The MSX Contamination Experiment team was responsible for establishing design and operational guidelines and the contamination control plan for the Midcourse Space Experiment (MSX), as well as for tracking hardware cleanliness prior to launch. The approaches taken and the results are described.

The Geostationary Operational Environmental Satellite (GOES) Sounder instrument uses radiant coolers to reduce the operating temperature of the detectors and filter wheel. GOES resides in an equatorial orbit 36,000 kilometers above the earth, and is stationary with respect to it. During the year, all sides of the spacecraft are exposed to the sun; the filter wheel emitter and detector radiators must be shielded form it to adequately cooled these components for nominal operations.Mirror Optical Solar Reflectors are used too reject sunlight before it can strike the radiators. Molecular outgassing from the Sounder instrument cavity, the filter wheel module, and the Sounder vacuum cooler housing have been demonstrated through mass transport modeling to contaminate the filter wheel sunshield panels during the in- orbit Radiant Cooler bakeout. Excessive molecular and particulate contamination can increase solar energy scatter, increase thermal emittance, and increase solar absorptance; all of which can increase the temperature of the components they serve, thus degrading nominal operations. After the GOES-K spacecraft thermal vacuum test, a haze was observed on and around the entrance aperture, and on the inside faces the filter wheel cooler sunshield. This paper documents the inspections, testing, and analysis used to: a) locate the likely sources for the contaminants, b) predict molecular contaminant accumulation on the filter wheel sunshields during the in-orbit bakeout, c) estimate the thermal effects from molecular build-up, and d) assess proposed hardware modifications and show the selection rationale used to maintain functionality for the GOES-K Sounder instrument.

At various times, concerns have been expressed that rapid decompressions of compartments of gas pockets and thermal blankets during spacecraft launches may have caused pressure differentials across their walls sufficient to cause minor structural failures, separations of adhesively-joined parts, ballooning, and flapping of blankets. This paper presents a close form equation expressing the expected pressure differentials across the walls of a compartment as a function of the external to the volume pressure drops, the pressure at which the rates occur and the vent capability of the compartment. The pressure profiles measured inside the shrouds of several spacecraft propelled by several vehicles and some profiles obtained from ground vacuum systems have been included. The equation can be used to design the appropriate vent, which will preclude excessive pressure differentials. Precautions and needed approaches for the evaluations of the expected pressures have been indicated. Methods to make a rapid assessment of the response of the compartment to rapid external pressure drops have been discussed. These are based on the evaluation of the compartment vent flow conductance, the volume and the length of time during which the rapid pressure drop occurs.

Many contamination lessons have been learned since the Midcourse Space Experiment satellite was launched on April 24, 1996. FOremost is the inclusion of spacecraft design and thermal engineers with the contamination team early in the program, which resulted in the placement of vents away from the optical sensors, the separation into clean and 'dirty' sections, the exclusion of thrusters, the use of non- perforated silver/Teflon on the optical sensor axis, and the outgassing procedures instituted for all subsystem suppliers. The use of contamination instruments as monitors during integration and testing enabled correct technical decision to be made during several contamination incidents. In space, the contamination monitoring instruments supported programmatic decisions during the early mission planning. During the warm-up of the cryogenic telescope, it was learned that a small gap between the multilayer insulation and the internal baffles contributed to mass redistribution of water vapor. Consequently, it was also learned that a careful warming procedure may potentially be used to clean space-based cryogenic mirrors of condensed water vapor. Particles brought from the ground can be released by mechanical motions such as door openings as well as by thermal shock induced by the Sun during umbra exit. Solar- induced water evaporation from multilayer insulation can dominate the gaseous environment over the spacecraft's lifetime of several years.

This paper describes the derivation of cleanliness requirements and the development of contamination control procedures for the Space IR Telescope Facility (SIRTF), the fourth of the great observatories. SIRTF, scheduled for launch in December 2001 and designed to explore the IR universe, will be placed into a heliocentric orbit and will perform background-limited imaging and spectroscopic measurements of celestial objects in the 3-180 micrometers spectral range. The main components of the facility are the three scientific instrument packages, a liquid helium dewar to maintain the instruments as 2 to 5 K, a telescope which is maintained at 5 K, and a spacecraft. The instrument/dewar assembly, known as the MIC, will be cooled before launch, while the telescope will be launched warm and will be cooled on orbit by a combination of radiation to space and helium boil off. SIRTF has several unique contamination control drivers and design challenges. The mission lifetime requirement of 2.5 years will be achieved by minimizing the thermal load on the telescope and helium dewar using very low emittance surfaces, most of which operate below the condensation temperature of water vapor, and are hence very vulnerable to emittance degradation due to contaminant deposition. Contaminant films and particles on optical surfaces will reduce optical throughput, increase off axis point source transmittance, and degrade the point spread function of the telescope and scientific instruments, and hence degrade the observatory performance. Quantification of these effects is hampered by the lack of contaminant optical effects data at long wavelengths and uncertainty about the structure of cryolayers at temperatures less than 20 K. The operational temperatures of the SIRTF telescope and the cryostat will be low enough to suppress equilibrium outgassing rates to negligible values, but outgassing of the telescope, baffle assembly, thermal insulation, etc. during initial cooldown is significant. Assessment of molecular contaminant transport during this phase requires modeling the effect of transient source and deposition surface temperatures, as well as the measurement of transient outgassing rates of materials at temperatures below ambient. The study completed so far facilitates subsequent contamination analysis and control at the optical component level.

