The paper will discuss recent work at ASTM and IEST to update existing standards and introduce new standards. Committee work on standards of interest to contamination control engineers will be discussed. IEST-STD-CC1246E was released in the last year, and changes from revision D will be highlighted. A new ASTM Standard Practice for Spacecraft Hardware Thermal Vacuum Bakeout will also be emphasized.
Ultraviolet light attenuation was measured by the Martin Marietta Corporation in 1980 between the wavelengths of
approximately 115 nanometers to 300 nanometers. These data are compared to the light attenuation calculated using
x-ray interactions with matter, pioneered by the Lawrence Berkley National Laboratories. The Center for X-Ray
Optics provides a methodology to calculate light attenuation from x-ray light to 124-nm ultraviolet light. There is a
slight overlap in the data, allowing for a comparison of commonly outgassed species from the base materials
reported in the Martin Marietta document.
Two Surface Acoustic Wave (SAW) quartz crystal microbalances were used to determine nonvolatile mass deposition from a nitrogen purge gas. The units were connected serially and both were evaluated in the first and second position. Similarities and differences will be discussed.
The GOES-12 spacecraft exhibits a loss of heat rejection capabilities over the long term. It also presents a
unique opportunity to evaluate this change at more than one temperature. The Imager and Sounder instruments use
passive radiant coolers to reject heat from their sensors, and operate near 200 Kelvin. The Solar X-ray Imager (SXI)
instrument mirror operates at or above 273 Kelvin. The mass deposition for SXI is substantially less than that of the
Imager and Sounder. The phenomenon is evaluated and reasons for it discussed. This paper follows other descriptions
by the author of the electrostatic return and introduces photolytic interactions at different temperatures.
The GOES-8 Imager and Sounder radiant coolers have lost some of its ability to dissipate heat over time. This is shown by the temperature increase over time of the vacuum cooler housings that are cooled with optical solar reflector (OSR) radiators. Contamination has a significant, well-documented effect on the solar absorptance of OSRs. This evaluation attempts to discern how much molecular contamination has collected on the Imager and Sounder radiant coolers by analyzing the increase in temperature of the vacuum cooler housing. In the first part, temperature change is transformed into solar absorptance units by a method devised by ITT. The second part transforms the solar absorptance gain into a molecular film thickness and correlates the film thickness to the number of days after radiant cooler cover deployment.
Molecular contamination of ultraviolet (UV) optics has been well characterized at wavelengths longer than 120 nm, and some tests have been done at very short wavelengths (10 nm). Characterization of molecular contamination effects at intermediate wavelengths is scarce. This paper draws a parallel between organic light filters and molecular contaminants, considering the contaminant as an extra organic film on an existing filter. This allows the analyst to use Henke light absorptance analysis to characterize extreme ultraviolet (EUV) light attenuation. This technique shows that peak light attenuation occurs near 73 nm (17 eV), rather than the previously assumed 121.6 nm, due to the preferential ejection of 2s electrons in carbon near 17 eV.
Molecular contaminants degrade the optical or thermal properties of spacecraft systems. In the case of grazing incidence mirrors, they may even increase the system throughput at certain wavelengths. Theoretical calculations using a semi-imprircal model developed by Henke, Davis and Gullickson at the Lawrence Berkeley National Laboratory show the effect of varying film thickness' on mirror reflectivity.<SUP>2</SUP> The reflectivity is a product of the base material and any thin films, including molecular contaminants. The effect on nickel, gold, and Zerodur substrates are evaluated with polycarbonate, polypropylene and poly(dimenthyl silicone) contaminants in the range of 5 to 100 Angstroms x-ray wavelength. X-rays pass through the film until they meet an atom; they are then scattered elastically or absorbed. Photoabsorption occurs when the photon energy is equal to or greater than the energy required to promote an inner shell electron out of the atom. Strategies for evaluating contaminant effects with different light sources are included, taking into account the scattering cross section, expressed as mirror reflectance, of the materials involved.
Contaminants degrade the thermal properties of spacecraft systems. This paper describes a method of quantified visual inspection, which assumes a minimum visible particle size that can be easily evaluated for any observer. It also assumes a log-normal particle size distribution, but not the slope of it. By counting the particles visible in a convenient unit area, the fractional area coverage can be calculated. The method can be applied to real systems during ground inspection. This paper discusses methods for assessing the radiant cooler sunshields of the Imager and Sounder instruments on the Geostationary Operational Environmental Satellite (GOES) spacecraft. Then the method for converting the percent area coverage to the amount of solar power, which is scattered, to the cooler patch is explained. The essence of this conversion is based on careful consideration of the paths of specularly reflected and scattered solar light within the sunshield enclosure. A three-dimensional Bidirectional Reflectance Distribution Function (BRDF) model of the radiant cooler sunshield was constructed in order to estimate the illumination of a contaminated spot on a sunshield wall based on relative sun orientation, direct illumination, and specularly-reflected illumination from other walls. Radiant energy reflected from the contaminated spot to the radiant cooler was estimated using a BRDF model based on data reported by Young in previous studies.
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.
The Hubble Space Telescope was designed to be periodically serviced on-orbit during its 15 year mission. Servicing carriers have been designed for these servicing missions and were previously flown during the Hubble Space Telescope Servicing Mission 1, Space Transportation System 61, December 1993. In preparation for the Hubble Space Telescope Servicing Mission 2, the Hubble Space Telescope contamination control philosophy was reviewed to determine its applicability to reflown hardware. The contamination control program currently in place for the Hubble Space Telescope Servicing Mission 2, evolved from the Hubble Space Telescope Servicing Mission 1 contamination control program. The challenge of the Hubble Space Telescope Servicing Mission 2 contamination control program was to maintain the integrity and outgassing certification of the reflown hardware while accommodating configuration changes to the hardware. Environmental control of the hardware, materials screening and outgassing certification of added hardware were the important features of the program.