Secondary electron emission (SEE) plays a key role in spacecraft charging [Garrett, 1981; Frooninckx and Sojka, 1992] . As a result, spacecraft charging codes require knowledge of the SEE characteristics of various materials in order to predict vehicle potentials in various orbital environments [Katz, et. al., 1986]. Because SEE is a surface phenomenon, occurring in the first few atomic layers of a material, the SEE characteristics of a given surface are extremely sensitive to changes in surface condition—e.g., the addition or removal of surface contaminants, or changes in surface morphology. That spacecraft surfaces can and generally do undergo significant evolution during their operational lifetimes is a fact well established by NASA\u27s Long Duration Exposure Facility (LDEF) [Crutcher, et al., 1991a]. Deposition and removal of contaminants can occur as a result of preferential adsorption of gases on cooler surfaces, the collection of ionized gases on negatively charged surfaces, atomic-oxygen-induced oxidation, photodissociation under vacuum uv bombardment, and ion-induced desorption. Since SEE is material-dependent phenomenon, it is reasonable to assume that as a spacecraft\u27s surfaces evolve, so too do it\u27s SEE characteristics. In order to determine whether or not charging models need incorporate the effects of changing surface conditions aboard operating spacecraft, data assessing the impact of these changes on the SEE characteristics of various surfaces are required. Measurements have therefore been made investigating the dynamic evolution of secondary electron (SE) yields resulting from energetic electron bombardment of typical spacecraft materials in a rarefied atmosphere representative of the microenvironment surrounding space vehicles. A detailed report of the experiment and results has been given elsewhere [Davies, 1996; Davies and Dennison, 1997]; what follows here is a brief summary
