David M.↗
NASA NTRS2002

The Evolution of Disks and Winds in Dwarf Nova Outbursts - FUSE

This project was a project to study the FUV spectra of two proto-typical dwarf novae, U Gem and SS Cygni, through an outburst cycle. The luminosity of the boundary layer in the two systems, as evidenced by earlier EUVE observations, is different in the two systems. Our intensive study of the two systems was intended to (1) probe the ionization and kinematic structure of the wind as a function of system brightness, (2) isolate the contributions of the disk to the FUV spectra, and (3) examine physical conditions and abundances of material just being accreted onto the disk from the secondary. The U Gem and SS Cyg observations took place in March and October 2000, respectively. The data obtained with FUSE was of excellent quality. Analysis of the both observations is now essentially complete, although some modeling of the SS Cyg spectra is ongoing, as we complete an ApJ manuscript on this object. Our main results for U Gem are as follows: The plateau spectra have continuum shapes and fluxes that are approximated by steady state accretion disk model spectra with an accretion rate 7x10(exp 9) Msolar/yr. The spectra also show numerous absorption lines of H I, He II, and 2-5 times ionized transitions of C, N, O, P, S, and Si. There are no emission features in the spectra, with the possible exception of a weak feature on the red wing of the 0 VI doublet. The absorption lines are narrow (FWHM approx. 50 km/s), too narrow to arise from the disk photosphere, and at low velocities (less than or equal to 700 km/s). The S VI and O VI doublets are optically thick. The absorption lines in the plateau spectra show orbital variability: in spectra obtained at orbital phases between 0.53 and 0.79, low-ionization absorption lines appear and the central depths of the preexisting lines increase. The increase in line absorption occurs at the same orbital phases as previously observed EUV and X-ray light-curve dips. If the absorbing material is in (near-) Keplerian rotation around the disk, it must be located at large disk radii. The final observation occurred when U Gem was about 2 mag from optical quiescence. The spectra are dominated by emission from an approx. 43,000 K, metal-enriched white dwarf (WD). The inferred radius of the WD is 4.95x10(exp 8) cm, close to that observed in quiescence. Allowing for a hot heated region on the surface of the WD improves the fit to the spectrum at short (less than 960 A) wavelengths. Our main results for SS Cyg are as follows: The first two of four observations of SS Cyg show disk dominated spectra with accretion rates of order 10(exp -8) Msolar/yr. Except for narrow interstellar features (atomic and molecular H), the lines are all broad consistent with a disk or wind origin. The O VI line in the spectra is mostly of wind origin as detailed modeling with our Monte Carlo code (developed in part using funds from this project) show. The continua from spectra in observations 3 and 4, observed during the decline phase, are not well fit with steady-state disks, and show considerable resemblance to quiescent spectra obtained with HUT. The most probable interpretation for the emission features seen in the spectrum in the last two observations is that they arise from a photo-illuminated choronosphere above the disk, rather than a wind.

Long, Knox↗
NASA NTRSDate not supplied

Spacecraft Materials Degradation Under Space-Simulated Low Earth Orbit (LEO) Environment

External spacecraft materials play an important role in satellite protection from the harsh space environment. Research has shown that the physical, chemical, and optical properties of matter change continuously as a result of exposure to solar radiation and aggressive chemical species produced in Earth’s upper atmosphere. Thorough knowledge of the material properties evolution throughout a planned mission lifetime helps to improve the reliability of spacecraft. Moreover, the establishment of correlation factors between true space exposure and accelerated space weather experiments at ground facilities enables accurate prediction of on-orbit material performance based on laboratory-based testing. The presented work aims to evaluate the radiation effects of low Earth orbit (LEO) environment, namely, exposure to the high-energy electrons, atomic oxygen (AO), and vacuum ultraviolet (VUV), of several modern spacecraft materials. The studied materials represent the “flight duplicates” of samples that are launched as a part of the 16th Materials International Space Station Experiment Flight Facility (MISSE-FF) mission in 2022.

Elena A Plis↗
NASA NTRSDate not supplied

Spacecraft Materials Degradation Under Space-Simulated Low Earth Orbit (LEO) Environment

External spacecraft materials play an important role in satellite protection from the harsh space environment. Research has shown that the physical, chemical, and optical properties of matter change continuously as a result of exposure to solar radiation and aggressive chemical species produced in Earth’s upper atmosphere. Thorough knowledge of the material properties evolution throughout a planned mission lifetime helps to improve the reliability of spacecraft. Moreover, the establishment of correlation factors between true space exposure and accelerated space weather experiments at ground facilities enables accurate prediction of on-orbit material performance based on laboratory-based testing. The presented work aims to evaluate the radiation effects of low Earth orbit (LEO) environment, namely, exposure to the high-energy electrons, atomic oxygen (AO), and vacuum ultraviolet (VUV), of several modern spacecraft materials. The studied materials represent the “flight duplicates” of samples that are launched as a part of the 16th Materials International Space Station Experiment Flight Facility (MISSE-FF) mission in 2022.

