A series of scientific spacecraft either in orbit or to be launched in the very near future carries ion emitters for active control of the electrostatic surface potential. The ion beam emission changes, via the associated reduction of the positive surface potential, the photoelectron sheath around the spacecraft which may result in the formation of a potential barrier. In this study, a spherically symmetric model of the photoelectron sheath around a spacecraft (with conductive surface) is analyzed. An algebraic equation is set up to describe the current equilibrium between the escaping photoelectrons, incoming ambient plasma electrons, and the artificially emitted charged particles at the surface of the spacecraft. The height of the potential barrier can be given by solving the algebraic equation, if the electrostatic surface potential of the spacecraft, the density and temperature of the ambient plasma electrons are known. The position of the potential barrier is estimated with the assumption that the electrostatic potential distribution is of a Debye form. The method has been applied to the data gained from the Geotail spacecraft at times when the ion emitter was operational. From the calculation, it is shown that the height of the potential barrier grows from 0.27(Tph/e) to 0.91(Tph/e), as the ion emitter current increases from 9.2(&mgr;A) to 38.3(&mgr;A). The maximum barrier height appears to be about 2 V, when the ion emitter operates at its maximum current about 50 &mgr;A. ¿ American Geophysical Union 1996