least typical for airline operations (the cost of operating an airliner is about half fuel cost). The calculation yields 16x0.05x2/0.05, $32/kg, again similar to the SPS cost estimates. The needed technologies are advanced structural materials, mature rocket engines, thermal protection, and subsystems technologies, and perhaps the use of control-configured design. These examples might tempt one to believe that the SPS cost estimates for solar arrays and space transportation were made in this way, but in fact they were not; they employed the parametric cost estimating methods typically in use in the aerospace industry. The NAS report notwithstanding, SPSs could someday produce economical energy. The concept is not intrinsically high cost. The only question is how much technology advancement is needed; that question was recognized as of paramount importance by the SPS studies. WHAT’S NEW? The SPS data base created by the studies and the reviews is now about two years old. It is appropriate to finish this essay by considering what we know today that might alter the views of two years ago. There are several things. Conservation High energy costs have stimulated conservation which has in turn stemmed the demand for energy. The urgency of developing permanent renewable solutions has been somewhat alleviated. We should not be so complacent as to think the urgency is gone, but if a system like SPS began coming on line in 2010 or 2020, rather than in 2000 as postulated by the reference program, it would still be timely. Carbon Dioxide In the background of the SPS reviews was coal: U.S. reserves are enormous, and the technology for generating baseload electricity from coal is in hand. Why spend huge sums to develop a new and uncertain electric generation technology when there is so much coal? Two years ago the carbon dioxide question was just that— a question. Since then, the weight of scientific opinion has shifted strongly toward accepting the predictions of the greenhouse models, that a doubling of the CO2 in the Earth's atmosphere will bring about climate changes with far-reaching effects (15,16). Limitation on consumption of fossil fuels now appears to be CO2 loading in the atmosphere rather than quantity of reserves. While this is a long-range problem, so is the development and implementation of nonfossil energy technology. The viable alternatives now known are breeder reactors, thermonuclear fusion, and solar— SPS would fill an important and otherwise unsatisfied need in solar technology. Given the uncertainties and risks in the nuclear technologies, should we not be paying at least a small insurance premium? The Space Shuttle When the judgements on SPS were being rendered, the shuttle had not flown and was two years behind schedule. It had been called "SPS's Three Mile Island.”
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