Jeffrey C. Lesho

Quartz crystal microbalance (QCM) flight measurements of contamination on the MSX satellite

The midcourse space experiment (MSX) satellite was launched into a 903 Km, 99.4-deg orbit April 24, 1996. It carries imaging spectrometers and radiometers that operate in the UV, visible, and infrared spectral ranges. In addition, it carries several contamination measuring instruments that are being used to characterize the contamination environment on, in, and around the satellite. Five are quartz crystal microbalances (QCMs), four of which are temperature- controlled (TQCMs). They are located on various external surfaces of the spacecraft and are operating at minus 40 degrees Celsius to minus 50 degrees Celsius to measure the condensation of silicone and organic molecules. One is a cryogenic quartz crystal microbalance (CQCM) which is located adjacent to the SPIRIT III infrared cryogenic telescope primary mirror. Its temperature followed the mirror which cooled from 28 to 20 K during the first week of operation. All QCMs recorded deposition in the 10 - 20 ng/cm

MSX Satellite: Flight Measurements of Contaminant Films*

2)-day (1-2 angstrom/day) range. Thermo-gravimetric analyses on the QCMs provided insight into the amount and species of contaminants condensed. Data from the QCMs and other instruments in the contamination experiment (CE) suite played an important role in determining when it was safe to open covers on some of the optical instruments.

Flashlamp measurement of the MSX particulate environment

The Midcourse Space Experiment (MSX) satellite was launched on April 24, 1996. Earlier, descriptions of the Ballistic Missile Defense Organization (BMDO) satellite and some of the early results were presented. This paper provides an update of the data accumulated through the end of the cryo period. The cryo period included the time from launch through the lifetime of the SPIRIT 3 cryogenic telescope. This period covered about 10 months and ended when the dewar containing solid hydrogen warmed up to a temperature above 12 K. The five QCMs onboard the satellite provided data that have been invaluable in characterizing contamination levels around the spacecraft and inside the SPIRIT 3 cryogenic telescope. One of the QCMs, the CQCM, was located internal to the SPIRIT 3 cryogenic telescope and was mounted adjacent to the primary mirror. Real-time monitoring of contaminant mass deposition on the primary mirror was provided by the CQCM, which was cooled to the same temperature as the mirror — -20 K. Thermogravimetric analyses (TGAs) on the CQCM provided insight into the amount and species of contaminants condensed on the SPIRIT 3 primary mirror. The four TQCMs were mounted on the outside of the spacecraft for monitoring contaminant deposition on the external surfaces. The TQCMs operated at -50°C and were positioned strategically to monitor the silicone and organic contaminant flux arriving at specific locations. These TQCMs were located near the UV instruments or positioned to monitor mass coming from specific contaminant sources such as the solar panels. Updated time histories of contaminant thickness deposition for each of the QCMs are presented. Changes in contaminant deposition were seen during the SPIRIT 3 end of cryo warm-up, and implications will be discussed.

Optical measurement of the MSX local H2O density

The xenon flashlamp is one of a suite of instruments that monitor the particulate and gaseous contamination environments of the midcourse space experiment (MSX) spacecraft. The near-field particulate measurement comprises the high intensity xenon flashlamp that illuminates a volume of space in the field of view of the UVISI wide field of view visible imager (UVISI IVW). Radiation scattered by illuminated contaminant particles is imaged by the IVW. The intensity of the radiation is related to a particles size and composition. The particles track yields information about its velocity and trajectory. From ground calibration data we estimate a sensitivity to detect particles smaller than 1 micrometer and to determine cross-field velocities from 1 mm/sec to 50 m/sec. The visible radiation measurement of the particulate environment provided by the xenon flashlamp and UVISI IVW is complemented by multiband IR, UV, and visible measurements by other MSX sensors. The early mission data from this experiment will quantify the relationship between ground contamination control measures, the on-orbit contamination environment, and the performance history of on-orbit sensors.

Long-term observations of the particle environment surrounding the MSX spacecraft

The krypton radiometer (KR) is one of a suite of instruments that monitor the gaseous and particulate contamination environments of the midcourse space experiment (MSX) spacecraft. The krypton radiometer measures the local water density in a volume of space approximately 50 cm from the spacecraft near its +X/+Y/+Z corner. The instrument comprises an array of krypton VUV lines source lamps that dissociate water and a near UV radiometer that detects the chemiluminescence from the OH dissociation products. Ground calibrations indicate that the instrument has sufficient sensitivity to detect water densities as low as 1.5 multiplied by 107 molecules cm-3. Water is the primary outgassing species during the early part of a spaceflight. Water deposition is also a particular concern to cryogenic sensors, such as the spatial infrared imaging telescope III (SPIRIT III) on this spacecraft. As the mission progresses, we will correlate the KR measurements of the water density with measurements by the neutral mass spectrometer, total pressure sensor and cryogenic quartz crystal microbalance. Using the MSX external contamination model we will create a complete description of the MSX water environment including outgassing, return flux and deposition, and effects.

