connotation that may adversely affect decisions regarding necessary near-term research and development programs if the most promising technology options are to be exercised in the 21st century, they are required to focus near-term space missions in such a way as to obtain information on which to base succeeding missions. Given such a focus, generic space technologies can be developed to meet the requirements of a variety of space missions that could lead to step-by-step advances in the broadbased uses of space. But there is a wide divergence of views regarding the long-term impacts of specific space missions: the influence of advances in science and technology on future space activities; their competitiveness with similar activities performed on Earth; and the scale, timing, and effectiveness of governments' and industries’ investments in space programs in response to idealistic visions, pragmatic considerations, and political realities. Therefore, as important as the evolution of space technologies is to the planning and execution of specific space missions, economic, environmental, and societal issues also must be considered; and to ensure public support, they must be studied early and in parallel. In recent times, profound changes have occurred in the human condition; the undesirable ecological impacts of industrialization have been recognized and population growth has increased pressures to tap available natural resources. As a consequence, there are few known but unclaimed terrestrial resources waiting to be tapped. The upsurge in the material aspirations of people everywhere has coincided with wide acceptance of the proposition that limited resources and environmental constraints inexorably prohibit continued economic growth. However, models of social organization accept growth as a prerequisite for progress; no realistic economic system has been proposed which will permit adaptation to a stagnant society. As a result, confidence in the future has been eroded by a pervasive sense of the fragility of civilization. While space utilization cannot meet all the challenges facing society, the human prospect becomes much more encouraging when its potential benefits are taken into consideration. In sight is the technology to provide access to the limitless energy and material resources of the solar system which could sustain economic growth and advances in living standards of all people for as far into the future as can be reasonable projected. The availability of abundant energy without undesirable environmental consequences and at an acceptable cost is vital to any realistic approach to future economic growth. There is a growing consensus that humanity will increasingly rely on renewable energy resources which have their origin in the energy radiated by the Sun. Although there are many ways of reducing the magnitude of the energy supply challenges, only a few energy conversion methods have the potential to generate power continuously (baseload). In addition to known methods based on coal and nuclear fuels and the experimental ocean thermal energy conversion methods, there are two not-yet-demonstrated methods for baseload power generation in the 21st century: fusion and the solar power satellite, SPS. RATIONALE FOR THE SPS CONCEPT As originally conceived (1), an SPS could utilize various approaches, e.g., photovoltaic and thermal-electric, to solar energy conversion. From these, photovoltaic conversion was selected as a useful starting point because solar cells were already in wide use in communication, Earth observation and meteorological satellites, both in
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