such other possibilities as the use of extraterrestrial construction materials to reduce the cost of transporting material to an SPS construction site in orbit. The SPS represents a fertile field for innovations. Few of the potentially interesting alternative technologies have been analyzed in detail. It would be premature to choose among them because the consequences of these technologies cannot be evaluated without a vigorous system study of the impact of advanced technologies on SPS designs at the system and subsystem levels. The development of the most effective SPS designs for intended uses represents significant challenges, and these challenges must be met realistically. But it is as inappropriate now to discount the SPS as a major option for the 21st century as it was for Simon Newcomb, American astronomer, to state in 1906 that “the demonstration that no possible combination of known substances, known forms of machinery and known forms of force can be united in a practical machine by which man shall fly long distances through the air seems to the writer as complete as it is possible for the demonstration of any physical fact to be.” AN APPROACH TO SPS DEVELOPMENT Since its inception, the SPS concept has been controversial, because it brought into focus technical, economic, environmental, and societal issues involving perceptions about the role of space technology, and centralized and decentralized energy conversion options. The SPS has also raised questions about the decision-making process in the face of uncertainties associated with projected system performance and costs, environmental effects, and continuing support of private and public investors. Criticism of the SPS focused on the fact that it is a large-scale project. Traditionally, a large-scale project is conceived and executed with a definite start date and agreed- upon performance objectives, budgets, and schedules, and with an identifiable management organization responsible for implementation, i.e., a monolithic project. Success or failure of a monolithic project is judged by whether or not the project objectives are met within budget and on schedule. Further, the time needed to complete such large-scale projects makes them vulnerable to changes in the regulatory environment, and, if they extend over a decade or more, possibly to changing economic and political conditions. Any such project requires a continuing consensus between public and private investors, as well as the support of appropriate interest groups and government agencies during the various stages. The implicit assumption in the CDEP program was that the SPS is a monolithic project requiring a massive commitment of funds over the next several decades. An approach can be devised for the development of the SPS that identifies the underlying generic technologies and their application to specific space projects as shown in Fig. 1. The “terracing” of space projects would reduce the challenges typically associated with large-scale projects including control of the project, the effects of technical uncertainties, maintenance of investor confidence, reduction of environmental impacts, and the difficulties associated with termination of the project if warranted. The increasing capabilities needed for planned space projects — free- flying carriers, manned space stations, and space transportation systems of higher performance and lower cost—will contribute to the industrial infrastructure that could be the foundation for SPS development. As shown in the figure, the SPS is only one potential application of space technology which could evolve from future space projects. However, projects such as the
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