Elena Plis↗
NASA NTRS1983

Sputtered coatings for protection of spacecraft polymers

Kapton polyimide oxidizes at significant rates (4.3x10(-24) gram/incident oxygen atom) when exposed in low Earth orbit to the ram atomic oxygen flux. Ion beam sputter deposited thin films of Al2O3 and SiO2 as well as a codeposited mixture of predominantly SiO2 with a small amount of polytetrafluoroethylene were evaluated and found to be effective in protecting Kapton from oxidation in both laboratory plasma ashing tests as well as in space on board shuttle flight STS-8. A protective film of or = 96 percent SiO2 and or = 4 percent polytetrafluoroethylene was found to be very flexible compared to the pure metal oxide coatings and resulted in mass loss rates that were 0.2 percent of that of the unprotected Kapton. The optical properties of Kapton for wavelengths investigated between 0.33 and 2.2 microns were not significantly altered by the presence of the coatings or changed by exposure of the coated Kapton to the low Earth orbital ram environment.

Banks, B. A.↗
NASA NTRSDate not supplied

Ground Testing of the MISSE-16 Materials

External spacecraft materials play an important role in satellite protection from the harsh space environment. Research has shown that the physical, chemical, and optical properties of matter change continuously as a result of exposure to solar radiation and aggressive chemical species produced in Earth’s upper atmosphere. Thorough knowledge of the material properties evolution throughout a planned mission lifetime helps to improve the reliability of spacecraft. Moreover, the establishment of correlation factors between true space exposure and accelerated space weather experiments at ground facilities enables accurate prediction of on-orbit material performance based on laboratory-based testing. The presented work aims to evaluate the radiation effects of low Earth orbit (LEO) environment, namely, exposure to the high-energy electrons and atomic oxygen (AO), of heritage and novel spacecraft materials selection. The studied materials represent the “flight duplicates” of samples that are launched as a part of the 16th Materials International Space Station Experiment Flight Facility (MISSE-FF) mission in 2022.

Elena A. Plis↗
NASA NTRS2000

Space Environmentally Stable Polyimides and Copolyimides

Polyimides with a unique combination of properties including low color in thin films, atomic oxygen (AO), ultra-violet (UV) radiation resistance, solubility in organic solvents in the imide form, high glass transition (T(sub g)) temperatures and high thermal stability have been prepared and characterized. The polymers were prepared by reacting a novel aromatic diamine with aromatic dianhydrides in a polar aprotic solvent. The solubility of the polymers in the imide form as well as the color density of thin films were dependent upon the chemical structure of the dianhydride. Several thin films (25-50 mm thick) prepared by solution casting of amide acid or imide solutions exhibited very low color and high optical transparency (approximately 90%) as determined by UV/visible spectroscopy. The polymers exhibited T(sub g)s >200 C depending upon the structure of the dianhydride and temperatures of 5% weight loss approximately 500C in air as determined by dynamic thermogravimetric analysis. Thin films coated with silver/inconel were exposed to a high fluence of AO and 1000 equivalent solar hours of UV radiation. The effects of these exposures on optical properties were minor. These space environmentally durable polymers are potentially useful in a variety of applications on spacecraft such as thin film membranes on antennas, second-surface mirrors, thermal/optical coatings and multi-layer thermal insulation (MLI) blanket materials. The chemistry, physical and mechanical properties of the polymers as well as their responses to AO and UV exposure will be discussed.

Watson, Kent A.↗
NASA NTRS2002

Facility and Methods Developed for Simulated Space Vacuum Ultraviolet Exposure Testing of Polymer Films

