The interpretation of space shuttle measurements of ionic species

Measurements of ionic species made in the space shuttle bay area during low‐earth orbit are interpreted within the framework of a kinetic analysis that incorporates shuttle cloud/ambient interactions, flow, and electromagnetic phenomena. This analysis indicates that a pressure of water vapor near the shuttle surface of 10 −6 torr is consistent with [H 2 O + ]/[O + ] and [H 3 O + ]/[H 2 O + ] ratios of 0.1 and 0.2, respectively, and the latter ratio is within the range of measurements. For CO 2 the analysis shows that a [CO 2 + ]/[O + ] ratio of 0.01 is consistent with a CO 2 pressure near the shuttle of 8 × 10 −8 . For these conditions the dominant formation of ions is by charge exchange from ambient O + ions, and the dominant loss of ions is by parallel flow along the magnetic field lines. In addition, Monte Carlo calculations of the slowing of the incident O + ions indicate that charge exchange with H 2 molecules occurs before the ions have lost a significant amount of energy when the H 2 O pressure near the shuttle surface is less than 10 −6 to 10 −5 torr.