Century—Freedom and Mir 2—are operational, more detailed and complex manned vs. robotic assembly productivity tests, of the sort that a solar power satellite might entail, need to be performed on these stations in order to determine which of the two construction options is best. From this standpoint, the 10 MW space to Earth SPS 2000 demonstration, proposed by ISAS and potentially operational in ten to twenty years, is also extremely interesting as a demonstration of robotic construction, since it relies upon the teleoperated assembly of 10 separate Ariane 5- launched components. Such a fully automated assembly would be unprecedented (even Space Station Freedom is not planning to use robots for assembly). Within the context of a program for space solar power, the automated assembly in and of itself would probably justify much of the cost of the demonstration, regardless of the amount of power actually delivered to the ground. Problems on Earth Returning to the figure of 100,000 tonnes for each 5 GW SPS, note that even if it were decided that smaller solar power satellites should be built, the overall tonnage/GW would remain approximately constant, since more satellites would be needed to produce the same amount of energy. To put this truly tremendous amount of mass into perhaps even better perspective, since the launch of Sputnik 1 in 1957, only 30,000 tonnes of payload have been placed in orbit. This means that the emplacement into orbit of one SPS will require more than a tenfold increase of the entire world's launch capabilities, from 1000 tonnes to at least 20,000 tonnes per year. [Hannigan, 1991] Furthermore, if several SPS are to be constructed simultaneously, then the global launch rate will have to increase by another order of magnitude. There are two problems with this increased terrestrial launch rate scenario. First of all, at current space transportation cost levels of 10,000 US$ per kg or 10 Million US$ per tonne, a 100,000 tonne SPS would cost 1 Trillion USS to launch into orbit, a figure almost equal to the annual US Gross Domestic Product. So not only does the actual launch rate have to increase by two orders of magnitude, but the effective launch costs have to decrease by at least two orders of magnitude in order for any space solar power system like the SPS reference system to be economically feasible. As mentioned in the previous chapter on Space Transportation, only a factor of ten decrease in launch costs can be realistically expected in the foreseeable future (see Sec. 8.5.1)— and even that much may be too optimistic. Secondly, there is the matter of the possible deleterious environmental effects of a hundredfold increase in the terrestrial launch rate on the upper atmosphere. This problem is discussed in more detail in Sec. 6.2.2. Because of these assorted technical, financial, and environmental potential “showstoppers” to the building of large SPS with terrestrial materials, the use of nonterrestrial resources has been seriously considered. The Lunar Solution Contemporaneous to the NASA/DOE studies, Gerard K. O'Neill, to whom this report is dedicated, first proposed the use of lunar materials for the construction of Solar Power Satellites [O'Neill, 1978]. He argued that the raw materials needed for construction of these satellites could be delivered to GEO from the lunar surface at one-twentieth the transport cost of their delivery from the Earth. This argument is based on both the Moon's substantially lower gravity well and its lack of atmosphere, which allows the use of electromagnetic launchers called mass drivers, whose viability O'Neill also helped demonstrate. The general idea is that these mass drivers, which would only have to be approximately 160 m long in order to impart lunar escape velocity, would propel these raw materials to a mass catcher located in a halo orbit at the Lagrangian point L2 [Farquhar, 1971], some 60,000 km behind the Moon, from whence the raw materials could be cheaply delivered to GEO. The NASA/DOE study itself was constrained to consider only the terrestrial resource option. However, NASA did commission two studies on lunar resource utilization for Solar Power Satellites from General Dynamics [Bock et al., 1979] and MIT [Miller and Smith, 1979]. More recently, the Space Studies Institute funded two studies by Space Research Associates [Kelso et al., 1985 and Tillotson, 1989] to determine what mass fraction of solar power satellites could be built from nonterrestrial materials. All four studies determined that at least 90% of solar power satellites could be built from nonterrestrial materials at great reduction to overall system cost. The 1985 SRA study even concluded that as much as 99% of solar power satellites could be built from lunar resources. Resources found on the Moon that are of potential use for solar power satellite fabrication include silicon, aluminum, glass, and iron.. Also, oxygen can be used as a propellant throughout the space solar power cislunar infrastructure. However, some of these
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