the 1 MW OTV's; this is the longest trip time for the OTV's. The payload for these missions is 11,400 kg. Lunar transportation may also represent a potential mid-term market for beamed power. With the potential of the Space Exploration Initiative, there will be a transportation system established between the Earth and the Moon. Initially, the missions to the Moon will be infrequent, with several robotic probes being sent to survey the mineral and topographical distributions on the lunar surface. This will help identify the best site for the lunar base. Beamed power may be able to supply power for orbital transfer vehicles from LEO to Low Lunar orbit (LLO). A lunar base construction program would have to deliver up to several hundred metric tons in LLO. This mass would include the lander masses for the descent to and ascent from the lunar surface. There will be potentially up to several missions per year to LLO using electric propulsion vehicles. These vehicles could use beamed energy from either laser or microwave sources. During the construction of a lunar base, there will be an opportunity to use electric propulsion for cargo delivery. [Palaszewski, 1988] An analysis of the transportation requirements from LEO to Low Lunar Orbit (LLO) has shown that 78 flights are required to deliver mass to LLO over a 19 year period. By using electric propulsion, there may be a potential market for power beaming. The analysis considered three power levels of OTVs: 100 kW, 300 kW and 1 MW. The predicted fleet sizes for these three power levels were 7 for a 1-MW power level, 18 for a 300-kW power level and 47 for a 100-kW power level. The longest round trip time required to travel from LEO- LLO-LEO was 257 days for the 1 MW system, and 770 days for the 300 kW OTV. The 100 kW OTV was not considered viable because of the extremely long trip time. The largest payload masses carried were 35,000 kg. It is difficult to imagine that an interplanetary spacecraft will used beamed energy for primary propulsion to get to a planet. The transmission distances are so long and there is the small problem of the Sun obscuring the Earth line of site for a beamed energy system. A more likely application is the use of beamed energy to provide power to the surface of a planet from an orbiting spacecraft. Interplanetary missions may also use high power solar arrays or nuclear reactors to conduct detailed measurements of the surface and atmospheres of planets and their moons, as well as comets and asteroids. Typical power levels that would be available for power beaming or experiments would be 10's of kilowatts for small planetary missions to dozens of megawatts for Mars missions carrying either humans or cargo. These vehicles would have an onboard power supply to power an electric propulsion system to get the vehicle to its destination. After its primary propulsion task is completed, the power supply would have much auxiliary or idle power to employ in other tasks. Assessment for Mid Term Of the space-based markets described above, none seem adequate to support the necessary constellation of power satellites by themselves. However, it does seem possible that a power satellite system servicing a mix of markets has some potential in the mid term. Because the eclipse power market is unlikely to saturate, servicing it would take only a small fraction of a power satellite's time each year. It is possible that this market could be filled while providing service to a LEO platform using the same satellites. For the small portion of time the satellite would provide power to the communications satellites the platform could survive on batteries if not in view of another power satellite. Conceivably, both of these markets could be exploited while also providing power for an orbital transfer service powered by satellites not in view of the platform. Note that this sort of market mixing could probably only be accomplished using a laser transmission system, as each satellite would have to be capable of sending power to a variety of locations. Earth Applications Identifying the mid-term applications of space solar power for Earth use, we considered the 2020-2040 time frame and assumed that space solar power delivery to the electricity grid will not yet be done on a large scale. Energy demand in the developing world has been growing in the preceding 3 decades, and more and more large scale grids are installed there. Still, the mid-term market is highly similar to the near term one, with the same basic assumption holding, that the
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