Critical surfaces on satellites operating int he low earth orbit can be contaminated by molecular species emanating from the spacecraft itself and from hypervelocity debris impacts. Consequently, on-board procedures are necessary to limit the contamination to levels below specified thicknesses in order to achieve the operational requirements of solar panels, thermal management materials, thermal radiators and optical systems. We demonstrate that our recently discovered phenomenon of the acceleration of the atomic oxygen erosion of organic molecular species by the simultaneous super-position of an electrostatic charge on the substrate, can be used to clean organic contaminants off critical surfaces in space.

The NASA Space SCience Program, in its on-going mission to study the universe, has begun planning for a telescope that will carry on the Hubble Space Telescope's exploration. This telescope, the 'Next Generation Space Telescope' (NGST), will be 6-8 meters in diameter, will be radiatively cooled to 30-60 Kelvin in order to enable extremely deep exposures at near IR wavelengths, and will operate for a lifetime of 5-10 years. the requirement will be to measure wavelengths from 1-5 microns, with a goal to measure wavelengths from 0.6-30 microns. As such, NGST will present a new contamination control challenge.

Quartz Crystal Microbalance (QCM) sensors have long been used in space to measure outgassing molecules being emitted by spacecraft materials or, alternately, erosion effects made by striking the spacecraft with some external molecular flux, e.g., atomic oxygen. However, the measurements produced by the QCM have been hard to meaningfully interpret because of solar thermal radiation effects. Normally, in a QCM, a sense crystal is exposed to space to measure the appropriate mass flux, but the reference crystal is hidden from this exposure. Crystals used in QCMs not only have a mass sensitivity but also have a temperature sensitive component. When the vagaries of spacecraft motion and thus QCM motion is considered, the sense crystal sees the sun at various times and at various angles. When exposed to sunlight, the QCM changes frequency because thermal radiation strikes and exposed crystal and not the reference crystal. We will report on the findings of a new Thermoelectric QCM with two exposed crystal, and the effects of sunlight on it. With both the sensor and the reference crystal exposed to thermal radiation and thus eliminating the offset frequency, the resulting beat frequency will reflect only the mass flux and the data will be easier to interpret.

There is a current need for a sensor which can measure minute outgassing or erosion over very long time spans in the space environment. One way of addressing this need is a QCM with very stable output and high mass sensitivity. In order to increase the mass sensitivity. In order to increase the mass sensitivity of the QCM, the crystal has to oscillate at a higher frequency. In the past, assurance that the mass sensitivity at 10MHz as predicted by theory has been provided by nine different experimenters using the same or different techniques. When 15MHz QCMs with an increased theoretical sensitivity became available, they were experimentally exposed to the same molecular source flow as the 10MHz QCMs, to measure their response. It proved to be identical to theory. Historically, QCM sensor discussions have dealt exclusively with plano-plano crystals, i.e., both sides flat and parallel. Now, however, increases in frequency beyond 15MHz call into question whether we still have plano-plano crystals or whether plano-convex now best describes the crystals. Since the diameter of the high frequency crystal has to be less in order for it to oscillate, it becomes harder and harder to assure true plano-plano crystal performance as the fundamental frequency is raised. In this paper, we will discuss experiments which have been performed comparing the mass sensitivity of 25MHz to 15MHz crystals, or the mass range that is available with these high sensitivity crystals. We will also address the plano-plano versus plano-crystals' sensitivities.

Space IR Telescope Facility (SIRTF) will carry three instruments to explore the sky in the IR: IRAC, IRS, and MIPS. SIRTF detectors have unprecedented sensitivity in all wavelength ranges. The particles, if present in the field of view (FOV) of the telescope, can cause significant imaging problems of the celestial objects if the thermal emission from the particles exceeds the background emission reaching the detector. Far-field particles may be identified as false targets. The near-field particles can light up the entire detector screen so that no other image may be observed while the particle stays in the FOV. Particles may be generated due to telescope aperture cover ejection, due to micrometeoroids colliding with the solar panel, or through other miscellaneous means. If the particle did not have a significant initial velocity component when it first entered the FOV, the only force to drive it out of the FOV is solar pressure. The time taken by a particle to traverse the FOV on account of the solar pressure alone may be several minutes. It is of interest to know the size and frequency of particles which can enter the telescope FOV without causing a substantial loss in the science data.

