Mark T. Boies

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.

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

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.

MSX Satellite: Flight Measurements of Contaminant Films*

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.

Laser radar instrument for the Near-Earth Asteroid Rendezvous (NEAR) mission

In 1999 after a 3-year transit, the Near-Earth Asteroid Rendezvous (NEAR) spacecraft will enter a low-altitude (approximately 50 km) orbit about the asteroid, 433 Eros. Five instruments, including a laser radar, will operate continuously during the one-year orbit at Eros. The NEAR laser rangefinder (NLR), developed at the Applied Physics Laboratory (APL), is a robust rangefinder and the first spaceborne altimeter to have continuous inflight calibration capability. A bistatic configuration, the NLR uses a diode- pumped Cr:Nd:YAG transmitter and a leading-edge receiver with a 3.5-inch aperture Dall-Kirkham telescope. Detection is accomplished using an enhanced-silicon avalanche photodiode. From system tests, the NLR is capable of ranging in excess of 100 km to the asteroids surface. Measurements of the time-of-flight between laser pulse firings and detection of surface backscatter are made using an APL- developed receiver having range resolution of 31.48 cm and accuracy of 2 m. Total mass of the NLR is 4.9 kg and its average power consumption is <EQ 15.1 W. This paper reviews specifications for the NLR instrument, provides overall design details, and presents system performance using prelaunch test results.

Laser rangefinder for the near-earth asteroid rendezvous (NEAR) mission

The near-earth asteroid rendezvous (NEAR) mission is the first of the NASA discovery programs. Discovery-class programs emphasize small, low-cost, quick turnaround space missions that provide significant science returns. The NEAR spacecraft and ground control system are currently being developed and tested at the Applied Physics Laboratory (APL). The NEAR spacecraft will orbit, 433 Eros, possibly the most studied of the near-Earth asteroids. Subsequent to a 3-year cruise, the NEAR spacecraft is inserted into a 50-km-altitude orbit about Eros for 1 year to permit data collection in the infrared, visible, x-ray and gamma-ray regions. One instrument, the NEAR laser rangefinder (NLR), will provide altimetry data useful in characterizing the geophysical nature of Eros. In addition, ranging data from the NLR will support navigation functions associated with spacecraft station-keeping and orbit maintenance. The NLR instrument uniquely applies several technologies for use in space. Our configuration uses a direct-detection, bistatic design employing a gallium arsenide (GaAs) diode-pumped Cr:Nd:YAG laser for the 1.064-micrometer transmitter and an enhanced-silicon avalanche-photodiode (APD) detector for the receiver. Transmitter pulse energy provides the required signal-to-noise power ratio, SNRp, for reliable operation at 50 km. The selected APD exhibited low noise, setting the level achievable for noise equivalent power, NEP, by the receiver. The lithium-niobate (LiNbO3) Q-switched transmitter emits 12-ns pulses at 15.3 mJ/pulse, permitting reliable NLR operation beyond the required 50-km altitude. Cavity aperturing and a 9.3X Galilean telescope reduce beam divergence for high spatial sampling of Eross surface. Our receiver design is an f/3.4 Dall-Kirkham Cassegrain with a 7.62-cm clear aperture -- we emphasized receiver aperture area, Arx, over transmitter power, Pt, in our design based on the range advantage attainable according to the simplified range equation, Rmax equals [(Pt(rho) BArx)/(SNRp NEP)]1/2. Asteroid reflectivity, (rho) B, is estimated to be 0.05 at our wavelength. A reasonable power signal- to-noise ratio for reliable operation, SNRp, was assumed. To minimize our noise equivalent power, NEP, we carefully designed and selected the receiver components. The receiver circuit uses leading-edge detection of the laser backscatter. Our detector circuit is an enhanced-silicon APD hybrid using a video amplifier, an integrating Bessel filter, and a high- speed programmable threshold comparator. We accomplish time-of-flight (TOF) measurements digitally with an APL-designed GaAs application-specific integrated circuit. A radiation-hardened FORTH microprocessor controls range gating, data collection and formatting, and operational modes. Implementation of control and data communications between the spacecraft and rangefinder uses the MIL-STD 1553-bus architecture. Functional testing and calibration indicate exceptional performance; return power levels were reliably detected over several thresholds with 71-dB attenuation, while observed range jitter was equivalent to the resolution determined by the TOF GaAs chip (31.5 cm). This paper discusses NLR performance requirements, design implementation, and qualification testing. It also provides preliminary results from calibration and performance testing.

Total pressure sensor results from the early operations phase of the MSX mission

The total pressure sensor (TPS) is one of ten contamination sensors aboard the midcourse space experiment (MSX) satellite. The TPS measures both the natural and spacecraft induced pressure environments. This paper presents a first look at the TPS data from the early operations phase of the MSX mission. Flight data are show to be in good agreement with the external contamination model predictions for MSX. TPS fluctuations are shown to be consistent with the venting characteristics of the Spirit III cryogenic cover. Data are presented which characterize and confirm the tumbling nature of the receding Spirit III cover upon its release. Finally, flight data over an orbital period are shown to conform to a bimodal pressure profile.

Long-term observations of the particle environment surrounding 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.

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

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.

NEAR laser rangefinder light-weight packaging design

The NEAR laser range finder (NLR) design is a compact, light weight design with a high power laser transmitter and a high performance mirror receiver system. One of the main objectives of the NLR is to provide the in-situ distance measurement from the spacecraft to a near earth asteroid. An on board computer will compile this information to provide necessary navigation requirements for the NEAR satellite. Due to the weight budget constraint, the maximum weight limitation of the NLR has been a critical issue from the beginning of the program. To achieve this goal and meet the system design objectives, innovative designs have been implemented in the development of light weight optical, mechanism, and electronic packaging hardware. This paper provides details of the NLR electronic packaging design, thermal and structural designs.