Approximate expressions for the electron current collected by a planar retarding poential analyzer (RPA) mounted on a moving, conducting, charged spacecraft are derived. They are utilized for the analysis of electron current data obtained by the RPAs on the Viking spacecraft in the ionosphere of Mars and in the disturbed and undisturbed solar wind near this planet. It is shown that contamination currents by photoelectrons emitted from the spacecraft can be distinguished and removed from the signal. Parameters deduced from the analysis of KPA electron sampling data are the multicomponent electron temperatures, the number densities, and the spacecraft potential. This paper seeks only to establish the usefulness of the technique by application to a representative data set. ¿ American Geophysical Union 1987