Gaseous environment of the Shuttle early in the Spacelab 2 mission

A cold-cathode ionization gage was flown on Space Shuttle flight STS-5IF as part of the Spacelab 2 payload. Neutral pressure data that were taken in the payload bay during the first few hours on orbit are presented. These data show that when the payload bay is oriented such that the atmospheric gases are ramming into it, the pressure rises to a peak of 4 x 10 to the -6th Torr. Pressure is also slightly higher during the sunlit portion of each orbit. Outgassing of the payload bay causes the pressure to be elevated to a few times 10 to the -6th Torr early in the mission. In addition, several effects on pressure have been identified that are due to chemical releases. Substantial increases (50-150 percent) are seen during another experiment's gas purge. Orbiter chemical-release effects include: pressure increases of 200 percent up to 7 x 10 to the -6th Torr due to Orbital Maneuvering System burns, minor perturbations in pressure due to vernier thruster firings and little or no increase in pressure due to water dumps. In the case of vernier thruster firings, effects are seen only from down-firing thrusters in the back of the Orbiter, which are probably due to reflection of thruster gases off Orbiter surfaces.