real future will bring. But they could have worked. Criticisms of these concepts produced by the NAS review, having to do with underdamped motion of space cranes, are equally valid for proving that tall buildings cannot be constructed by the methods used today. The high voltage solar arrays of the reference systems undoubtedly would have suffered immediate breakdown of the coverslips due to plasma and charge collection on their surfaces. Potential workarounds include conductive surfaces to dissipate the plasma charge and use of low-voltage arrays with a.c. power transmission on the satellite. Our practical knowledge of high power, high voltage space systems is rudimentary. This technology gets my vote as one of the most challenging. My second vote goes to microwave power transmission and control. The reference systems were very sophisticated compared to anything that has flown in space, but not sophisticated enough. Power transmission poses many unique challenges that will not be explored by data transmission technology. Our studies of the flight control challenge indicated that today's state of the art in algorithms and sensors is adequate. There are a number of challenges in designing machinery, but if these are met, flying the SPS does not appear to be a major challenge. Recommendations presented below for increased sophistication of the spaceborne part of the power transmission link will considerably simplify the ground station problem. I do not see the development of low-cost space transportation as a serious challenge. It will evolve naturally over the next two decades whether or not anything is done about SPS. The shuttle has demonstrated the critical technologies. Commercial demands will drive them to maturity. The electric-propelled orbit transfer systems included in the reference systems were technically dubious — some might even say flaky. But new concepts of space transportation described below obviate the need for electric orbit transfer. The new concepts represent a natural evolutionary path that will be far advanced by the time SPSs need the capability. ENVIRONMENTAL ISSUES The SPS studies identified several environmental issues. Most emotional of these was the use of microwaves for power transmission. Fueled by scathing indictments like Brodeur’s The Zapping of America, microwave power transmission was branded by the opponents of SPS as an evil the equivalent of nuclear war. Never mind that there was no concrete evidence of harmful effects from microwave power, except at power levels like those used in ovens. The SPS power beam was described in the “anti” literature as a “death ray.” Many of the pro-SPS advocates were, of course, dealing from a position of selfinterest. SPS opponents often claimed to speak for the “public,” which they did not. It is important to understand the origin of this opposition. Its power center was the counterculture movement that gained much notoriety in the latter 1960s. Their prophet was E. F. Schumacher, who wrote Small Is Beautiful, a tract on counterculture economics. Their high priest is Amory Lovins of “soft technology” notoriety. Their philosophy opposed any energy technology that was centralized or sophisticated. Its spokesmen were most vocal in opposing SPS because it was not only a high-technology, centralized, large-scale concept, but also because it was solar. The counterculture had adopted solar energy technology as their rallying cry, as “soft,”
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