Brush and forest fires, both naturally occurring and anthropogenic in origin, in proximity to space flight hardware
processing facilities raise concerns about the threat of contamination resulting from airborne particulate and molecular
components of smoke. Perceptions of the severity of the threat are possibly heightened by the high sensitivity of the
human sense of smell to some components present in the smoke of burning vegetation.
On August 26th, 2009, a brushfire broke out north of Pasadena, California, two miles from the Jet Propulsion
Laboratory. The Station Fire destroyed over 160,000 acres, coming within a few hundred yards of JPL. Smoke
concentrations on Lab were very heavy over several days. All Lab operations were halted, and measures were taken to
protect personnel, critical hardware, and facilities. Evaluation of real-time cleanroom monitoring data, visual inspection
of facilities, filter systems, and analysis of surface cleanliness samples revealed facility environments and hardware were
Outside air quality easily exceeded Class Ten Million. Prefilters captured most large ash and soot; multi-stage filtration
greatly minimized the impact on the HEPA/ULPA filters. Air quality in HEPA filtered spacecraft assembly cleanrooms
remained within Class 10,000 specification throughout. Surface cleanliness was minimally affected, as large particles
were effectively removed from the airstream, and sub-micron particles have extremely long settling rates. Approximate
particulate fallout within facilities was 0.00011% area coverage/day compared to 0.00038% area coverage/day during
normal operations. Deposition of condensable airborne components, as measured in real time, peaked at approximately
1.0 ng/cm2/day compared to 0.05 ng/cm2/day nominal.
Gas-phase contamination modeling for space systems typically looks at the free molecular flow regime, Knudsen number » 1, wherein transport is characterized by collisionless motion of contaminant molecules and deposition proportional to grey- or black-body view factors. Such an approach, however, was not applicable to the contamination transport environment [to be] encountered by the NASA Mars Science Laboratory (MSL) during surface operations on the Red Planet. For MSL, we required an understanding of contaminant transport under the Mars-ambient conditions of an approximately 8 Torr CO2 atmosphere in order to provide traceability between hardware outgassing limits and the allowable vapor-phase contaminant concentrations in the vicinity of atmospheric sampling sensors and deposition to prospective solid sample sites on the Martian surface.
In setting outgassing requirements for the MSL surface system, an engineering upper-bound estimate--rather than a precise result based on an all-inclusive simulation of the dynamic flow field--of the local contamination density was needed. Here we describe a 3-D, low-speed computational fluid dynamics approach, including molecular diffusion, to determine mixing ratios of contaminants at the atmospheric sample inlets and solid sample inlet deposition rates. Turbulence enhances the effective diffusion, leading to the dilution of the volatile contaminants, which reduces contamination concentration at a distance far from the source in comparison to inviscid or laminar flow fields: Therefore, the approach employed here results in a conservative upper bound compared to one in which turbulence is explicitly addressed. Because contaminant transport in this environment (Peclet number in the range of 50-1000) is advection dominated, spatial contamination concentration is a strongly-peaked function of the wind direction. Results of sample calculations for expected Mar wind speeds (u = 1-20 m/s) and several wind directions are presented.
The Galaxy Evolution Explorer (GALEX), a NASA Small Explorer Mission planned for launch in Fall 2002, will perform the first Space Ultraviolet sky survey. Five imaging surveys in each of two bands (1350-1750Å and 1750-2800Å) will range from an all-sky survey (limit mAB~20-21) to an ultra-deep survey of 4 square degrees (limit mAB~26). Three spectroscopic grism surveys (R=100-300) will be performed with various depths (mAB~20-25) and sky coverage (100 to 2 square degrees) over the 1350-2800Å band. The instrument includes a 50 cm modified Ritchey-Chrétien telescope, a dichroic beam splitter and astigmatism corrector, two large sealed tube microchannel plate detectors to simultaneously cover the two bands and the 1.2 degree field of view. A rotating wheel provides either imaging or grism spectroscopy with transmitting optics. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the UV-global star formation rate relationship in galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the red shift range zero to two. The GALEX mission will include an Associate Investigator program for additional observations and supporting data analysis. This will support a wide variety of investigations made possible by the first UV sky survey.
Proc. SPIE. 3427, Optical Systems Contamination and Degradation
KEYWORDS: Optical fibers, Image processing, Particles, Silicon, Image analysis, Scanning electron microscopy, Digital imaging, Semiconducting wafers, Binary data, Picture Archiving and Communication System
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.
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.
This paper describes a finite-element numerical approach for predicting the time-dependent particulate motion of released particulate that complements the standard particulate redistribution model for predicting contamination levels of exposed payload surfaces during the ascent-phase. The particulate redistribution model is used to give a spatially and time averaged estimate of the contamination level during ascent that is considered to be very conservative. All particulates are assumed to be retained in the fairing/payload volume with none venting out of the volume. The new numerical model takes more comprehensive approach and attempts to estimate the time-dependent particulate surface-impingement flux by considering the key physical mechanisms believed to govern the gas/particulate transport problem. It solves the time-dependent fairing/payload gas transport (venting) and particulate transport problem during the ascent-phase. The gas transport model is used only up to point where gas flow no longer significantly impacts the partiality motion. Thus, only the continuum fluid regime is considered in the model. It is possible to treat particulate release beyond this point, but it has not been considered in the modeling effort described in this paper. Booster acceleration, gas motion during venting and particulate drag (due to gas flow) are treated. Released particulate trajectories and particulate impingement fluxes are predicted, and subsequent surface contamination levels are estimated. Results are presented for both Pegasus/TOMS and EOS during ascent. The final contamination levels predicted are considered to be the initial on-orbit particulate contamination levels.