hydrogen- or hydrocarbon-burning rockets could cause temporary removal of ions and electrons from the ionospheric F-region. At heights of 150 to 500 km, the normally occurring O+ ions transfer their charge to the combustion products H2O and CO2, forming polyatomic ions that recombine rapidly with free electrons. Severe depletions in ionospheric electron-number density can result. These plasma depletions can produce serious telecommunication effects and may trigger climate modifications. The proposed HLLV launch trajectory currently involves only limited propellant emissions in the F-layer of the ionosphere. The scope and magnitude of associated environmental impacts is being investigated. Ionospheric Effects on SPS Operation The microwave power beam responsible for transmitting energy from space to the ground-based rectenna is designed to operate at 2.45 GHz. This frequency represents a convenient compromise between the ionosphere-microwave interactions that are more easily excited at lower frequencies and the increased scattering losses from atmospheric hydrometeors such as rain and hail that occur for higher frequencies. Although no major changes in this operating frequency are anticipated, small changes may be made to reduce interference effects on other electromagnetic systems. An alternative option of laser power transmission is also being considered. To avoid nonlinear ionospheric interactions, the maximum power flux density within the SPS microwave beam was originally limited to 23 mW/cm2. Therefore, to deliver 5 GW of power, the downcoming beam would have to be at least 5 km in diameter. The rectenna, equally as large as the beam, then becomes a substantial part of the SPS system cost (estimated at approximately 42%). Any significant change in the maximum power-flux density in the microwave beam results in an associated change in SPS cost efficiency. The SPS economics, thus, are closely related to the ionosphere-microwave interaction thresholds and the magnitude of associated effects. A research program to determine these thresholds accurately and to evaluate accompanying environmental impacts is well underway. In addition to economic considerations, the ionosphere can also affect the SPS operational performance. The downcoming microwave power beam is directed onto the ground-based rectenna by an upgoing pilot beam. This retrodirective system operates at a frequency close to that of the power beam and is centered in the rectenna. Thus, the upgoing pilot beam propagates through the same ionospheric plasma as does the downcoming power beam. Any disturbances generated in the ionosphere could then lead to pilot beam scintillation or scattering, resulting in wandering or defocusing of the microwave power beam. A detailed study of beam control is an important part of the SPS ionospheric research program. Microwave Beam Effects on the Ionosphere Electromagnetic radiation propagating through the ionosphere is collisionally damped by free electrons. For microwave frequencies, the fraction of wave energy absorbed by the plasma is expected to be extremely small. Nevertheless, because this absorbed energy goes directly into the free electrons, which have a very small effective heat capacity, the resulting ohmic heating can significantly affect the local ionospheric thermal budget. Strong radiation can initiate significant enhancements in electron temperature, also affecting ionospheric densities and structure. Furthermore, the resulting thermal forces can drive additional ionosphere-microwave in-
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