It may sound like "Star Trek" several generations removed, but technology is moving toward a long- harbored dream of gathering the sun's energy in space and beaming it to Earth for electric power.
In space, where there are no clouds and few shadows, solar-collecting cells could capture more sunlight than on Earth. And now, as improvements in photovoltaic technology make harnessing solar power increasingly feasible, and as concern mounts about the environmental impact of burning fossil fuels, some scientists and engineers are devoting serious thought to the notion of electricity from outer space.
Research on another facet of the project -- beaming the power to earth in the form of microwaves -- also is moving gradually ahead. In August, dozens of scientists from around the world gathered in Paris to discuss power from space, with solar satellites a major focus of interest.
"There is a lot of progress in microwave-beamed power, and a lot of work has been done, sort of in fits and starts," said Dr. Alton D. Patton, a professor of electrical engineering at Texas A&M and the director of NASA's Center for Space Power and Communications on the campus in College Station, Texas.
Proponents of power from space talk about progress over the comingdecades. In the energy field, only power from nuclear fusion is commonly discussed with anything near such a long period in mind. But there is a difference with solar satellites, advocates say, because each element of the technology has already been demonstrated, albeit at nothing remotely approaching a commercially feasible price.
The idea was first floated in the 1960s by Peter E. Glaser, now vice president of space operations at Arthur D. Little Inc., a consulting firm in Cambridge, Mass., and still the theory's most energetic proponent.
With the energy crises of the 1970s, interest grew in Mr. Glaser's pet notion. But enthusiasm waned after $20 million in studies by the government. One, by the National Academy of Sciences in 1981, estimated that the cost could be $3 trillion over 50 years.
By the mid-1980s, the Soviets, who had built the world's largest rocket boosters and may have been exploring uses for them, were also expressing interest. But with the turmoil in that country, a Soviet role would now seem a moot point.
In the last few years, has come new concern about whether burning coal and other fuels for electric generation is leading to a "greenhouse effect" and global climate change.
And photovoltaic technology, which turns sunlight directly into electricity, is moving steadily forward, with cells that cost less, require less surface area and, most important from an astronautical point of view, weigh less.
As sketched by Mr. Glaser, the basic elements are "1960s technology," consisting of photovoltaic cells, microwave transmitters and receivers, and launching vehicles. But 21st-century engineering refinements would be needed.
His vision calls for a cluster of satellites in geosynchronous orbit, staying at a fixed point above receiver stations on the ground. At 22,000 miles up, the satellites would avoid the Earth's shadow except for a brief period around the equinoxes.
Each spacecraft would carry several square miles of solar cells and a microwave generator. The microwaves would be focused on the ground receiving station, also several square miles in area.
Mr. Glaser said a receiving antenna, some five miles in diameter, could handle 5 billion watts, or about as much as is generated by five large nuclear power plants. The antenna, called a "rectenna," would consist of metal twigs in which the microwaves would generate an electric current, and a rectifier to turn this from alternating current into direct current.
In an era of pervasive environmentalism, the idea of bathing the world in microwaves might be expected to raise hackles. But Mr. Glaser said that the heart of the beam would have only one-quarter the intensity of sunlight, on the order of 100 milliwatts per square centimeter. "Compared to the energy released in a thunderstorm, this is peanuts," he said.
The technology also calls up visions of beams running amok, frying hapless towns in their path. But the beams would have no harmful effect if they briefly strayed off target, he said, and would be kept continuously aimed at the receiving antenna by a signal sent from the center of the antenna; if that failed, the beam would shut down.
In any case, other researchers note, a malfunction would probably cause the beam to become less focused, rather than more focused.
"The beaming part is actually well proven, well within the state of the art," said Ivan Bekey, a space power expert at the National Aeronautics and Space Administration. "There's nothing new there."
He pointed out that researchers in the 1970s successfully beamed 10 kilowatts, enough power to serve several large houses, over several miles of California desert. Putting the giant satellites in orbit would be the hard part, he said.