of this concept as a primary, baseload electrical power source. Laser power transmission has a significantly lower efficiency for longdistance power transmission than is estimated for microwave power transmission. Atmospheric attenuation is substantial compared to microwave frequency transmission. Therefore, this concept is presently less attractive than the microwave concept for transmitting power from geosynchronous orbit. Alternate system concepts such as solar collectors and laser transmitters in low earth orbit with relays in geosynchronous orbit have received preliminary consideration. Another SPS concept is being evaluated that makes use of materials derived from the moon to construct the SPS. The moon's lower gravitational force (onesixth of earth's) would allow much less propulsion energy to move payload to geosynchronous earth orbit. This idea appears to have merit in terms of conserving earth resources and possibly reducing the cost of space transportation; however, it would require development of moon-based mining, manufacturing and launch facilities. Consequently, the research and development requirements for such an approach would be greatly increased. While the above options offer interesting possibilities, the present DOE/ NASA program focuses evaluation on the SPS concept using terrestrial materials and deployed in geosynchronous orbit as illustrated in figure 3. This evaluation does not exclude the possibility of future consideration of the alternatives and options such as identified above.
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