STS Compared with SPS As the previous discussion illustrates, STS should receive priority over SPS for a variety of good reasons, and can be expected to ultimately grow into an ambitious SPS programme. Fig. 5 shows SPS in a similar format to that in which Fig. 2 shows STS. This can be used to compare the two systems conceptually. The space power satellite itself is the largest challenge in an SPS programme, and its major difference from an STS programme. The difficulties inherent in transporting thousands of tons of material to geosynchronous orbit and assembling the SPS there are enormous. Add to this the fact that assembly will no doubt require many people for a long period of time and it is clearly a great technical challenge. While photovoltaic based SPSs are usually discussed, review committees with experience in space power generation [3] point out that thermodynamic power generation is likely to be more attractive for the NASA space station, and by implication for the SPS. This highlights the difficulties of SPS - if we are to use the most appropriate technology, thermodynamic coversion, we must build an 8,000 MW plant in space. The largest plant built on earth to date is much less than this (nuclear plants are typically 1,100 MW, and new coal plants are usually 2,000 MW or so). The largest solar-poivered generating plant on earth today is 100 MW-and is thermodynamic. The progress of science and technology will eventually lead us to be able to build 8,000 MW SPSs easily, but we cannot do that now. STS uses proven, economical,
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