Two general concepts have been proposed for delivering br solar power to Earth from space. In one, Peter Glaser of br Arthur D. Little, Inc. (Cambridge, MA), proposed in 1968 br that a huge satellite in geosynchronous orbit around Earth br could dependably gather solar power in space. In the sec- br ond concept, discussed here, solar power would be col- br lected on the moon. In both ideas, many different beams br of 12-cm wavelength microwaves would deliver power to receivers at sites located worldwide. Each receiver would br supply commercial power to a given region. Such a receiv- br er, called a rectenna, would consist of a large field of small br rectifying antennas. A beam with a maximum intensity of br less than 20 of noontime sunlight would deliver about br 200 W to its local electric grid for every square meter of br rectenna area. Unlike sunlight, microwaves pass through br rain, clouds, dust, and smoke. In both scenarios, power br can be supplied to the rectenna at night.