affect a soft landing in water. Thus the vehicle is available for reuse, contributing to the required low operational cost. Another significant auxiliary system is the orbital construction facility required to provide the necessary production rate for satellite power stations. A concept for such a station is shown in Figure 1-5. 1.2.3 Energy Overview and the SPS Ever increasing rates of consumption of the Earth's available fossil and nuclear fuel stores are characteristic of this latter half of the twentieth century. Global population is increasing, and so also is the fraction of that population which forms the energy consuming "middle class." This is true not only in the U.S., Russia, Japan, etc., but in the so called emerging nations. As a consequence, we may expect these existing global energy sources to last only to these rather approximate dates: oil, 1995 to 2005; coal, 2030 to 2080; uranium (without breeder reactors), 2020 to 2050. As they are consumed, four additional factors come into play: first their cost steadily increases as remaining quantities become more difficult to obtain (e.g., thinner coal veins). Second, their consumption releases additional pollutants to the biosphere (for example, CO2 removed from the atmosphere millenia ago by plants, which formed coal, is now being returned). Third, since energy sources are geographically concentrated (e.g., most coal in U.S., most oil in Middle East), a potential for great international tension and even war may be created as reserves dwindle. Fourth, nuclear fission involves byproduct materials which may be used for weapon production by either governments or outlaws. Thus some attention is now turning to "renewable" or "non-depletable" energy sources. Primary candidates for electric power appear to be nuclear breeder reactors, nuclear fusion and solar energy. These are characterized by varying degrees of complexity, technical risk, pollution, cost, etc. Each could reduce our dependence on imports, and if adopted by other nations, serve to reduce international tensions. Solar power may be used directly for heating and cooling; it may also be used for the production of Fig. 1-5. Orbital Construction Facility
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