The development of quartz crystal instrumentation for space research resulted from investigations on the use of AT-cut and Y-cut quartz crystal oscillators to detect very small changes in mass and temperature. These novel methods of using the different cuts of quartz to make mass and temperature measurements led to the design and construction of seven new instruments that are used to study the space environment.

Area Function, also variously referred to in spaceflight contamination control parlance as 'percent obscuration' or 'percent area coverage' (PAC), is an important parameter in the evaluation of witness plates used to monitor cleanliness in hardware ground processing environments. Computed-based image analysis tools can provide a rapid, accurate, and reliable means by which to obtain area fraction information from such witness plates. We present an example of measuring area fractions using A Cambridge 360 scanning electron microscope (SEM) in combination with a PGT/IMIX image analysis system to examine silicon wafer witness plate specimens. The SEM/Image analysis system sued in this work was shown to measure area fractions within 1 to 3 percent of the true PAC, with a precision of +/- 7.7 percent. Image collection and processing operations such as background equalization, erosion and dilation, were performed on secondary electron emission SEM source images. Secondary emission was found to produce source images most amenable to image processing given the material composition of the fallout on the specimen witness pates examined most amenable to image processing given the material composition of the fallout on the specimen witness plates examined here, but the application of other signal types are also discussed. The results presented provide the basis for a generalized discussion of issues basic to the use of computer-based image analysis tools, i.e., accuracy, precision, background equalization, contrast, and magnification.

The capability to monitor airborne hydrocarbon compounds is essential in order to protect sensitive optical payloads from performance degradation caused by the deposition of surface films. Commonly used hydrocarbon monitoring instrumentation such as flame ionization detectors yield no information about the source or identity of compounds they detect. The Fourier Transform IR Spectrometer (FTIR) with its inherent ability to discriminate a large number of compounds offers a tremendous advantage over other types of instrumentation. The contamination monitoring laboratory at John F. Kennedy Space Center has developed an airborne hydrocarbon monitoring system based on FTIR technology to support the AXAF payload. This system consist of a portable cart suitable for use in Class 1 Division 2 environments. This paper describes the system in detail.

Two separate real-time particulate fallout monitoring instruments have been developed by the contamination monitoring Laboratory at NASA John F. Kennedy Space Center. These instruments monitor particular fallout contamination deposition rates in cleanrooms and allow certification of cleanliness levels as well as proactive protection of valuable flight hardware.

Monitoring of cleanroom and spacecraft cleanliness during ground processing operations is essential in order to verify performance requirements for optical systems prior to launch. The objective is to replace manual particle counting with automated particle counting in order to reduce the processing time for the witness plates and to improve precision and accuracy of the measurements. A modified silicon wafer inspection instrument, using a HeNe laser light source, was used to count and size particles deposited on wafers exposed in the cleanrooms. The previous paper discussed measuring particles on silicon wafer witness plates during operations cleanrooms and discussed analytical methods for calculating percent area coverage. This paper describes the optical performance of the ESTEK instrument and test on the instrument.

Wafer surface scanners, developed and long used in the microelectronics industry for detecting defects on silicon wafers during the semiconductor manufacturing process, have been more recently employed on a limited basis in the aerospace industry to assess particulate debris fallout in cleanrooms and clean work area environments. One use of a wafer scanner in this context is to scan witness plates to obtain data from which to calculate the fraction of a contamination-sensitive surface that is obscured by particulate fallout. Wafer surface scanners have been found to be fast, precise, and straight-forward to sue, but questions about the accuracy of surface area fractions derived rom scanner data have generated controversy in the spacecraft contamination control community. We have examined some commonly used methods for calculating fractional area coverage form scanner data. Geometric midpoint, log-log, and shape-factor models were evaluated for accuracy against a reference standard in the form of area fractions measured using a computer-based image analysis systems, Area fractions calculated using geometric mean diameters of the wafer scanner particle size data produced errors of 0 and +10 percent. Using the log-log mean diameters produced errors of -15 and -20 percent. Shape factor models were found to be inappropriate for use with scanner data.

