Design of a Low-Cost, Earth to Space Power Beaming Demonstration KEITH ROGERSt Introduction The concept of collecting the sun's energy in space and transmitting the energy down to the Earth's surface has now been around for about a quarter of a century. In that time, many studies have been done, but little actual experimentation has been completed. Power beaming experiments and demonstrations have so far covered distances of at most a few hundred meters, and only one such demonstration has been conducted in space. [1] One of the reasons for this lack of hardware experimentation is the formidable size of the proposed solar power stations. Even the more modest examples, such as Japan's SPS 2000 program, have price tags measured in billions of dollars. [2] At various times a terraced approach to developing the necessary technologies has been suggested, but even these plans soon progress to the point where expensive space-based demonstrations are necessary. [3] This paper will examine a simpler, cheaper way of demonstrating some of the important technologies for the eventual goal of providing beamed power for space and Earth. What really needs to be examined before taking the leap to megawatt level power satellites is the effects of beaming power through the atmosphere. Doing this with space to Earth power beaming experiments is expensive; one needs to put into orbit not only a fairly hefty power source but also a transmitting antenna with an aperture large enough to focus the beam so that it can be received in a reasonably sized area on Earth. Combine these factors with the control needs for pointing such a ponderous structure and fuel requirements for maintaining its orbit, and the costs really begin to add up. Yet if we reverse the equation, and beam power from Earth to space, things look quite different. Now our hefty power source and large aperture antenna do not need to be sent into orbit; in fact they have already been built. Our target can be a small, cheap microsatellite with minimal control systems and almost no fuel. Associated with this vastly reduced cost is a corresponding decrease in the time and organizational work necessary to carry out the project. Clearly, an Earth to space power beaming experiment would appear to be cheaper than a corresponding experiment in the other direction. But would it accomplish the same scientific objectives? A space to Earth power beaming experiment would test a number of different things. It would measure atmospheric absorption under different weather conditions. Atmospheric scattering effects would be examined, as would sidelobe strengths and distributions. All of these things could be assessed equally or nearly as well going from Earth to space. In addition, an Earth to space power beaming experiment could accumulate data on rectenna efficiencies in space and their degradation over time, information that would be useful to future space t MIT, 790 Main St., #10, Cambridge, MA 02139
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