Large manned spacecraft release and carry up huge amounts of water in their orbital motion. Some of the neutral water molecules undergo charge exchange with the ambient ionospheric plasma and become what are called pickup ions at low-Earth orbit (LEO) altitudes. In this paper the velocity distribution of the pickup ions is studied on the basis of the analysis of the trajectories of the pickup ions, the influence of thermal movement, and the influence of collisions between the pickup ions and other particles. The integral models are developed to calculate the velocity distribution of the pickup ions in the frame moving with the orbiter. The comparison between the simulation result and actual measurement data from the spherical retarding potential analyzer (SRPA) on mission Tethered Satellite System-1 Reflight (TSS-1R) and earlier missions on which the SRPA was flown shows that the models obtained are correct for the special environment of a large space platform at LEO altitudes and are useful in analyzing the data of the ion probes mounted on similar spacecraft. With the models obtained the influence of some factors on the speed distribution of the ions collected by the payload SRPA of a large spacecraft orbiter is analyzed. ¿ 2001 American Geophysical Union