Large structures in low earth orbit will release neutral water or ions through outgassing, water dumps or thruster firings and thus perturb the ambient ionosphere. Neutrals and ions within the perturbed environment will undergo chemical reactions to form a contaminant cloud. It is assumed that the contaminant cloud consists of ions such as O+, H2O+, H3O+ and neutrals such as O, H, OH and H2O. A two-dimensional model for the motion of the contaminant cloud perpendicular to the magnetic field lines is developed. Numerical solution of the derived model equations examines the effects of Alfv¿n wave coupling, neutral water density, ion temperature and initial conditions on the cloud motion. In low density neutral water clouds (≤109 cm-3) the shielding of the electric field is small. In neutral water clouds with densities of interest for shuttle conditions (~1010 cm-3) the shielding is predicted to be of the order of the motional electric field and the ratios of the line averaged densities of the ions is consistent with experimental data. The effect of ion temperature is negligible for the range of densities and temperatures considered. It is predicted that for symmetric initial conditions, the drifting clouds will develop fingerlike instabilities. Plasma depletions are predicted in the wake of the moving structure with enhancements in the ram direction. Asymmetric initial conditions, such that could result from a thruster firing, rotate the plasma cloud which undergoes a highly asymmetric distortion. ¿ American Geophysical Union 1989