David F. Hall

CONTAMINATION EXPERIMENTS IN THE MIDCOURSE SPACE EXPERIMENT

The midcourse space experiment satellite is a space-based sensor platform designed to collect earth and atmospheric remote sensing and astronomy data in support of ballistic missile defense and civilian science objectives. Because of the potential adverse effects of contamination on the main optical sensors, the satellite contains a suite of contamination-monitoring instruments designed to completely characterize the source, generation, and mechanisms of contamination of space opticsand to validatepree ight contamination models. Descriptions of these contamination instruments, their calibration and testing, and the ground test data collected are presented. The sensitivity to contamination of the main optical sensors, which are imagers and interferometers in the infrared, visible, and ultraviolet spectral regions, are also discussed.

Midcourse Space Experiment Contamination Measurement During Cryogen Phase

In-orbit measurements with contamination-monitoring instruments were used to validate the Midcourse Space Experimentcontaminationmodelandtoinvestigatethephenomenonofmolecularand particlegenerationinspace. Measurements from the e rst orbit contact through the e rst 10 months showed water vapor as the largest gaseous species, with argon gas from a venting source important only during the e rst week in orbit. Simple reporting tools were used for rapid assessments of the spacecraft environment during early operations. The contamination levels and the decay rate of water vapor around the spacecraft were found to be in excellent agreement with prelaunch predictions. Future measurements include validation of the model of the aging spacecraft and investigation of the degradation of thermal radiators.

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

Flight Experiment To Measure Contamination Enhancement By Spacecraft Charging

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.

Local Environment Surrounding the Midcourse Space Experiment Satellite During Its First Week

The ML12 experiment was launched on January 30, 1979, on the United States Air Force (USAF) Space Test Program P78-2 spacecraft, which is sometimes called SCATHA. It was designed to determine if spacecraft charging contributes significantly to the rate that contaminants arrive at exterior spacecraft surfaces, and to establish some of the characteristics and effects of these contaminants. Two sensor types are used in the experiment. One type is a combination retarding potential analyzer (RPA) and temperature controlled quartz crystal microbalance (TQCM). With it, distinction can be made between charged and uncharged arriving molecules, and information can be obtained concerning the temperature dependence of contaminant adsorption and desorption rates. The other sensor type is a tray of calorimetrically mounted thermal control coating (TCC) samples. Samples of different spacecraft surface materials are exposed to arriving contaminants, and the solar absorptances (as) of these materials are continuously measured. The two RPA/TQCMs are both accumulating mass, but the accumulation rates and characteristics of the mass differ, probably because of the locations of the RPA/TQCMs on the spacecraft. Of the 16 TCC samples, two quartz fabric samples showed .01 to .05 increases in as during the first 50 days on orbit.

Flight measurement of molecular contaminant deposition

The environment measured surrounding the complex Midcourse Space Experiment spacecraft during its e rst weekonorbitisreported.Asuiteofinstrumentsincludingapressuresensor,aneutralandanionmassspectrometer, quartz crystal microbalances, and e ashlamp-based water and particle detectors were activated within hours after launch. These instruments measured the gaseous composition, particulate, and e lm accretion temporal histories. Spacecraft environment cleanliness and response to operational activities were used to guide decisions about sensor operation. As a result of careful material selection and ground preparation procedures, the measured levels of condensible species were sufe ciently low to permit safe sensor operation after only a few days in orbit.

Optical effects of photochemically deposited contaminant films

A spacecraft was instrumented with four temperature controlled quartz crystal microbalance (TQCM) contamination detectors. One TQCM, located inside the vehicle, recorded contaminant deposition that was orders of magnitude higher than did the three TQCMs located in various positions outside the vehicle. The deposition rate on the interior TQCM varied with the temperatures of interior spacecraft cavity surfaces. In particular, there is clear evidence of condensation on these surfaces and re-evaporation from these surfaces by previously outgassed contaminant molecules. The e-folding time constants of the deposition on two of the exterior TQCMs held at -50 degree(s)C are approximately 1.4 years, with extrapolated final equivalent thickness of the deposition in the 20 - 25 nm (200 - 250 angstroms) range. The third exterior TQCM, which has a significant field of view of a segmented thermal blanket, collected contamination at a greater rate. The data enable the ranking of the several contamination transport mechanisms at work and the drawing of general recommendations for spacecraft design.

Effects of molecular contamination on triple junction solar cells

Two aromatic hydrocarbons bibenzyl (BB) and dodecahydrotriphenylene (DTP) have been studied in an ongoing contaminant effects measurement program. Photochemical deposition of these molecules did not proceed quickly under conditions which result in the deposition of dark tenacious films from phthalate siloxane and alkene precursor molecules. Additional measurements show that DTP probably does not deposit photochemically at a substantial rate because the quantum yield for photodeposition is small not that a DTP molecule does not reside on the surface long enough to absorb light. The initial sticking coefficient of DTP appears to depend on surface temperature. Films of DTP scatter visible and near-ultraviolet light very efficiently which is consistent with the observed heats of vaporization and desorption for this molecule. 1.

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

Space borne high efficiency triple junction solar cells are calculated to be about twice as sensitive to degradation from deposition of films of molecular species (similar to those typically outgassed by spacecraft materials) than are lower efficiency silicon cells. This is a consequence of the facts that 1) sub-cells of multijunction cells are connected in series, so that one of them limits the current through the stack, 2) the current in each sub-cell is lower in multijunction cells than in single junction cells, and 3) the absorptance of the molecular films increases rapidly as wavelength is decreased, effectively concentrating its effect in one sub-cell.

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.