In-Space Use of Beamed Power
The initial advantage for power beaming is that the effective cost of power in space is considerably greater than on Earth.
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 minutes of the 90 minute orbit. A significant market is the commercially-important geosynchronous earth orbit ; for GEO the eclipse is confined to a short (70 minute) 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 and Deschamps . In principle this space power utility is the nucleus of a SPS.
Providing power for a lunar base or to roving exploration parties on the moon might be another application of beamed power. Night power for a photovoltaic powered moon base is an important consideration; 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 , 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 and Faymon . 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 a SPS. The transportation requirements will be orders of magnitude more than needed for known commercial applications such as communications satellites. A significant boost would be identification of near-term, large-scale commercial applications of space*. Pending such an as-yet unknown commercial application, however, I see little prospect for commercial space enterprise to develop transportation on the scale required.
The SPS infrastructure is thus dependent on development of the required space infrastructure by space-exploration missions conducted by the various national governments of Earth. Any of the various manned missions proposed in the near term (space station Freedom, return to the moon, manned Mars mission, "Mission to Planet Earth") could provide elements of the necessary experience. Various unmanned missions, such as planetary probes (e.g., Cassini) and exploratory missions to the smaller bodies of the solar system such as asteroids and comets (CRAF) contribute little to the transportation infrastructure needed, although they are important preliminary elements to the long term exploitation of space resources.
An aggressive planetary exploration policy has additional long-term applications to SPS. The projected cost of a SPS could be considerably reduced if extraterrestrial resources are employed in the construction . One often-discussed road to lunar resource utilization is to start with the mining and refining of lunar oxygen, the most abundant element in the Moon's crust, for use as a component of rocket fuel to support the lunar base as well as exploration missions. Once the mining and refining process is in place to produce oxygen, the next-most abundant elements, aluminum and silicon, can be refined to produce solar array. Such lunar- manufactured solar arrays could have many applications (figure 2): not just to support growth of manufacturing capabilities on the moon, but also in LEO, GEO, and to support planetary missions, as well as to support solar-electric inter-orbital transportation and to serve as primary power supplies for the beamed transportation systems discussed in the previous section.
Thus, with the development of the component parts of a mature photovoltaic technology, beamed power for in-space use, and a space infrastructure, the implementation of a solar power satellite consists only of integrating the pieces.
*space tourism has been suggested as one such application.