they are most appropriate to meet social and economic criteria with acceptable environmental impacts and be of the greatest overall benefit to society. One of these criteria pertains to the material requirements for construction and maintenance of solar systems which are designed to generate power continuously as an alternative to power plants requiring nonrenewable fuels. Such solar systems are capital intensive because of the low flux density of solar energy compared to other energy forms and interruptions of solar energy availability by weather and diurnal variations. Furthermore, these systems must operate in a variety of natural environments and must have sufficient structural stability to ensure operation and survival under occasional extreme conditions of wind, rain, snow and earthquakes as well as continuous routine environmental conditions such as dust deposition, corrosion, seasonal temperature extremes and the effects of solar radiation. The requirement to withstand these environmental effects translates into a minimum mass density of at least 10 kg/m2, indicating that unless advanced and lightweight solar systems with extended lifetimes can be developed, enormous amounts of commodity materials would be required for the construction and maintenance of solar systems on a global scale. The challenge, therefore, is to develop solar system designs which are very low in mass and possess an extremely long lifetime. These considerations point to the advantages which can be gained in constructing a solar system in space, where the absence of gravity and environmental effects could significantly reduce the material requirements and achieve the desirable long-life characteristics for the solar energy conversion system. 3. THE SOLAR POWER SATELLITE CONCEPT In view of these considerations of the potential of solar energy to meet global requirements, solar energy conversion methods which can meet the demands of modern societies for the continuous supply of power will need to be developed. Such methods will have to have a significant global impact, be conserving of materials resources, be economically competitive with power generation methods based on the use of nonrenewable energy sources, be environmentally benign, and be acceptable to the nations of the world. One of the major options for meeting this goal is embodied in the solar power satellite (SPS) concept. It is widely acknowledged that man's conquest of space had a most profound influence on technological advances. It demonstrated that evolutionary progress need not be confined to the Earth's surface. For example, satellites for Earth observations and for communications already significantly affect the lives of the Earth's population — and the indications are that there is no limit to the uses of space technology for the benefit of society. Therefore, a logical extension of the efforts to harness the Sun was to use space technology to overcome terrestrial obstacles, such as inclement weather and the diurnal cycle, for the large-scale conversion and application of solar energy. If satellites could be used for communications and for Earth observations, then it was also logical to develop satellites that could convert solar energy and place them in Earth orbits, particularly geosynchronous orbits (GEO), where they could generate power continuously during most of the year. With their year-round conversion capability, such satellites could overcome some of the major obstacles to large-scale installation on Earth, i.e. the huge conversion area requirements and means for energy storage. Thus the demonstrated capability of industrized
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