Fig. 26. Evolutionary program phasing decision for the very large commitment to full-scale system development and commercial operations. Estimating the costs of electricity from a Solar Power Satellite network is a challenging task. The economic viability of the concept will depend significantly on the ability to develop techniques for the mass production of solar cells at extremely low costs, and on the ability to reduce space transportation costs through complete system reusability and high traffic volume. Environmental considerations of the concept have focused primarily on the microwave transmission beam and its possible effects on the ionosphere, and on the low-level microwave energy that will exist outside the ground receiving antenna. Microwave energy levels in these two areas have been kept purposely low to avoid concern; however, the paucity of definitive data on microwave effects makes it difficult to provide absolute answers to all questions at the present time. Preliminary estimates of natural resource requirements and energy payback intervals are generally encouraging, recognizing possible specific problems with particular resources and with critical assumptions within the energy payback estimates. 5. SUMMARY OF SYSTEMS DEFINITION STUDIES TO DATE The systems definition studies have considered photovoltaic and thermal energy conversion systems and found both to be technically feasible, with the photovoltaic approach believed to offer certain advantages. Analysis of the microwave transmission system has emphasized the development of techniques for forming and steering the beam to the required accuracy and the performance required of the various subelements of the system. Ballistic and winged launch vehicles have been defined for the transportation of construction materials to low Earth orbit, and chemical and electric propulsion systems have been analyzed for the low Earth orbit to geosynchronous orbit transfer. The relative merits of constructing in low Earth orbit or geosynchronous orbit have been evaluated, and construction concepts have been defined. Based on the various trade-off studies conducted, a reference system has been defined to provide a basis for assessing alternate technical approaches and for evaluating the concept from the standpoint of environmental and other consid-
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