A TQCM coated with graphite was flown aboard a Spartan carrier in January 1996. During a flight of about 46 hours at an altitude of 305 km, the graphite reacted with the atomic oxygen (AO) in the environment and was eroded away. The 15-MHz TQCM's frequency dropped from 6800 to 4000 Hz in about 15 hours of exposure and was shown to be a strong function of the TQCM's orientation to the ram direction. The erosion rates for four different ram angels was measured and found to be both consistent and repeatable. The average graphite volume loss for the 61 degree and -62 degree ram angles was calculated to be about 2 X E-08 cm³/hr and for the 18 degrees and 19 degrees angles to be about 8.5 X E-08 cm³/hr, which is slightly less than previous flight data. The erosion data was then correlated with AO density numbers for the particular times and positions of the spacecraft in orbit. From this analysis, an equation was derived that shoed the carbon volume loss as a function of both atomic oxygen density and ram angle. For example, 1.59 E-07 cm³/hr would be the calculated carbon volume loss for a ram angle of 0- degrees and an AO fluence of 3.52 E+17 atoms/hr. The results of this data and analysis may lead to the development of a sensor capable of monitoring the AO fluence on a spacecraft.

Particle occurrence rates, velocities, size distributions, and trends in the environment have been measured above the Midcourse Space Experiment satellite using optical sensors. Results from the spacecraft's first year on orbit are presented. Particles were detected during relatively quiescent times and as a result of distinct particle release events. On 11 November 1996, we observed a discrete particle release even that is not attributable to spacecraft activity. We hypothesize that this event was caused by an impact by either orbital debris or a micrometeoroid. We present the particle size and velocity distributions from that event and compare them to the quiescent distributions and to previous model predictions.

The passive optical sample assembly-I (POSA-I), part of the Mir Environmental Effects Payload (MEEP), was designed to study the combined effects of contamination, atomic oxygen, UV radiation, vacuum, thermal cycling, and other constituents of the space environment on spacecraft materials. The MEEP program is a Phase 1 International Space Station Risk Mitigation Experiment. The payload was attached by EVA to the exterior of the Mir docking module during the Space Shuttle mission STS-76. It was removed during STS-86 after exposure to the Mir space station environment for 18 months. POSA-I consists of nearly 400 samples of thermal control paints, chemical conversion coatings, mirrors, optics, and baseline materials for International Space Station. POSA-I also flew state-of-the-art materials and passive instruments for monitoring the atomic oxygen and UV radiation dose to the experiment. Pre- and post-flight characterization of candidate spacecraft materials is discussed. Contamination was detected on the POSA-I experiment. On the side facing space, visible contamination was observed. The contamination was uniform as would occur from a slow photodeposition process. A very definite film was deposited on the optical samples. The deposition appears to have directionality with definite shadowing effects. On the side facing the main Mir core, no visible contamination was noted.

The Total Pressure Sensor (TPS) on-board the Midcourse Space Experiment (MSX) Spacecraft has continuously measured the ambient local pressure since launch of MSX on April 24, 1996. The primary goals of the sensor are: 1) to monitor the ambient pressure surrounding the spacecraft's optical telescopes and to indicate when environmental conditions are acceptable for opening the protective covers, and 2) to monitor the long-term decay of the species outgassed from the spacecraft. The water-induced environment was expected to rapidly decay over the first few months to elves more closely approaching the natural environment. The data generally shows decay toward this level, however, the pressure is quite variable with time and can be influenced by discrete illumination and spacecraft orbital events. Several experiments, conducted approximately one year into the mission, indicate that the thermal blankets retain significant quantities of water. The local pressure due to water vapor is shown to increase by a factor of 100 from direct solar illumination of the blankets. Moreover, the multi-layer construction of the blankets causes them to form a deep reservoir, which continues to be a source of water vapor several tens of months into the mission. Additionally, the TPS has monitored numerous events in which the measured ambient pressure on the optics deck has exceeded 10^-9 Torr. Several of these events did not include solar illumination of the blankets. These events indicate that sources other than the MLI blankets are the cause for certain high-pressure transients. Finally, these events are not limited to the early mission, outgassing phase of the program. They have been witnessed over a year into the mission. The results documented herein indicate that special consideration must be given in the design of optical sensors to account for long term outgassing of a spacecraft.

The stability of materials used in the space environment continues to be a limiting technology for space missions. This technology is important to all users of space and particularly the International Space Station (ISS). The optical properties monitor (OPM) and the space portable spectroreflector (SPSR) experiments were performed on the Russian Mir Station to study the long term effects of the natural and induced space environment on materials. The OPM was deployed on the exterior of the Mir Docking Module on April 29, 1997 and remained until retrieval in January, 1998. The OPM exposed test materials to the Mir space environment and measured the effects of this exposure using on-board optical instruments. These instruments included an integrating sphere spectral reflectometer and a two color Total Integrator Scatter (TIS) instrument. The OPM also monitored selected components of the environment including molecular contamination using a pair of temperature- controlled quartz crystal microbalances. The SPSR is a hand- held extra vehicular activity (EVA) instrument that was used by an EVA crew to measure the optical properties of Mir thermal radiator surfaces after many years of operational use in space. The SPSR is an integrating sphere spectral reflectometer similar to the OPM reflectometer.