Measurement of long-term outgassing from the materials used on the MSX spacecraft

We present a summary of the particle environment surrounding the Midcourse Space Experiment (MSX) satellite after 32 months on orbit, including two discrete particle releases produced by micrometeoroid or debris impact. We report on the characteristics of that environment, including particle occurrence rates, velocities, size distributions and trends in the environment. To our knowledge, the long term particle contamination observations that we have made on MSX are the first of their kind. The particle occurrence rate decreased steadily during the first year on orbit, but then remained at a constant level after 32 months on orbit. Our estimate of the total number of particles on the spacecraft surfaces at launch. We conclude that environmental effects such as UV, radiation, thermal cycling, and micrometeoroid impacts are a significant and continuing source of particles on orbit.

Optical effects of condensates on cryogenic mirrors for the Midcourse Space Experiment (MSX)

The Midcourse Space Experiment (MSX) spacecraft was specifically designed and processed to minimize contamination. This spacecraft represents a best case scenario of spacecraft induced environment. The contamination instrument suite consisted of 10 sensors for monitoring the gaseous and particulate environment. The Total Pressure Sensor (TPS) has continuously measured the ambient local pressure surrounding MSX since its launch on April 24, 1996. The sensors primary goal was to monitor the early mission (less than one week) ambient pressure surrounding the spacecrafts optical telescopes and to indicate when environmental conditions were acceptable for opening the protective covers. However, the instrument has illustrated that it is quite robust and has successfully measured the long-term decay of the pressure environment. The primary constituent of the atmosphere is water outgassed from the thermal blankets of the spacecraft. The water-induced environment was expected to rapidly decay over the first few months to levels 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 yearly 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. Moreover, the multi-layer construction of the blankets causes them to form a deep reservoir that continues to be a source of water vapor 3+ years into the mission. We will present pressure data from several experiments, each separated by one orbital year, that exhibit these water vapor induced pressure busts. The decay and longevity of these bursts will also be discussed.

QCM flight measurements of contaminant films and their effect on midcourse space experiment (MSX) satellite optics

The effect of condensates on optical surfaces is a continuing concern for space-based optical systems such as the Midcourse Space Experiment. Many such systems contain cryogenic optical surfaces that operate on low temperatures where gases such as nitrogen, oxygen, carbon dioxide, and water will condense. This study presents the effects of these gases on mirror surfaces at temperatures as low as 15 K under high vacuum conditions. The bidirectional reflectance distribution function was determined for these condensates in various film thicknesses up to 8 mm. Optical scatter, thickness, and density measurements were obtained simultaneously with the superpolished quartz crystal microbalance (SPQCM). Correlations between thin film deposition, as determined by the SPQCM, and the expected increase in optical scatter are shown. These correlations are important in determining launch decisions in cases where various degrees of condensation may have occurred on cryogenic optical systems during ground processing.

Measurement of the particulate and water vapor contamination environments of the Midcourse Space Experiment (MSX) spacecraft

The Midcourse Space Experiment (MSX) is a Ballistic Missile Defense Organization (BMDO) demonstration and validation satellite program that has both defense and civilian applications. MSX has UV, visible, and infrared instruments including the SPIRIT 3 cryogenic telescope. It also has several contamination measuring instruments for measuring pressure, gas species, water and particulate concentrations and condensable gas species. A cryogenic quartz crystal microbalance (CQCM) and four temperature controlled microbalances (TQCMs) are part of this suite of contamination measuring instruments. This paper describes some of the flight QCM data obtained and analyzed to date. The CQCM is located internal to the SPIRIT 3 cryogenic telescope and is mounted adjacent to the primary mirror. Real-time monitoring of contaminant mass deposition on the primary mirror is provided by the CQCM which is cooled to the same temperature as the mirror -20 K. The four TQCMs are mounted on the outside of the spacecraft and monitor contaminant deposition on the external surfaces. The TQCMs operate at -50°C and are positioned strategically to monitor the silicone and organic contaminant flux arriving at the UV and visible instruments, or coming from specific contaminant sources such as the solar panels. During the first week of flight operation, all QCMs recorded deposition in the 10-20 ng/cm2-day (1-2 A/day) range. These TQCM deposition rates have continuously decreased, and after 270 days mission elapsed time (MET), the rates have fallen to values between 0 and 0. 15 A/day depending on TQCM location. Thermogravimetric analyses (TGAs) on the CQCM and TQCMs have provided valuable insight into the amount and species of contaminants condensed.

Space environmental and contamination effects on cryogenic and warm optical surfaces: a review

We have designed, fabricated, and tested two flashlamp-based instruments that will characterize the particulate and water vapor contamination environments aboard the Midcourse Space Experiment (MSX) spacecraft: the Xenon Flashlamp and the Krypton Radiometer. These instruments will operate as part of suite of instruments to monitor the MSX contamination environment over its five-year mission. The Xenon Flashlamp illuminates particles in the field of view of the UVISI Wide Field of View Visible Imager, which in turn measures the scattered radiation. The particle measurement can detect particles smaller than 1 micrometers and can measure cross-field particle velocities from 0.5 cm/sec to 50 m/sec. The Krypton Radiometer measures the local water vapor density. VUV radiation from an array of RF-excited krypton lamps photodissociates H2O in the fields of view of a filtered radiometer and one of the UVISI Spectrographic Imagers. The radiometer and the spectrograph simultaneously measure the intensity of the resulting OH chemiluminescence. The H2O density is proportional to that intensity. The spectrograph will provide a positive identification of the radiating species. Instrument descriptions as well as ground test and simulation data are presented.