(2) Extent of resupply needed during construction: This item can vary over a wide range, depending on the atmospheric composition needed at the construction station, and on whether food is brought in water-loaded or dry form. (3) Atmospheric composition: The structural mass is proportional to the internal atmospheric pressure. To provide an earth-normal amount of nitrogen would cost in two ways in space-colony construction, because structural masses would have to be increased to contain the increased pressure, and because nitrogen would have to be imported from the earth. O'Neill claims (Ref. 13) that using the fastest possible timetable for construction (major construction of the first colony begins in 1982), the program could pay back all of the total investment plus 10 percent interest in 24 years. Here, the economic output of the program is measured solely in the sale of power at initial rates of 15 mills/kWh, gradually declining to 10 mills/kWh. If the total material output of the space colony industries is assigned an added value, per pound of finished products, equal to the lift cost of bringing similar products from the earth, the added value is in the range of 40 to 160 billion dollars per year. This is about the total cost of constructing the first colony and assumes the use of shuttle-derived heavy lift vehicles and productivities typical of heavy industry on earth. The prime product would, of course, be satellite solar power stations. O'Neill finds the following objections to earth-launched satellite power stations (Ref. 14): (1) That they can demonstrate economic feasibility only if a whole series of goals can be reached, each within close limits. (2) That since those achievements could at best only be reached by pushing the stateof the art very hard, there is no room for dramatic reductions of energy cost with further development. (3) Ground-launch methods depend critically on the achievement of very low lift costs to geosynchronous orbit. This would require development costs of some tens of billions of dollars, and the technology involved is not well enough understood that success would be certain.
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