The Ballistic Missile Defence Organization is flying the Space Active Modular Materials Experiments (SAMMES), test of contamination and space environment effects on materials on board the Space Test Research Vehicle-2. This paper describes the experiment architecture, the instruments, and the sample suite. Notional descriptions of operations are provided to highlight the objectives and capabilities of SAMMES.

This paper discuses a general sticking coefficient theory for both neutral molecules and photochemically excited molecules in a solar vacuum UV (VUV) radiation environment. For neutral-molecule problems, the theory is based on the classical adsorption kinetics concept for either a single- component or multi-component gas on a surface with a constant incident flux. The theory has been verified satisfactorily with available DC-704 test data. For photochemically induced adsorption problems, a sticking coefficient model modified from that in the literature is proposed. This model asserts that photochemical reactions can take place only if the neutral-molecule residence time is longer than the photoexcitation dwell time. The total sticking coefficient is the sum of the neutral-molecule sticking coefficient and the photoexcited-molecule sticking coefficient. The photochemically induced deposition enhancement predicted by the present theory shows reasonable agreement with data presented in the literature.

This paper discusses a comparison of molecular transport predictions between the Molecular Transfer Kinetics (MTK) model based on Gebhart's enclosure theory for thermal radiation and the Molecular Flux (MOLFLUX) model based on the ray-tracing concept. The analysis has shown that the two molecular transport models are equivalent if the 'infinite number of molecular reflections' condition is imposed on MOLFLUX calculations. The general multi-node molecular transport expressions for MTK and MOLFLUX are derived, followed by proving hue MTK/MOLFLUX equivalence through comparisons of the MTK and MOLFLUX solutions for a two-node problem and a special three-node problem. Finally, the MTK/MOLFLUX equivalence is demonstrated by numerical solutions for a 169-node satellite configuration using the two models.

Contaminant deposition measurement have been made on species content and depth profiles on three experiment trays from the long duration exposure facility (LDEF), Auger, Argon sputtering, ESCA and SEM analysis was used to define the contaminant deposits. The Integrated Spacecraft Environments Model (ISEM) was used to predict the deposition levels of the contaminants measured on the three trays. The details of the modeling and the assumptions use dare presented along with the predictions for the deposition on select surfaces on the trays. These are compared to the measured results. The trays represents surfaces that have a high atomic oxygen flux, an intermediate oxygen flux, and no oxygen flux. All surfaces received significant solar UV flux. It appears that the atomic oxygen is necessary for significant deposition to occur. Surfaces that saw significant contaminant flux, solar UV and no atomic oxygen did not show any appreciable levels of observable deposition. The implications of the atomic oxygen interaction with contaminant deposits from silicon contaminant sources is discussed. The primary contaminant sources in the LDEF analysis are DC6-1104 adhesive and Z-306 paint. The result and interpretation of the findings have a potential significant impact on spacecraft surfaces that are exposed to solar UV and atomic oxygen in low Earth orbit.

The quantitative prediction of on-orbit molecular contamination effects is a difficult task that must rely on substantially simplifying assumptions about the rates and mechanisms of contamination production, transport, and deposition processes. Therefore, there is clearly room for a diversity of approaches within the spacecraft design community for the execution of contamination sources and effects analyses. This paper provides the initial description of the approach being taken for the development of a new contamination prediction code. In this development we intend to exploit the growing ASTM E1559 data base for outgassing and desorption measurements and to incorporate physical, kinetic models of condensation and photochemical deposition that are still sufficiently simple as to admit to successful parameterization.

This paper describes a model for the return flux of neutral and charged particles to a satellite in geosynchronous earth orbit. For neutral particles, the main return flux mechanisms is back-scattering via self-collisions among molecules outgassed or vented from the satellite; whereas for charged particles, the main mechanism is electrostatic re-attraction of ionized outgassed or vented molecules to a negatively charged satellite. Computer codes that simulate spacecraft charging typically contain a 3D charged particle trajectory-tracking procedure that, in principle, could be used for contamination studies. In practice, however, it is difficult to obtain quantitative results on the return flux distribution by this method. This makes such a code impractical to use as an engineering tool for identifying contamination problems reliably and evaluating corrective measures through simulation. To achieve a practical engineering tool, we prose an alternative to the particle tracking technique. We treat the problem for both neutral and charged particles in a unified manner by direct numerical solution to the Boltzmann equation in the BGK approximation. The feasibility of this approach is demonstrated by favorable numerical results presented for the simplified geometry of a spherical spacecraft.

