There are two major opportunities for beamed power in space. The first opportunity is to replace the batteries which are required on solar powered satellites to provide power during the eclipse portion of the orbit. For low-earth orbit (LEO), the eclipse typically runs for about 35 min of the 90 min orbit. A significant market , is the commercially-important geosynchronous earth orbit [10]; for GEO the eclipse is confined to a short (70 min) daily period centered on midnight near the vernal and autumnal equinox. A single power station might be able to provide power for several such GEO satellites. Such power stations could be satellite-to-satellite, or could also be Earth-to-satellite. Such a power system, ‘an electric utility for space’, has been discussed in some detail by Grey & Deschamps [11]. In principle this space power utility is the nucleus of an SPS. Providing power for a lunar base [12] or to roving exploration parties [13] on the moon might be another application of beamed power. Night power for a photovoltaic powered moon base is an important consideration [14]; such power could well be provided by beamed power systems. (However, it should be noted that such beamed power systems, although being studied by NASA [12-14], are not an element of current baseline plans for a lunar base.) The second opportunity for beamed power in space is for orbit-to-orbit transportation by electric propulsion. This has been discussed, for example, by Brown [15] and Faymon [16]. Space transportation systems typically deliver payload into low orbit; raising the orbit to commercially valuable orbits such as GEO is done by an orbital transfer vehicle. Clearly, the higher the specific impulse of the orbital transfer vehicle, the less propellant mass is required to be brought to orbit. Electrically energized rocket engines such as the ion-thruster or magnetoplasmadynamic thruster have the advantage of extremely high specific impulse, and thus low propellant usage (or, equivalently, high payload fraction); the disadvantage is that they have correspondingly high power consumption (in fact, the power consumption is proportional to the specific impulse squared). Use of beamed power is likely to evolve from other applications demonstrating the applicability of electric propulsion to a wide variety of missions. Initial applications are for station-keeping for geosynchronous satellites; slightly further term applications may be solar-electric propulsion for planetary probes. Since the advantage of high specific impulse is diluted if the vehicle must carry a heavy power system, electric propulsion provides a natural application for beamed power. An additional advantage of transportation use for beamed power is that continuous power is in general not required. The thrusters are used when power is available, and can be turned off when the power is unavailable. By maintaining an aggressive policy of pursuing applications of beamed power in space, the technology of power beaming can be commercially ready by the time that photovoltaic technology has been brought to technological maturity. These two technologies will be sufficient for SPS construction, however, only if the third element is in place: large-scale space infrastructure. Large-scale Space Infrastructure Development of SPS will require a large infrastructure for space transportation and space construction. This will present a large risk element unless the transportation infrastructure is developed and tested well before commitment to an SPS. The transportation requirements will be orders of magnitude more than needed for known
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