ever thermally. Until recently, this has been an emotional argument, with little substantiation. Reports of nonthermal effects have been anecdotal, with about as much scientific substance as flying saucer sightings. Attempts to reproduce them in the laboratory have not been successful. The problems in assessing microwave effects are many, including considerable difficulty in dosimetry. Perhaps more significant is the lack of adequate models of their effects on living tissue. The thermal model is well-understood. The main problem in investigating nonthermal effects is that there has been no model. Recently, models have been postulated for nonthermal effects (8,9). This provides a foothold for scientific investigation, i.e., the models make specific predictions of effects that can be measured. No such models found their way into the SPS research, probably because they were too new. Those with which 1 am familiar predicts that nonthermal effects will be found with high-level or pulsed sources. Further exploration of the predicted effects would have great significance to SPS. Most of the anecdotal reports of nonthermal effects relate to exposure to radar, a pulsed source. If it can be confirmed that nonthermal effects (if they exist) are associated with pulsed rather than continuous sources, concerns about SPS microwave power would be greatly alleviated. Land Use The SPS reference system involved use of large contiguous receiving sites, elliptical in shape, and roughly 100 square kilometers in area. Whereas the total land use per unit of delivered energy is comparable to energy sources in common use today, the large size of the contiguous land area was seen as a serious problem, especially in Europe. Hypothetical siting studies found adequate parcels in the U.S., but such large parcels may not be available in other industrialized states. The tailoring of the power beam in the reference system was relatively unsophisticated. While the peak beam intensity was 230 watts per square meter, the average was barely one-fifth of this. A primary goal of further SPS research should be to improve this situation. Peak levels of 300 to 350 watts may be feasible. The 230-watt limit was based on ionosphere heating theory now outdated, and experimental evidence indicates somewhat higher levels could be permitted. Sophisticated beam shaping should considerably improve the average-to-peak ratio. We should aim for a land use of at least three times less area per unit power received than the reference system. Space Transportation Effects In my view, the effects of space transportation operations must be regarded as a more serious environmental issue than biological effects of the microwave power beams. In the beginning, space transportation effects were not regarded as a problem at all. The rockets would use clean-burning methane and hydrogen, and their consumption of fuels would be miniscule compared to automotive and airline uses. Only late in the SPS study program was it recognized as important that a large part of the rockets' effluent would be discharged into the upper atmosphere. In the mesosphere and ionosphere, water and hydrogen (the predominant effluents) can be pollutants. There is little water at high altitudes, because of the “cold trap" at about 80 km. Water up there might form noctilucent clouds; it might alter the Earth’s albedo enough to cause climate alterations. Water or hydrogen in the ionosphere will
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