Solar power satellites ( SPS) were originally proposed as a solution to the oil crises of the 1970s by Czech-American engineer Peter Glaser, then at Arthur D. Little. Glaser imagined 50-square-kilometer arrays of solar cells deployed on satellites orbiting 36,000 kilometers above fixed points along the equator. A satellite at that "geosynchronous" altitude takes 24 hours to orbit the earth and thus remains fixed over the same point on earth all the time.

The idea was elegant. Photovoltaic cells on a satellite would convert sunlight into electrical current, which would, in turn, power an onboard microwave generator. The microwave beam would travel through space and the atmosphere. On the ground, an array of rectifying antennas, or "rectennas," would collect these microwaves and extract electrical power, either for local use or for distribution through conventional utility grids.

The technology, as originally envisioned, posed daunting technical hurdles. Transferring electrical power efficiently from a satellite in geosynchronous orbit would require a transmitting antenna on board the satellite about one kilometer in diameter and a receiving antenna on the ground about 10 kilometers in diameter. A project of this scale boggles the mind; government funding agencies shied away from investing immense sums in a project whose viability was so unclear. NASA and the Department of Energy, which had sponsored preliminary design studies, lost interest in the late 1970s.

In the last few years, however, the communications industry has announced satellite projects that suggest the time has come to revisit the solar power satellite idea. By early in the next century, swarms of communications satellites will be orbiting the earth at low altitude, relaying voice, video, and data to the most remote spots on earth. These satellites will relay communication signals to earth on beams of microwaves. The transmission of electrical power with a beam of microwaves was demonstrated as early as 1963, and projecting power and data along the same microwave beam is well within the state of the art. Why not use the same beam to carry electrical power?

The new communications satellites will orbit at an altitude of only a few hundred miles. Instead of hovering above a spot on the equator, low-orbiting satellites zip around the globe in as little as 90 minutes, tracing paths that oscillate about the equator, rising and dipping as many as 86 degrees of latitude. Because they are closer to the earth's surface, the solar collectors on the satellite can be a few hundred meters across rather than 10 kilometers. And because the microwave beams they generate would spread out much less than those from geosynchronous satellites, the ground rectennas could be correspondingly smaller and less expensive as well. By piggybacking onto these fleets of communications satellites--and taking advantage of their microwave transmitters and receivers, ground stations, and control systems--solar power technology can become economically viable.

Low earth orbit poses its own difficulties, though. Because they whip around the planet so quickly, low-orbiting satellites must possess sophisticated computer- controlled systems for adjusting the aim of the microwave beam so that it lands at the receiving station. These satellites will have to use sophisticated electronic systems, called phased arrays, to continuously retarget the outgoing beam.

The demand for space-based solar power could be extraordinary. By 2050, according to some estimates, 10 billion people will inhabit the globe--more than 85 percent of them in developing countries. The big question: How can we best supply humanity's growing energy needs with the least adverse impact on the environment?

Dependence on fossil fuels is not the answer because burning coal, oil, and gas will pour carbon dioxide into the atmosphere, raising the risk of global climate change. (And of course these resources will not last forever.) Nuclear fission reactors avoid the greenhouse problem but introduce the so-far intractable problem of disposing of nuclear waste. Controlled nuclear fusion might someday provide an inexhaustible supply of clean energy--but after forty years of continuous funding, a practical fusion reactor is still not in sight.

That leaves the menu of renewable energy sources. But terrestrial renewables pose environmental problems because of their relatively large land requirements. Hydropower, the most exploited renewable thus far, has significantly disrupted ecosystems and human habitats. Solar, biomass, and wind farms would similarly compete with people, agriculture, and natural ecosystems for land were they the basis of a global energy system.

Moreover, ground-based renewable energy systems, such as terrestrial photovoltaics and biomass fuels, generate fewer than 10 watts of electricity per square meter, on a continuous basis. To generate enough electricity to meet demand could require developing countries either to divert land from agricultural use, and thus diminish the supply of food, or to destroy natural ecosystems, a move that could hasten the onset of global warming.

Solar power satellites would require far less land to generate electricity. Each square meter of land devoted to the task could yield as much as 100 watts of electricity. And the power-receiving rectenna arrays--a fine metallic mesh--would be visually transparent, so their presence would not interfere with crop growth or cattle grazing.