Vacuum ultraviolet (VUV) radiation of wavelengths between 115 and 200 nm produced by the Sun in the space environment can degrade polymer films, producing changes in their optical, mechanical, and chemical properties. These effects are particularly important for thin polymer films being considered for ultralightweight space structures, because, for most polymers, VUV radiation is absorbed in a thin surface layer. The NASA Glenn Research Center has developed facilities and methods for long-term ground testing of polymer films to evaluate space environmental VUV radiation effects. VUV exposure can also be used as part of combined or sequential simulated space environmental exposures to determine combined damaging effects with other aspects of the space environment, which include solar ultraviolet radiation, solar flare x-rays, electron and proton radiation, atomic oxygen (for low-Earth-orbit missions), and temperature effects. Because the wavelength sensitivity of VUV damage is not well known for most materials, Glenn's VUV facility uses a broad-spectrum deuterium lamp with a magnesium fluoride window that provides output between 115 and 200 nm. Deuterium lamps of this type were characterized by the National Institute of Standards and Technology and through measurements at Glenn. Spectral irradiance measurements show that from approximately 115 to 160 nm, deuterium lamp irradiance can be many times that of air mass zero solar irradiance, and as wavelength increases above approximately 160 nm, deuterium lamp irradiance decreases in comparison to the Sun. The facility is a cryopumped vacuum chamber that achieves a system pressure of approximately 5310(exp -6) torr. It contains four individual VUV-exposure compartments in vacuum, separated by water-cooled copper walls to minimize VUV radiation and any sample contamination cross interactions between compartments. Each VUV-exposure compartment contains a VUV deuterium lamp, a motor-controlled sample stage coupled with a moveable cesium iodide VUV phototube, and two thermocouples for temperature measurement. The vacuum chamber and exterior equipment is shown. Each VUV lamp is located at the top of the chamber with its projection-tube pushed through an O-ring compression fitting. The lamp assemblies are located on ports that can be isolated from the rest of the vacuum chamber, permitting maintenance or replacement of the lamps without breaking vacuum in the main chamber where the samples are located. A view of two of the four interior VUV-exposure compartments, including the moveable sample stages and detector holders is also shown. Glenn is using this facility to support testing of Next Generation Space Telescope sunshield materials that is being led by the NASA Goddard Space Flight Center and to develop an understanding of the wavelength, intensity, and temperature dependence of VUV-induced polymer degradation.

Dever, Joyce A.↗
NASA NTRS1995

LDEF's contribution to the selection of thermal control coatings for the Space Station

The design of the Space Station presented new challenges in the selection and qualification of thermal control materials that would survive in low Earth orbit for a duration of up to 30 years. Prior to LDEF, flight data were obtained from Orbiting Solar Observatory (OSO) satellites, a number of Orbiter flights, and limited ground tests. The excellent data obtained from the OSO satellites were based on calorimetry and temperature measurements which were transmitted to Earth; these satellites were not recovered. For some of these flight experiments it was difficult to distinguish between changes due to contamination, atomic oxygen (AO), ultraviolet radiation (UV), particle radiation and the synergistic effects between them. The data from Shuttle flights were primarily focused on developing a better understanding of atomic oxygen (AO) effects. Although UV and AO were present, the relatively short duration of the Orbiter flights, about one week, was viewed as too short to show the effects from UV or possible synergistic interactions with AO and contamination. At the beginning of the program in 1989 there was no established design data base for AO resistant thermal control coatings for the Space Station. Then came the Long Duration Exposure Facility (LDEF). It provided the first long life data for materials exposed and recovered from space with a characterized environment. Post flight analysis proved data on the effects of contamination on optical properties in the ram (velocity) and wake directions and the erosion of Teflon and multilayer insulation (MLI) covers. The results from LDEF confirmed and, in some cases, modified the approach used for the Space Station, as well as helped to focus our development activities. These development activities resulted in a number of new technical solutions which are applicable to many spacecraft surfaces and missions. LDEF also showed the detrimental effects that could occur from silicone contamination, an issue that has not been completely resolved. An investigation was initiated in 1993 on the effects of silicone contamination and was continuing at the time this paper was prepared.

Babel, Henry W.↗
NASA NTRS1989

Velocities and rotational excitation of interstellar H2 toward Pi Scorpii

A spectrum of Pi Sco showing numerous atomic lines and 70 absorption features from the Lyman and Werner transitions of interstellar H2 in rotational level J from zero to five is presented. Their shapes of the composite column density profiles are very nearly Gaussian with a one-dimensional rms velocity dispersion of 3 km/s. The behavior of shifts in the inferred N(H2) as a function of velocity are consistent with the overall profiles being composed of nearly symmetrical, tightly paced assemblies of about seven unresolved components. The relative overall column densities in the higher J levels of H2 are consistent with a model where these states are populated by optical pumping through the Lyman and Werner transitions, powered by UV radiation from nearby stars. The slight narrowing of the high-J profiles may be due to small clumps of H2 at radial velocities some 5-8 km/s from the core of the profile are exposed to a pumping flux about 10 times lower than that for the material near the profile's center.

Jenkins, Edward B.↗