The scope and size of the environmental implications assessment of the SPS concept were unprecedented in the history of energy-option evaluations. It also created a precedent for future assessments of major space initiatives. Although different space technologies may be required for other space missions with varying operational requirements and criteria, the SPS concept assessment can serve as a model for evaluations of other large-scale space projects. ASSESSMENT OF SPS ENVIRONMENTAL IMPLICATIONS The objectives of the environmental implications assessment of the SPS concept were: o To identify environmental issues associated with the SPS reference system; o To prepare an assessment based on existing data; 0 To suggest mitigating strategies for guiding the design of the SPS and for p lanning long-range research to reduce the uncertainties identified during the assessment. Although the potential consequences of SPS technology were assessed, they could not all be quantified because of the lack of data on environmental effects or uncertainties associated with the performance of space technologies developed only to a conceptual stage. The key environmental issues that were the subject of the CDEP assessment included: o Microwave exposure effects on health and ecosystems; o Nonmicrowave effects on health and ecosystems; o Effects on the atmosphere; o Atmospheric heating effects; and, o Electromagnetic compatibility. The results obtained from the SPS assessment are summarized in the following sections. MICROWAVE EXPOSURE EFFECTS Microwave radiation falls in the band of frequencies from 30 MHz to 300 GHz. This radiation does not have sufficient energy to ionize biological molecules, but instead agitates them. When microwave radiation impinges on skin tissue, it may be reflected or absorbed or may pass completely through it. The specific effect will depend on the frequency of the radiation and on the orientation, composition, and thickness of the tissue. At the frequency of 2.54 GHz selected for the SPS reference system, absorption will take place primarily in skin tissues. 2 Microwave power densities at a receiving antenna site range from 23 mW/cm at the antenna center, to 1 mW/cmz at the antenna edge, to 0.1 mW/cnr at the antenna site exclusion boundary. If 60 receiving antennas in the continental United States were spaced^an average of 300 km apart, the minimum microwave power density at any point would be about 10”H mW/cm . At present, 1% of the U.S. population is potentially exposed to microwave power densities of 10" mW/cirr as a result of radar, television ^ignals^ and appliances. The U.S. population is experiencing a median exposure value of about 10” mW/cin for a time-averaged microwave power density. The immaturity of the theoretical understanding of microwave effects and the complexity of experimental conditions and biological systems contribute greatly to the difficulty in quantifying microwave biological risks. Exposure conditions will depend on operating frequency, polarization of the microwaves, and laboratory chamber characteristics. Any object introduced into the microwave radiation field tends to distort the field pattern in an unpredictable manner. The field strength measured by a probe at a particular location will change when an experimental subject replaces the probe, making environmental monitoring, personal dosimetry, and depth-dosimetry di fficult. International standards on safe exposure for microwave radiation levels have not yet been developed. World opinion on the choice of levels ranges over four orders of magnitude, and dffferent countries are applying different criteria. The United States and Western Europe have adopted 10 mW/cm^ ^s a guide for both public and occupational continuous exposures. Canada adopted a limit of 1 mW/cm for exposure-of the public, while the Soviet Union and Eastern European countries allow only 0.001 mW/cnr for long-term public exposure. Occupational exposure standards are about an order of magnitude higher than those for the public. Also, the United States and Western European exposure standards are primarily guided by risk avoidance of thermal biological effects, while the Eastern European countries have considered nonthermal effects of microwave radiation. There have been considerable controversy and skepticism about the health hazards of prolonged low-intensity microwave exposures and the validity of the observations. A theoretical understanding of possible nonthermal mechanisms and a quantitative assessment of scientific evidence are essential before conclusions can be arrived at regarding the potential biological implications of the SPS microwave power transmission system and the effects of microwave radiation from an increasing number of industrial, scientific, medical, and domestic microwave radiation sources. Occupational exposure to microwave radiation would occur at the receiving antenna on Earth (up to 23 mW/cmz) and at the transmitting antenna in GEO (about 2200 mW/crrT). Thus, occupational exposure to the SPS microwave power densities must be controlled both in space and on Earth. Protection
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