tivity of SPS unit costs to assumed lifetime should be studied. Further, uncertainty of the SPS energy ratio estimates make comparison with other technologies unclear as to the advantages of SPS: the question of energy ratio is clearly an area for important further analysis. Turning to the environmental effects of SPS, the protection of the large population of space workers from ionizing radiation is a matter of quite a different degree from what we have been able to accept with the astronauts. There is a much greater worker population and the occupational exposure limits are much lower than they are for the astronauts. As for the microwave health issue, one can easily think of an ideal resolution of that: first, one needs clear cut research results on the nature and the severity of the microwave long-term health effects. Those populations which are at risk could be identified and a quantitative assessment of what risks would be incurred at various exposure levels could be made; a quantitative assessment of the impact on SPS design and operation of various control measures could be made and then full information should be provided to the public so they can understand this problem. Clear cut research results and quantitative risk assessment for low level long-term exposures are not likely to occur in the next five years simply because it takes that long or longer to derive those results. The SPS program is going to have to operate in an atmosphere of great uncertainty in this area. Allocation of spectrum and orbit will be severely competitive; this will have to be recognized early and planned. Communication's use of the orbit and the frequency is increasing and in 30 years there will be a lot of vested interests to consider. Turning to the social areas, one can see several motives other than economic favoring an SPS system. There surely will accrue an advantage to society from a renewable energy source with promise of long-term stability, quite apart from simple economic benefit. It is also possible that SPS may be more benign in an environmental sense than other large scale energy generating technologies. An SPS system suitable redesigned on an international scale could offer benefits there as well as domestic benefits. An important question that the reference system does not address is how much more can the SPS be expanded; is the limit of its supply 300 gigawatts or can we go to 600 or 3,000 gigawatts? That's rather important for the international benefit question. The degree of international cooperation and participation will depend on further political exploration of this question. We think the next step of those associated with SPS is to give guidance to the government on planning and on its next investment in R&D. Therefore, it is clear that the SPS has to be assessed in comparison with other electrical energy technologies; but which ones? Clearly those that are competing for public R&D funds. In the period under consideration for SPS it is viewed not as a substitute for conventional energy sources, but as a supplement to coal and fission, at least early on. So comparison with coal does not advise you whether you should stop coal and build SPS, but it will provide an important benchmark that ought to be measured. The merits of SPS also need to be considered relative to new technologies that might supplement our conventional technologies and these have been mentioned before; they are ground-based photovoltaics with storage capacity, liquid metal fast breeder, nuclear fusion. These technologies are presently competing for federal R&D funds.
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