THRUSTER FIRING EFFECTS IN THE SHUTTLE ENVIRONMENT .2. POSITIVE-ION COMPOSITION

Measurements of positive ion composition in the payload bay of the Spa ce Shuttle confirm that the vernier reaction control system rocket eng ines cause major changes in the shuttle plasma environment. Previously reported measurements of increases and decreases in ambient and conta minant ion fluxes are corroborated and are shown to depend on the atti tude of the shuttle, the direction of exhaust plume flow, and the poin ting direction of the mass spectrometer sampling orifice. The column d ensity of exhaust neutrals and the length of the effective column are both roughly estimated from the magnitude and time response of the red uction in ambient O+ flux. The measurements are consistent with contro l of the contaminant ion motions by the Earth's magnetic field. Contam inant ions are detected only if the mass spectrometer is approximately aligned with the plane of ion motion perpendicular to the magnetic fi eld vector, and only if the plume gases expand into a region of space from which the pickup trajectories are directed back into the instrume nt sampling orifice. The degree to which contaminant ions are picked u p by the magnetic field depends on the chemical dynamics of the ion-mo lecule reaction that formed them. The appearance of large fluxes of io ns at mass 28 cannot be explained by the chemistry of O+ reacting with the principal exhaust neutral species. Electron impact ionization of neutral N2 to give N2+ is proposed as a possible source for this mass 28 ion.