prohibition of “large space engineering projects” was included in his space policy. This prohibition does not appear in the recent Reagan policy, but neither does endorsement of any new space programs. In 1977, responsibility for SPS was assigned to the Department of Energy. DOE and NASA carried out a program of concept definition and evaluation. This program included extensive paper studies and limited experimentation in a few critical areas. A unique feature of the program was emphasis on environmental, social, and economic issues. All aspects of the concept and its implications were scrutinized to seek out valid reasons not to proceed. This would preclude initiation of a costly project that might later turn out to have unacceptable consequences. The DOE-NASA studies found no technical, environmental, social, or economic reasons to discontinue study and research on SPS (4). The final study reports made no recommendations, despite intent to do so in the program plan. Independent reviews were conducted by the congressional Office of Technology Assessment (OTA) (5) and by the National Academy of Sciences (6). OTA studies are not intended to make program recommendations. The policy options they presented included a high option of technology development, a median option of selected research, and a low option of no further work. The NAS report found no insurmountable barriers, but recommended no further work. There is, at present, no SPS-dedicated research being funded by the U.S. government. The remainder of this paper synopsizes current knowledge of the SPS concept and reviews new circumstances not considered in the evaluations of 1980 and 1981. THE TECHNICAL CHALLENGES A discussion of the technical challenges of SPS must begin with its awesome scale. The reference SPSs of the DOE-NASA studies were structures the size of Manhattan Island, as massive as a nuclear aircraft carrier, and were to generate thousands of megawatts of electricity, compared to about twenty kilowatts for the largest space arrays thus far constructed. Space transportation operations to support their construction were to he the equivalent of about ten shuttle flights per day, and the space operations were to occupy hundreds of space workers in low Earth and geosynchronous orbits (7). This issue of scale has been the source of a great deal of incredulity. The scale is about two orders of magnitude greater than the space operations now contemplated for a mature shuttle system. With the presumed advances in space transportation technology, however, the annual space operations cost projected by the studies is about half that presently experienced by the U.S. commerical airlines. Hence the issue is not whether industry in general can deal with such scales, but only whether space industry can. There are several candidates for the greatest technical challenge of SPS, including [1| assembly of large space structures, [2] operation of the high power, high voltage electrical power system, [3] design and operation of the high-precision microwave power transmission system, [4] flight control of the satellites, (5] operation of the ground receiver stations, and |6| development of space transportation systems that can operate at the requisite low cost. Assembly of the space structures does not seem to be much of a problem, based on the concepts developed by the DOE-NASA studies. These concepts were undoubtedly underautomated and in some ways cumbersome, compared to what the
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