deplete ionization and affect communications systems that depend on ionosphere reflection (10). The reference SPS system for cargo delivery from low Earth orbit to GEO orbit relied on electric propulsion systems using argon as a propellant. Argon is a noble (inert) gas, and was seen as totally nonpolluting. Yet the number of argon ions to be expelled by the propulsion systems was vastly greater than the number of ions now present in the magnetosphere. The chemical inertness of argon is immaterial in the magnetosphere; the argon atoms are charged particles. Our knowledge of the potential effects of operating these propulsion systems is rudimentary, but indications are that ion injection into the magnetosphere could be a serious environmental problem. Newer concepts for large-scale orbit transfer operations have averted this issue. Sky Pollution SPSs could pose a serious threat to astronomy. The satellites themselves would be quite bright in visible and IR spectra and might create significant interference in the RF bands near the 2.45 GHz of the power transmitters. A flippant but perhaps accurate response to the astronomy issues is to aver that no serious astronomy will be conducted from the Earth’s surface by the time SPSs could pose a real problem; this point of view is supported by historical trends. SPSs would be visible from the surface of the Earth; various estimates of brightness range from first magnitude to about the brightness of Venus. If a number of SPSs were emplaced in geosynchronous orbit this would, of course, represent a great alteration of the night sky as compared to that seen by humans dating back to prehistoric times. The significance of this is easily overestimated. The night sky has been greatly altered in the past two hundred years. Formerly dominated by stars, planets, and the Moon, the night sky for most humans is now dominated by reflections of city lights. Stars are rarely seen. For ordinary people, the change introduced by SPS would be little noticed. For Earth-based astronomy, it could be devastating. But astronomy is moving into space. THE REFERENCE SYSTEM The DOE-NASA studies created a much-maligned concept for SPS called the reference system (11). Most of the criticism of this concept has arisen from the thought that it represented a design to someday be constructed. In fact, the creators of the “reference system” had no such illusions. It was essential, in order to meet the objectives of the DOE-NASA assessment, to have available a description of a more or less representative SPS system. The “reference system” served this need. It was never intended as a “to be built" design. The reference system, although never intended as something to be built, offers numerous lessons: 11] The reference system was too big for the early phases of an SPS program and perhaps too small for later phases. The system design should adapt to a range of power levels and sizes. [2] The reference system was only crudely cost-optimized. Any system to be actually built will be meticulously cost-optimized. The effort spent on design optimization will be from 100 to 1000 times that spent on the reference system. The results will bear little resemblance to the reference system.
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