This paper describes a series of analyses undertaken to estimate the operational neutral direct flux contamination environment within the transition Region and Coronal Explorer telescope. These analyses included details concerning vent effectiveness. TQCM measurement response during instrument and observatory-level thermal-vacuum testing, and indirect location of unidentified sources. In particular, this paper describes analytical and computational model development and the level with which model results corroborated with a and supplemented data collected during instrument and observatory-level thermal vacuum testing. One issue successfully dealt with a recognized problem associated with TRASYS calculations of internal geometries through viewfactor normalization. Confidence in model result affected telescope design, with removal of a previously unrecognized source from the primary mirror base, and redesign and replacement of the telescope ascent vents.

EOS AM-1 is the first in the series of the EOS spacecraft developed to advance the understanding of the biological and geophysical processes of the Earth's climate on a global basis. The fully integrated spacecraft is EOS AM-1 flight- phase contamination analysis has been performed to verify that the design of the spacecraft is compatible with limiting contamination to the level required for optical instruments and sensor, thermal control surfaces and solar array as well as to identify modifications if needed. This paper summarizes the approach and assumptions used in performing this contamination source and effects analysis for the EOS AM-1 spacecraft. Molecular and particulate contamination potential during the flight segment from launch through completion of orbital mission has been analyzed. Potential contamination sources examined include materials outgassing, instrument and spacecraft bus venting, spacecraft plume and other sources such as mechanisms, moisture absorption, and atomic oxygen. The modeling result have been used to confirm outgassing materials selection, to verify venting designs, and to develop bakeout requirements for components of the spacecraft bus and the instruments.

Understanding contaminant transport is fundamental to predict a possible contamination of spacecraft optics or other subsystem. A numerical modeling of EOIM-III STS-46 experiment environment was performed in order to understand how contaminant transport could be responsible for some of the observations. A DSMC model for collisions was developed to simulate collisions, and an empirical lobular model were used to simulate reflections on shuttle surfaces. Molecular dynamics was then modeled on two different scales: 1- a large computation box allowed to simulate return flux from outgassing into shuttle bay, 2- a detailed mesh of shuttle bay allowed to simulate local processes at materials and mass spectrometer level. In spite of the lack of experimental data about contaminant production rates, some quantitative predictions and comparisons could be performed using EOIM data about the flux into the bay and the fluxes from the materials, with or without baffle, which proved quite conclusive.

Laboratory measurements of photochemical deposition rates of outgassing products from Tefzel insulation have been conducted. We show that outgassing products from Tefzel insulation photodeposit under conditions of surface temperature and arrival rates for which bulk condensation will not occur. Normalized to the sample size, the photodeposition rate exceeds the reported condensable material outgassing rate. The result reported here strongly support the conclusion that photochemical deposition of contaminants from Tefzel is potentially a significant mechanism for degradation of thermal control surfaces on spacecraft.

Outgassing products originating from white siliconic paint were deposited on optical surfaces. The contamination layer was then exposed to vacuum UV (VUV) radiation. The volatile fragments generated by the paint outgassing as well as the UV irradiation were studied using a residual gas analyzer. The VUV effect on the contamination layer was studied by measuring the etching rate, measuring changes in the chemical state and measuring degradation in the optical transparency by UV-VIS photospectroscopy. Compiling the data obtained from the various analytical techniques it was concluded that the main erosion mechanism was crosslinking of the siliconic polymer chains. The crosslinking behavior induced fixation of the contamination layer, caused 'crystallization' of its liquid like form and degradation in its optical transparency.

The primary objective of the Earth Observing System (EOS) Solar Stellar Irradiance Comparison Experiment (SOLSTICE) is to accurately measure the absolute value of the solar UV irradiance at the top of the earth's atmosphere for a minimum mission lifetime of 5 years. To meet this objective, SOLSTICE employs a unique design to determine changes in instrument performance by routinely observing a series of early-type stars and comparing the irradiances directly with the solar value. Although the comparison techniques allows us to track instrument performance, the success of the SOLSTICE experiment depends upon photomultiplier detectors which have graceful degradation properties. Therefore, we have established a laboratory program to evaluate the characteristics of photomultiplier tubes which are exposed to long term fluxes similar to those we expected to encounter in flight. Three types of Hamamatsu photomultiplier tubes were tested as candidates for use in the EOS-SOLSTICE project. The results of these studies: pulse height distribution; quantum efficiency; surface maps,; and lifetime analysis are presented in this paper.