And the flow of power from terrestrial renewables is intermittent. Clouds blot out the sun; the wind stops blowing; lack of rainfall nullifies a hydro generator. Because these technologies do not deliver power continuously, they require some means of storing energy, adding to overall cost and complexity. A network of solar power satellites in low earth orbit could provide power to any spot on earth on a virtually continuous basis because at least one satellite will always be in "view" of the receiving station.

Unfortunately, solar power from space is not yet on the official menu of twenty- first century energy options. Since the 1970s, NASA and the U.S. Department of Energy have provided only token funding for the technology. A recent study by the National Academy of Sciences of potential strategies to mitigate global warming analyzed a wide range of nonfossil energy alternatives--including nuclear, hydroelectric, geothermal, solar photovoltaic, solar thermal, wind, and biomass energy--but did not include space power as an option.

Despite the funding desert in the United States, work on solar power satellites has continued elsewhere. In Japan, for example, leaders of the New Earth 21 program at the Ministry of Technology and Industry (MITI) view space solar power as "an essential part in the proper control of CO2 levels." MITI has sponsored the design of a kite-like orbiter that would travel in low earth orbit above the equator, with transmitting antennae on the earthward face and solar collectors on spaceward faces. In the United States, commercialization of space power will become a reality only if it can attract investment capital and succeed as a business. Fortunately, the private sector seems eager to invest in the communications satellites that could provide the vehicles for a solar power satellite. Motorola, for example, is putting $3.8 billion into Iridium, a venture comprising 66 communications satellites in low earth orbit. Teledesic Corp.--a joint venture of Microsoft chairman Bill Gates and cellular phone tycoon Craig McCaw of Mobile Telecommunications Technologies--plans to spend $9 billion to deploy 288 satellites.

One important consideration in planning space power is the expense of putting a satellite into orbit. Right now, it costs a thousand times more to put an object into space than to fly it across country by commercial airliner, even though the two jobs require roughly the same amount of energy--about 10 kilowatt-hours per kilogram of payload. Two factors account for the extra cost: the army of engineers and scientists required for a successful space launch, and the practice of discarding much of the launch vehicle after each flight.

Launch costs are likely to drop, however, as the demand increases for hoisting large volumes of material into space on a regular basis: the more frequently a launch system is used, the lower the cost per use. Moreover, NASA is seeking a new generation of reusable launch vehicles. The agency recently sponsored a competition among aerospace contractors for a space vehicle with the potential for airline-like operation. The winner was Lockheed Martin Skunk Works, legendary innovators in aircraft design from the U-2 to the Stealth fighter. Lockheed Martin plans to build and test the $1 billion wedge-shaped reusable X-33 --a one-half size, one-eighth mass version of a launch vehicle called Venture Star that would replace the space shuttle for ferrying cargo into low orbit. The target launch cost is $2,200 per kilogram--one-tenth that of a shuttle launch. At that price, space power could become cost-effective if satellites pull double-duty as communications relays and solar-power sources.

A solar power satellite should quickly pay back the energy needed to put it into orbit. Start with the conservative assumption that solar power satellite technology would produce 0.1 kilowatt of electricity on the ground per kilogram of mass in orbit. In that case, the energy expenditure of 10 kilowatt-hours per kilogram to lift the satellite into orbit would be repaid in electricity after only 100 hours--less than five days.

One way to keep launch costs down is to use an inflatable structure as the solar collector. Doing so would maximize the collector's surface area--important to gathering the greatest amount of solar energy--without imposing a major weight burden on the launch vehicle. Deflated solar collectors could be folded into a compact space on board the spacecraft; once in orbit, gas from a pressurized container would inflate the structure.

Balloons in space are an old story. In fact, the 1960-vintage satellite known as Echo I was a balloon used to bounce radio waves back to Earth. NASA is now studying the feasibility of inflatable structures in space for antennae, sunshades, and solar arrays, although not explicitly for solar power satellite systems. An important experimental milestone was the successful deployment by Space Shuttle Endeavour astronauts in May 1996 of the Spartan Inflatable Antenna Experiment--a 14-meter antenna inflated by a nitrogen gas canister in orbit.

