Ambient pressure environment surrounding the MSX spacecraft during the first year on orbit

The Total Pressure Sensor (TPS) on-board the Midcourse Space Experiment (MSX) Spacecraft has continuously measured the ambient local pressure since launch of MSX on April 24, 1996, The primary goals of the sensor are: 1) to monitor the ambient pressure surrounding the spacecraft's optical telescopes and to indicate when environmental conditions are acceptable for opening the protective covers, and 2) to monitor the long-term decay of the species (predominantly water from the thermal blankets) outgassed from 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 approximately one year into the mission, 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 of the blankets. Moreover, the multi-layer construction of the blankets causes them to form a deep reservoir, which continues to be a source of water vapor several tens of months into the mission, Additionally, the TPS has monitored numerous events in which the measured ambient pressure on the optics deck has exceeded 10(-9) Torr. Several of these events did not include solar illumination of the blankets. These events indicate that sources other than the MLI blankets are the cause for certain high-pressure transients, Finally, these events are nor limited to the early mission, outgassing phase of the program. They have been witnessed over a year into the mission. The results documented herein indicate that special consideration must be given in the design of optical sensors to account for long term outgassing of a spacecraft.