which determines the abundance of chemical species involved in the ozone equilibrium. Because vertical mixing in the stratosphere is very slow (about 2 years at 20 km and 4 to 20 years at 50 km), and declines with increasing altitude, gases injected into the stratosphere will accumulate even at a low annual rate of injection and could yield a large equilibrium value at very high altitudes. (The region from 50 to 100 km contains only 0.1% of the total mass of the atmosphere.) Although the chemistry of water vapor in the upper stratosphere has been studied, there is uncertainty regarding the possible consequences of incremental additions of water vapor. Water vapor is photodissociated to form radicals and molecules which will react with ozone and molecular and atomic oxygen. Furthermore, changes in the water vapor content could influence the natural flux of NOX to the level of the ozone layer. Consequently, the effects of the space transportation system on water vapor injection, particularly in the upper stratosphere, require further investigation. Studies of the effects on the Earth's upper atmosphere and ionosphere of rocket exhaust products from the launch of a heavy-lift launch vehicle (HLLV) have indicated that long-lasting ionospheric plasma depletion could occur. But this may be modified by choosing a second-stage flight trajectory that reduces the amount of propellant injected in the upper ionosphere (33). The HLLV will produce atomic hydrogen in massive quantities and contrails 10 to 20 km wide and extending several hundred kilometers downrange persisting for several hours in the mesosphere at an altitude of about 85 km. The addition of atomic hydrogen to the upper atmosphere, at about 300 km altitude, would double the concentration after six HLLV launches and may have a possible impact on global climate. The ionospheric plasma at an altitude of 200 km is projected to be depleted by a factor of four, 12 h after launch, and a factor of two, 24 h after launch. Such depletion could have a significant impact on night-time radio frequency communications. These considerations indicate that theoretical investigations and experimental data will be required to establish stratospheric pollution by space vehicle exhaust products, whether used for the construction of the SPS or any other significant scientific or technological purposes. In this context, consideration should be given to the possibility that after the SPS has been demonstrated to be a desirable alternative energy technology, a global SPS system could be constructed using, where possible, extra-terrestrial resources rather than terrestrial resources. Studies of this alternative indicate that the changeover from terrestrial to extra-terrestrial resources may be cost-effective (34). 8.2.8. Microwave biological effects. The designs of the transmitting antenna and receiving antenna are strongly influenced by the choice of the power distribution within the microwave beam and determine the level of the microwave beam power density at the edges of the receiving antenna site. Acceptable guidelines for continuous low-level exposure to microwaves must be used. At present, there is a considerable difference of scientific opinion on the appropriate exposure levels to microwave radiation. Exposure guidelines adopted by several nations differ sharply. In the United States, microwave radiation protection guidelines, first proposed in 1953, were based on physiological considerations — i.e. continuous whole body exposure of a human subject resulting in a maximum equilibrium temperature rise of 1°C. This guideline was accepted by U.S. Government agencies and industry as tolerable on a long-term basis without risk of irreversible
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