space. The initial space station is projected to require about 75 kWe for its operation to meet science and commercial payload mission requirements. As the space station programme proceeds, projected power requirements will reach the 400-kWf range. At these power levels, it may not be possible to integrate the power generation system using solar arrays because of the drag in low-Earth orbit and the disturbing forces generated by the large area of solar arrays. One approach to meet the space power requirements is to separate the non- interruptible power requirements for life support and other mission-critical systems from the industrial-type power supply. An orbiting Powercraft could supply industrialtype power to the space station and co-orbiting platforms when placed in the same orbit as the space station. The Powercraft could occupy either a leading or trailing position, depending upon the magnitude of the drag exerted on the solar arrays. When the Powercraft drag exceeds the drag of the space station, a trailing position will be preferable to avoid potential collisions with a space station in case of failure of the attitude or station-keeping system. The Powercraft could be positioned as close as 500 metres from the space station to maintain its desired position. Power from this position could be transmitted to the space station either by a tether or by a beam. The tether would be designed to eliminate forces acting on the space station to a level specified by the microgravity environment in a laboratory or materials processing module. Microwave or laser beam power transmission could be employed to decouple the Powercraft completely from the space station. This would also make it possible to beam power to several coorbiting platforms where low levels of microgravity have to be maintained over extended periods. The Powercraft for the space station would provide an opportunity to demonstrate technological advances and construction and assemble techniques that would provide important information that would be applicable to the development of the SPS concept. SPS The SPS concept represents a long-term goal for the development of solar space power. The goal of the SPS is to provide an economically viable and environmentally and socially acceptable option for power generation on a scale substantial enough to meet a significant portion of future world energy demands. The realization of this goal will rely on increasing capabilities demonstrated in space power projects, such as Powercraft. As currently envisioned, the SPS would be placed in geosynchronous orbit, where solar cell arrays would convert energy from the sun directly into electricity and feed it to microwave generators forming a part of a transmitting antenna. The antenna would precisely direct a microwave beam of very low power density from the SPS to one or more receiving antennas 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 orbiting satellites, each beaming power to one or more receiving antennas. During the early 1970s when the SPS concept was being evolved by NASA, [2] the space technologies required were in an early stage of development. Since then, significant advances in a wide range of technologies have been achieved and are being successfully applied to expanding space activities. The resolution of issues associated with the implementation of the SPS, including electrical power demand, power network interfaces, load management, receiving antenna siting, availability of material
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