The medium resolution imaging spectrometer (MERIS), developed under European Space Agency (ESA) contract, for the Envisat 1 Polar Orbit Earth Mission belongs to a new generation of Ocean Color sensors which aim to improve the knowledge of some crucial processes of our planet. The instrument currently in the final stages of development is built by an international team led by AEROSPATIALE under ENVISAT prime contractor-ship of DORNIER. MERIS is a 'pushbroom' type instrument which measures the radiance of the Earth in 15 programmable spectral bands between 390 nm and 1040 nm over a 1150 km swath width. During the duration of the MERIS mission, radiometric in-flight calibration sequences are carried on a regular basis by the observation of Spectralon diffusers illuminated by the sun. The high accuracy required over the 4 years mission duration necessitates the precise knowledge of the calibration reference and the stability of the reference over the mission has to be controlled. This presentation details the influence of cleaning procedures on optical stability of the Spectralon flat plate diffusers calibration reference under space conditions and sun illumination. This paper will also define the BRDF characterization performances achieved with the Flight Model flat plate diffusers following implementation of the selected cleaning procedure.

A durable protected silver coating was designed and fabricated for possible use on flashlamp reflectors in the National Ignition Facility to avoid tarnishing under corrosive conditions and intense visible light . This coating provides a valuable alternative for mirror coatings where high reflectance and durability are important requirements. This paper describes a protected silver coating having high reflectance from 400 nm to 10,000 nm. The specular reflectance is between 95 percent and 98 percent in the visible region and 98 percent or better in the IR region.

Catastrophic damage has been observed in ZERODUR mirrors used as first mirrors in two beam lines at the National Synchrotron Light Source. ZERODUR was selected as a substrate for these uncooled, grazing incidence mirrors because of its superior thermal expansion properties near room temperature. Despite the high reflectivity of the coatings used on these mirrors, a significant flux of high energy photons penetrates below the coating and is absorbed mainly in a thin layer at the top of the substrate. Over a long time period the absorbed flux causes the glass material to compact, leading to a build-up of surface stress, gross figure changes, and the growth of fractures. The total dose of absorbed radiation for these mirrors is estimated to be in the range of 10⁶ MRads, i.e. 10¹² Rads, which is orders of magnitude greater than the dose used in conventional radiation damage studies for space-borne optics and deep UV lithography systems.

We present an assessment of bulk fused silica and calcium fluoride, and of antireflective coatings for 193-nm lithographic applications. In the course of extensive marathon studies we have accumulated 1-5 billion laser pulses on over 100 bulk material samples at fluences from 0.2 to 4 mJ/cm²/pulse. The result show large variation in both initial and induced absorption of fused silica and in densification of fused silica. For antireflective coatings, there are samples that undergo no appreciable degradation when irradiated for > 1 billion pulses at 15 mJ/cm²/pulse. However, initial losses in some coatings may be unacceptably high for lithographic applications.

The responsivities of the Landsat-5 Thematic Mapper (TM) reflective bands are characterized over the lifetime of the instrument using its internal calibration system. This system illuminates only the focal planes and aft optics of the TM so that it does not capture changes in the telescope. The observed changes are quantified and categorized as to whether they are likely to be true instrument responsivity changes or changes in the internal calibrator system itself. Changes observed that are likely to be true instrument changes are: (1) 7 percent, 5 percent, 8 percent and 7 percent exponential-like decreases in responsivity with decay half lives of 250, 180, 60 and 110 days in bands 1 to 4, respectively, during the initial on-orbit period and (2) an oscillation in response of about 5 percent peak-to-peak in bands 5 and 7. The first effect is believed to originate in the TM spectral bandpass filters and the second effect is believed to be due to an icing build up in the cold focal plane window. Two rapid apparent responsivity changes, one a decrease and one an increase, which are peculiar to particular internal calibrator lamps are believed to be due to changes in the lamp assemblies themselves as is a gradual increase in all detector's responsivities with time. An annual oscillation of up to 2 percent peak-to-peak in all bands is likely the product of both a temperature sensitivity of the IC and the TM primary focal plane.

All but one of the backscatter UV (BUV) instruments have used solar reflective diffusers made of ground aluminum to maintain instrument calibration after launch. These diffusers have been sued throughout mission life-times, which range from less than 1 years to over 14 years. Means for monitoring diffuser reflectance include mechanisms on the instruments as well as methods to infer reflectance using earth radiance data. We compare changes in diffuser reflectance for the various instruments and find some common behavior as well as significant differences. Changes which appear to occur at different rates are actually quite similar when corrections are made for the amount and direction of incident solar irradiation. However, a class of instruments, the SBUV/2, has significantly lower degradation rates. We find, as have previous authors, that spacecraft self-contamination is the most likely cause of diffuser changes and observed differences. Observed changes suggest that contaminant layer thickness is the main reflectance degradation mechanism in the first few years of operation.

