design have been established at one one-hundredth of the U.S. occupational guideline. It is anticipated that future maximum permissible U.S. occupational standards will be lower by a factor of 2-10; population standards, if established, may well be lower than the occupational standards. Even more stringent microwave standards could increase land requirements and system cost or alter system design and feasibility. In light of the widespread proliferation of electromagnetic devices and the current controversy surrounding the use of microwave technologies, it is clear that increased understanding of the effects of microwaves on living things is vitally needed even if SPS is never deployed. Exposure of space workers to ionizing radiation is a potentially serious problem for SPS systems which operate in geosynchronous orbit (GEO). Recent estimates indicate that the radiation dose for SPS reference system personnel in GEO would exceed current limits set for astronauts and could result in a measurable increase in cancer incidence. However, there are a large number of uncertainties associated with quantifying the health risks of exposure to ionizing radiation. More research would be required to reduce these uncertainties and to identify and evaluate system designs and shielding techniques that would minimize risks at an acceptable cost. In addition, acceptable SPS radiation limits would have to be determined. If GEO SPS systems are to be considered, an assessment of the health risks associated with space radiation is a top priority. The potential for interference with other users of the electromagnetic spectrum could constitute a severe drawback for the microwave option. Satellite communications and optical and radio astronomy would be seriously affected. The effects on radio and optical astronomy would be the most difficult to ameliorate. The minimum allowable spacing between geosynchronous power satellites and geosynchronous communications satellites is not well known. The optical interference effects of either the mirror or laser transmission options would be of great concern to ground based astronomers. Any of the SPS options would alter the appearance of the nighttime sky. Some may find this aesthetically objectionable. SPACE CONTEXT The hardware, experienced personnel, and industrial infrastructure generated by an SPS project would significantly increase U.S. space capabilities and could, along with other elements of the U.S. civilian space program, lay the groundwork for the industrialization, mining, and perhaps the settlement of space. NASA is likely to play a major role, especially in the initial stages of development. Non-SPS programs could be aided by accelerated development of transportation and other systems; on the other hand, they could be harmed by the diversion of funds and attention to SPS. An SPS research and development program would be in accord with current space policy, which calls for peaceful development of commercial and scientific space capabilities. Given the current absence of long-term programs and goals for the U.S. civilian space program, it is difficult to predict the effects of an SPS project on NASA plans or on private-sector capabilities. These effects will need to be carefully considered. Acknowledgements — The assessment was guided and assisted by an advisory panel composed of 16 distinguished individuals chosen from industry, academia, government, and the public at large. Their aid and sound advice is gratefully acknowledged. In addition, some eighty other individuals made substantive contributions as well. In particular we would like to thank David Claridge, the assessment's first project director, and Richard Rowberg, OTA Energy Program Manager, for their thoughtful guidance.
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