22, page 39, Figure 29). Similarly, atmospheric transmission at 35 GHz is approximately 90 to 95% (our calculation was based on Ref. 22, page 39, Figure 28a). The overall power link efficiency will be lower at 35 GHz at this point in time as both DC-to-RF and RF-to-DC conversion efficiencies are lower than at 2.45 GHz. With time, 35 GHz technology will probably approach 2.45 GHz efficiencies, but the attenuation problem cannot be done away with. Thus, either the total SPS generating capacity would have to be increased, and a ground based energy storage system developed, or additional rectennas would have to be constructed. The latter proposal would involve building more than one rectenna for each SPS unit, so that the beam can be redirected on rainy days. This would involve additional land area, added complexity in the SPS units themselves, as well as additional transmission losses along the ground as power is transmitted from a rectenna located where rain is not falling to users located where rain is falling. Thus, further development of 35 GHz power transmission does not appear to be desirable for SPS, although it may have space-to-space or Earth-to-space applications and other potential applications. The NASA-DOE SPS study was terminated in 1980, with the National Research Council estimating that the cost of an individual SPS would be very large. Since then, several important studies on the feasibility of constructing an SPS using lunar materials have been conducted by the Space Studies Institute.19 In addition, SSI has also conducted several experiments on mass-drivers, telerobotic assembly, manufacturing lightweight composites from lunar composite materials, and is in the process of designing a lunar probe to investigate the availability of water on the Moon. In addition, the strides made in solar cell efficiencies and concentrators can reduce the mass of the solar panels on an SPS by 50% and the overall mass by 20%. Power beaming technology today is much more mature than it was ten years ago. Carbon fiber composites developed for the space programs might further reduce the mass of the SPS. As SSI has demonstrated, the transportation costs to LEO can almost be eliminated if the SPS is built of lunar materials. In the short-term, CO2 emission reduction treaties should be signed. Such treaties should be linked to the adoption of alternative energy sources by the signatories to displace their fossil fuel consumption. The world can either continue to pump CO2 into the atmosphere and wait and watch the globe warm or can cooperatively pool its resources together to secure a peaceful and prosperous future for its citizens. As we have demonstrated, the time to act is now! Conclusions While the economy of the world can become somewhat less energy-intensive through conservation, the increase in the world's population, especially in developing nations, will necessitate extensive increases in energy production in the coming years. Climatological studies show that if this is done through continued reliance on fossil fuel combustion, a global greenhouse warming of several degrees is likely to result by the end of the 21st century. Therefore, new energy sources must be developed. Those considered in this paper include terrestrial photovoltaics, nuclear fission, nuclear fusion, and the solar power satellite (SPS). Of these
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