It is not such a very large step from such an experiment to a solar-collecting satellite that could be assembled in orbit from inflated segments. Were NASA to make research on inflatable space structures a high priority, the knowledge base to make cost-effective low-mass power satellites could evolve rapidly.

At first, the solar energy relayed from space would be used only to provide the minimal electrical power needed to run the electronics of the receiving station on the ground--much the way that line current powers conventional telephones. Ultimately, the satellites would beam down larger amounts of power, which could provide the megawatts of electricity that would contribute substantially to powering a village or even a city.

Scaling up to higher power levels would be straightforward, entailing simply the deployment of a larger amount of solar-collecting area in space. Power would be transmitted through the infrastructure of transmitters and receivers that will then be in place for the satellite communications systems. In this regard, microwave transmission has a decided advantage over conventional cable methods of transmitting power. A microwave system that is 80 percent efficient at sending 1 kilowatt will still be 80 percent efficient at sending 1 megawatt. This is fundamentally different from an electric utility transmission line, where you need thicker, and costlier, wires to carry more power. If too much power is put through a cable, it will melt the insulation.

Some fear that a network of solar power satellites could turn the atmosphere into one big microwave oven, cooking whatever wanders into the beam's path. In reality, the microwave intensities that we propose would be orders of magnitude below the threshold at which objects begin to heat up. People would be exposed to microwave levels comparable to those from microwave ovens and cellular phones. While some critics speculate that microwaves pose nonthermal threats to human health, there is no reliable epidemiological evidence for adverse effects from microwaves at these low levels. Higher levels of microwave radiation would be found at the rectennas on which the beams are focused, but fences and warning signs could demarcate these areas of possible danger. But according to our calculations, microwave intensities even at the perimeter of the rectenna would fall within the range now deemed safe by the Occupational Safety and Health Administration.

A bigger potential problem is that of sharing the limited frequencies in the microwave spectrum. Motorola has come under fire, for example, because its planned system will employ frequencies in the 1.616-to-1.626-gigahertz range, which almost overlaps the 1.612-gigahertz frequency that astrophysicists tune to when gathering data about the cosmos. Radio astronomers worry that interference from a solar power satellite will overwhelm the comparatively weak signals they are seeking to detect. Motorola promises to limit spillover of its communications beams into the radio astronomers' frequency niche, but the issue underscores the fact that the microwave spectrum is a limited resource jealously guarded by commercial and nonprofit users alike. Allocation of the spectrum must be addressed promptly and effectively to avoid preemption of space power technology before it's born.

Whether solar power satellites become a reality will ultimately depend on the willingness of telecommunications and electric utility companies to enter the space power business. So far, neither industry has shown much interest. But then, they are for the most part unaware of the commercial possibilities. One has to know that an option exists to choose it. Thirty years ago, communications satellites were a novelty. Ten years ago, no one had heard of the Internet.

What is certain is that the present push for deregulation has led to a scramble on the part of telecommunications, computer, cable TV, and utilities industries to enter each others' markets. Some electric power companies want to enter the telecommunications business as a way of capitalizing on the huge investment in wire and cable that reaches virtually every building in the country. It makes equal sense to propose that communications companies enter the power business. In practice, consortiums of power and communications companies might develop the proposed technology together.

No single piece of this technology poses a fundamental stumbling block. The physics of photovoltaic cells and microwave generation are well understood. To move to the next stage, though, will require a demonstration that all the pieces of this system can work together: the solar panels, the phased-array microwave antennas, the receiving stations that separate the data signals from the power beams, and the computers that tell the satellites where on the ground to aim the beams. NASA could accelerate this development tremendously by placing into orbit a prototype of a solar power satellite.

The benefits are too large to walk away from. A network of solar power satellites such as what we propose could supply the earth with 10 to 30 trillion watts of electrical power - enough to satisfy the needs of the human race through the next century. Solar power satellites thus offer a vision in which energy production moves off the earth's surface, allowing everyone to live on a "greener" planet. Consider the philosophical implications: no longer need humankind see itself trapped on spaceship earth with limited resources. We could tap the limitless resources of space, with the planet preserved as a priceless resource of biodiversity.