The' Satellite power System (SPS) is an emerging concept for capturing solar energy in space for use in producing electrical energy on earth. To develop an understanding of the technical and economic feasibility and of the environmental and societal acceptability of the SPS is an enormous challenge. The Department of Energy and the National Aeronautics and Space Administration are engaged in a three-year assessment of the SPS that began in the fall of 1977 and will be completed in the summer of 1980. The DOEINASA assessment is engaging the efforts of many organizations i n the United States and is developing a large body of information. A t approximately the mid-point of the assessment, this preliminary project assessment report describes what has been done and what has been learned with an emphasis on. the overriding issues. The document is a preliminary report.