The Solar UV Spectral Irradiance Monitor (SUSIM) aboard the Upper Atmosphere Research Satellite (UARS) has been measuring the solar spectral irradiance from 115 nm to 410 nm daily since October 1991. The primary difficulty in maintaining the calibration of these long-term measurements is the correct accounting of degradation in the instrument's responsivity. Accordingly, SUSIM was equipped with redundant optical elements and four stable deuterium lamps, any one of which can replace the sun in the optical path. Periodic calibration of the responsivity of the daily used optical paths is accomplished by comparison of the solar signals from these paths with the solar signals from redundant optical paths containing less-frequently used elements which, in turn, are calibrated using the lamps. Measurements of optical responsivity changes during the first 6.3 years of the SUSIM UARS mission are presented in this paper. The degradation is found to be almost entirely dependent on UV exposure. Consequently, most of the measured degradation takes place in optical elements which precede dispersion. Of these, degradation of transmission elements is strong and has a wavelength dependence similar to that of the lens on the calibration lamps on which contaminant layers apparently also form. The time dependence of the degradation also appears to follow a particular functional form for most wavelengths. Degradation in reflective elements is more moderate and, for some wavelengths, has been observed to reverse, i.e. to increase in responsivity. As an example, the standard channel responsivity at Lyman-(alpha) has increased from its minimum by a factor of more than three.

The Solar Stellar Irradiance Comparison Experiment is a three channel spectrometer designed for measuring the solar UV irradiance from 119 to 420 nm with a spectral resolution of 0.1 to 0.3 nm. The three channels are designated as the G, F, and N channels that cover the 119-190 nm, 170-320 nm, and 280-420 nm regions respectively. The SOLSTICE is aboard the NASA Upper Atmosphere Research Satellite (UARS) which was launched on September 12, 1991. The degradation of the SOLSTICE sensitivity is primarily tracked in-flight by measuring a set of bright, early-type stars with the same optics and detectors and by only changing slit sizes and integration times. While the Sun changes by 1 percent in the near UV and by as much as a factor of 2 in the far UV, early-type main sequence stars are not expected to change by more than 1 percent in the UV for long time periods. The ensemble average of the SOLSTICE stellar observation indicate that these stars are indeed stable to 2 percent or better. Since the launch of UARS, the SOLSTICE sensitivity has decreased by a few percent per year. We attribute the degradation primarily to again effects for the photomultiplier tubes for all three channels and to diffusion between layers in the broadband interference filters for the F and N channels. There also appears degradation for the G channel diffraction grating related to excessive heating of the grating on a few days during the UARS mission. There appears only minor degradation associated with optical contamination, mainly because of the strict use of low-outgassing materials in the SOLSTICE instrument and maintenance of class 10,000 clean rooms and oil-free vacuum systems for all pre-flight testing of the SOLSTICE instrument.

Degradation of sensitive satellite surface scan adversely effect the accuracy, lifetime and mission effectiveness of a spacecraft or payload. More sophisticated and complex space systems have increased the concern about contamination. Thus, it has become necessary to develop better prediction tools and testing techniques for use in contamination prevention and control. This paper discusses the effect of the induced contamination environment on the long-term degradation of two remote sensing instruments. Both instruments were the subject of contamination control programs. The Stratospheric Aerosol Gas Experiment (SAGE II) was launched by the shuttle on the Earth Resources Budget Satellite in 1984. The result of a throughput degradation contamination assessment performed prior to launched compares actual results acquired through March, 1998. The SAGE II instrument still continues to produce data within the limits predicted. A Total Ozone Mapping Spectrometer was launched in 1991 on a Russian Meteor-3 spacecraft. Degradation of the solar calibration diffuser plates have been observed and reported earlier. A new instrument, SAGE II, will be launched in 1999 on another METEOR-3 spacecraft from the Baikonur Cosmodrome in Kazakhstan. The METEOR- 3M/SAGE III is currently undergoing an intense contamination control program in order that data of the same quality as the SAGE II instrument will be realized.

A spaceborne spectral irradiance Monitor (SIM) is being developed at the Laboratory for Atmospheric and Space Physics, University of Colorado, to measure solar spectral irradiance and its variation over a 5 year period with a precision of 0.02 percent. The SIM consists of two independent and identical prism spectrometers. Each channel is equipped with an electrical substitution radiometer and ancillary photodiodes to prove spectral coverage from 0.3 to 2.0 micrometers with (lambda) /(Delta) (lambda) > 30. To meet this irradiance specification, the entrance slit must be characterized to give the effective slit width. Additional work is being performed to characterize how the space environment changes the diffractive properties of the slit over the course of the mission. The slit edges skim off the wavefront and then the broken edges diffract, losing energy at angle wider than the prism. The net effect at the detector is to reduce the effective slit width. Calculations using the Fraunhofer approximation show that most of the percentage slit area reduction is due to diffraction from the slit width. For a sharp cut-off at the edge of the slit the effective slit width decreases as 3.242(lambda) , where (lambda) is the wavelength of the incoming radiation. An experimental apparatus was constructed to test this calculation and to study the effects of heating and annealing on etched stainless steel entrance slit edges.