decentralized, and low technology. Hence they regarded SPS as especially venal. Terrestrial solar technologies are, of course, well-adapted to decentralized use, but they are not “low” technology nor are they cheap, as often advertized. The opposition of this movement to SPS makes SPS neither right nor wrong. But the ideological background behind much of the vocal opposition to SPS is important. Given this understanding, we can address the environmental uncertainties of SPS with less ideological baggage. Microwave Power Microwaves are a form of radiation, a part of the electromagnetic spectrum that spans a range of wavelengths measured in miles, for VLF radio, to fermis, for gamma rays. Microwaves are so called because in the early days of radio, when first investigated, they were much shorter in wavelength than “shortwave,” the latter designating wavelengths of tens of meters. The wavelength for the proposed SPS power transmission system is 12.24 cm, about five inches. The risks of exposure to electromagnetic radiation differ with wavelength and corresponding energy per photon. If the wavelength is somewhat less than one micron, a single photon has enough energy to make or break a chemical bond or to ionize an atom or molecule. Such radiation has more dangerous bioeffects than longer- wavelength radiation. In this sense, sunlight is more dangerous than microwaves; ask anyone with a severe sunburn about the danger of sunlight. (Of course, if sunlight were not photochemically active, there would be no photosynthesis and no life on Earth.) The hazards of ionizing radiation can be compared to those of nonionizing radiation by noting that current U.S. industrial standards for allowable energy incident on humans for the former are roughly a billion times less than for the latter. This comparison is very rough because of differences in the measurement of allowable exposure. The risks of exposure to radiation also depend on penetrating power. Sunlight is ordinarily not dangerous because clothing affords adequate protection, but microwaves readily penetrate clothing and penetrate flesh to a distance on the order of two wavelengths. The public health risks of the SPS microwave power transmission concept have been highly publicized. They were thoroughly researched by the DOE-NASA studies of the concept. This research found no evidence that the anticipated exposure levels would represent a risk to the public or to the environment, but the results were incomplete inasmuch as there are large gaps in our knowledge of the effects (if any) of long-term, low-level exposure to microwave energy. The SPS microwave issue highlights a controversy over microwave biological effects that has raged for years. This is the question of nonthermal effects. The conventional view of microwave effects is that the electromagnetic fields associated with microwaves agitate molecules, heating them, and causing thermal effects. This is what happens in a microwave oven. Microwaves at high intensities induce sufficient heating to cause cooking. The SPS power beams would not have high enough intensities to be dangerous from the thermal standpoint, even if one were exposed to the maximum beam intensity. The exposure levels expected outside the receiver antenna area (from which people would be excluded) are completely insignificant from the viewpoint of thermal effects. For years, microwave alarmists have argued that there are nonthermal effects, effects that are biologically significant at levels that would be of no concern whatso-
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