  This paper presents an overview of the roles played by incoming and outgoing
electrons in spacecraft surface and stresses the importance of surface
conditions for spacecraft charging. The balance between the incoming electron
current from the ambient plasma and the outgoing currents of secondary
electrons, backscattered electrons, and photoelectrons from the surfaces
determines the surface potential. Since surface conditions significantly affect
the outgoing currents, the critical temperature and the surface potential are
also significantly affected. As a corollary, high level differential charging
of adjacent surfaces with very different surface conditions is a space hazard.
