Atmospheric absorption of laser radiation will be reduced when the receiving sites are located at high elevations. But even in such locations, unfavorable weather will require that the laser radiation be beamed to other receiving sites with favorable weather conditions and fed into a common transmission grid. The dimensions of a laser power receiving site, including a safety zone, will be measured in hundreds of meters vs the thousands of meters needed for a microwave power receiving antenna. Although laser power transmission is in an early stage of development and significant technology advancement will be required, there is considerable promise in a laser power transmission system for the SPS. Environmental impacts, including heating of the atmosphere and meteorological implications, are not expected to be significant, although the plasma chemistry of the upper atmosphere and induced photoreactions deserve further study. Although requirements for safety and security of laser power transmission may be adequately met, the potential for misuse of laser power transmission may be perceived as either dangerous or, under certain conditions, provocative, which could lead to political and societal opposition. 5. SPACE TRANSPORTATION SYSTEM To be commercially competitive, the SPS will require a space transportation system capable of placing large and massive payloads into synchronous orbit at low cost. The cost of transportation will have a significant impact on the economic feasibility of the SPS. The space transportation system which will be available during the early phases of SPS development for technology verification and component functional demonstration will be the space shuttle, now well along in development. Compared to the previously used expendable launch vehicles, it will not only significantly reduce the cost of launching payloads, but will also be a major step towards the development of space freighters of greatly increased payload capability — and substantially lower costs. The space freighter, which may be either a ballistic or winged reusable launch vehicle, represents an advanced space transportation system with a planned capability to place payloads ranging from 100 to 500 metric tons into LEO. The space freighter will be recoverable and repeatedly reusable. The fuel for the lower stage will be liquid oxygen and a hydrocarbon; liquid oxygen and liquid hydrogen will be used for the upper stage. Both offshore and onshore launch facilities could be developed for the space freighter. Frequent launches (e.g. ten launches per day) will necessitate maintenance and overhaul procedures similar to those employed in commercial airline operations. Personnel and cargo will be transported from LEO to GEO with chemically or electrically propelled vehicles specifically designed for this purpose. The material required for the SPS construction and assembly will be transported by a cargo orbital transfer vehicle which could be powered by ion thrusters of high specific impulse. Although the transit time to GEO would be measured in months, ion thrusters would minimize the amount of propellant to be transported to LEO. Transportation costs of ballistic or winged-launch vehicles to LEO will be about $20/kg, including amortization of the vehicle fleet investment, total operations manpower, and propellant costs (10). The total cost per flight will be about $8 million, with vehicle production and spares accounting for 40%, manpower for 35%, and propellants for 25%.
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