The solar energy collected by an SPS would be converted into electricity, then into microwaves. The microwaves would be beamed to the Earth's surface, where they would be received and converted back into electricity by a large array of devices known as a rectifying antenna, or rectenna. (Rectification is the process by which alternating electrical current, such as that induced by a microwave beam, is converted to direct current. This direct current can then be converted to the "slower" 50 or 60 cycle alternating current that is used by homes, offices, and factories.) At geostationary orbit (36,000 kilometers or 22,000 miles high), the SPS would have a 24-hour orbital period. It would therefore always hover over the same spot on the equator and can keep its beam fixed on a position at a higher latitude. Since the Earth's axis is tilted, an SPS orbiting over the equator wouldswing above or below the Earth's shadow during its daily orbit. Sunlight would not be blocked, except for a period of about an hour eachnight within a few weeks of the equinoxes.

Image of 'traditional' SPS: SSI

It is interesting to compare the availability of sunlight in space with that on Earth. A solar panel facing the sun in near-Earth space receives about 1400 watts of sunlight per square meter (130 watts per square foot). (Of course, only a fraction of this is usable due to conversion inefficiencies.) On Earth, the day-night cycle cuts this in half. The oblique angle of the sun's rays with respect to the ground (except at noon in the tropics) cuts this in half again for a typical spot on the Earth. (Solar panels on the ground can be angled upward to circumvent this, but they must then be spread out over more ground to avoid casting shadows on each other.) Clouds and atmospheric dust cut the available sunlight in half again. Thus, sunlight is about eight times more abundant in geostationary orbit than it is on the Earth. Although the microwave beam from an SPS would also be dilute, it would be converted to electricity at a greater efficiency than sunlight. However, the largest cost savings in SPS versus terrestrial solar collectors may be the elimination of the need for storage at night (or transmission from the day side of the Earth).

Spurred on by the oil crises of the 1970's, the US Department of Energy and NASA jointly studied the SPS during that decade. The result of this study was a design for an SPS which consisted of a 5 x 10 kilometer (3 x 6 mile) rectangular solar collector and a 1-kilometer-diameter (0.6 mile) circular transmitting antenna array. The SPS would weigh 30,000 to 50,000 metrictons. The power would be beamed to the Earth in the form of microwaves at a frequency of 2.45 GHz (2450 MHz), which can pass unimpeded through clouds and rain. This frequency hasbeen set aside for industrial, scientific, and medical use, and is thesame frequency used in microwave ovens. Equipment to generate themicrowaves is therefore inexpensive and readily available, though higherfrequencies have been proposed as well. Therectenna array would be an ellipse 10 x 13 kilometers (6 x 8 miles) insize. It could be designed to let light through, so that crops, or evensolar panels, could be placed underneath it. The amount of poweravailable to consumers from one such SPS is 5 billion watts. (A typicalconventional power plant supplies 500 million to 1 billion watts.) Thepeak intensity of the microwave beam would be 23 milliwatts per squarecentimeter (148 milliwatts per square inch). The US standard forindustrial exposure to microwaves is 10 milliwatts per square centimeter,while up to 5 milliwatts per square centimeter are allowed to leak frommicrowave ovens. US standards are based on heating effects. Stricterstandards are in effect in some countries. So far, nonon-thermal health effects of low-level microwave exposure have beenproven, although the issue remains controversial. Nevertheless, even the peak of the beam is not exactly a death ray. Underneath the rectenna,microwave levels are practically nil.

Tyhe reason that the SPS must be so large has to do with the physics of power beaming. The smallerthe transmitter array, the larger the angle of divergence of thetransmitted beam. A highly divergent beam will spread out over a greatdeal of landarea, and may be too weak to activate the rectenna. In order to obtain asufficiently concentrated beam, a great deal of power must be collectedand fed into a large transmitter array.

Interest in the SPSconcept waned after the 1970's due to the end of the oil crisis and the failure of inexpensive launch systems to materialize. In recentyears, there has been a renewed interest in the SPS, due to concerns abouta possible global warming resulting from carbon dioxide emissions from fossil fuel combustion. A study commissionedby the Space Studies Institute (SSI) has shown that about 98% of the mass of the SPS can consist ofmaterials mined from the moon. A lunar infrastructure would have to existfor this to occur. My own SSI-sponsored work, based on earlier work byGeoffrey Landis and Ronald Cull at the NASA Lewis Research Center, hasshown that an SPS could be built using thin-film solar cells deposited onlightweight substrates. Such an SPS could deliver perhaps ten times as much power per unit mass as older designs. The combination oflightweight materials, inexpensive launch systems, and a spaceinfrastructure can make the SPS a reality. No breakthroughs in physicswould be required. However, a significant commitment to technologydevelopment would be needed.

Seth D Potter was a Research Scientist in Physics at New York University when this article was written. He is currently an engineer at The Boeing Company in Seal Beach, California, USA, and serves on the Board of Directors of the National Space Society Education Chapter.