low Earth orbit (LEO) and in geosynchronous orbit (GEO). Since then, an added incentive has been the substantial progress being made in the development of advanced photovoltaic materials and the increasing confidence in the ultimate achievement of significant cost reductions. In the SPS concept, solar cell arrays would convert solar energy directly into electricity and feed it to microwave generators forming part of a planar, phased-array transmitting antenna. The antenna would precisely direct a microwave beam of very low power density to one or more receiving antennas (i.e., rectennas) at desired locations on Earth. At the receiving antennas, the microwave energy would be safely and efficiently reconverted into electricity and then transmitted to users. An SPS system could consist of many satellites in GEO, each beaming power to one or more receiving antennas. The SPS concept challenged the view prevalent in the 1960s that solar energy conversion could not make a significant contribution to the global energy economies, and demonstrated that there are no « priori limits to the development of energy resources in space. Although not a panacea for the increasingly complex economic, environmental and societal problems, the SPS concept represents an alternative direction for developing renewable energy resources and for engaging in sustainable human activities in space in the 21st century. The energy which would be available on Earth from space could overcome the physical, societal, and institutional limitations to which future generations would be subjected by dwindling resources on Earth. Energy from space could break open the closed ecological system of planet Earth so that humanity need not be destined to live from generation to generation facing the threats of resource shortages and the resulting social upheavals. Extraterrestrial resources could satisfy some of the requirements of the global population in the 21st century; if these resources are not used, people’s insistence for improvements in living standards may be very difficult to meet. There is increasing confidence that, even with present space technology, the resources of the Moon — for example, oxygen, silicon, and aluminum — and possibly those in the asteroids could be the raw materials not only for the construction of the SPS, but also for other orbital industrial complexes so that they would no longer need to depend exclusively on terrestrial resources. The SPS concept has the unique advantage of not having to rely on a thermodynamic cycle to generate electricity on Earth, as is the case for fossil fuels and nuclear fission and fusion. It also can be the impetus for the development of space transportation systems and construction technologies to support a broad variety of industrial uses of space with useful intermediate and long-term applications. Furthermore, the SPS presents an opportunity for peaceful cooperation among nations in space. And finally, it has the potential to reduce conflict by eliminating the need for exploitation of energy resources at the expense of others; energy from space can be supplied on a global scale and thus be of benefit to all people. The SPS concept was conceived with the following objectives in mind: • To convert solar energy in space for baseload power generation on Earth; • To be of global benefit; • To conserve scarce resources; • To be economically competitive with alternative power generation methods; • To be environmentally benign; and • To be acceptable to the nations of the world.
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