Scientists are eyeing the jet stream, an energy source that rages night and day, 365 days a year, just a few miles above our heads. If they can tap into its fierce winds, the world's entire electrical needs could be met, they say.

The trick is figuring out how to harness the energy and get it down to the ground cost-effectively and safely.

Dozens of researchers in California and around the world believe huge kite-like wind-power generators could be the solution. As bizarre as that might seem, respected experts say the idea is sound enough to justify further investigation.

The jet stream typically blows from west to east 6 to 9 miles over the northern hemisphere at speeds up to 310 mph.

By lofting generators into the upper atmosphere, scientists theorize they could capture the power of the jet stream and transmit the electricity along cables back to Earth.

A wind machine, floated into such a monstrous force, would transmit electricity on aluminum or copper cables -- or through invisible microwave beams -- down to power grids, where it would be distributed to homes and businesses. Unlike ground-based wind generators, the high-altitude devices would be too high to be heard and barely visible against the blue sky.

"My calculations show that if we could just tap into 1 percent of the energy in high-altitude winds, it would be enough to power all civilization. The whole planet!" said atmospheric scientist Ken Caldeira at the Carnegie Institution's Department of Global Ecology at Stanford University.

Research into high-altitude wind machines began in the 1980s. Bryan Roberts of the University of Technology in Sydney, Australia, was an early pioneer. Working with a team of researchers, he has field-tested a small, two-rotor prototype device tethered a short distance above the ground, successfully generating the electricity from low-level winds and transmitting it to Earth.

Creating a much larger, commercially viable system envisioned by scientists would take millions of dollars of research. Scientists need to figure out the structural materials that could stand up to the jet stream's buffeting winds and find a way to adjust the generator's position as the jet stream meanders back and forth across the sky.

Perhaps more vexing is determining the appropriate size and composition of the cable that would act like the string on a child's kite to keep the machine from blowing away while it functions as an electrical transmission line.

Obstacles aside, some optimists think the jet stream could supply commercial electricity within a decade or two.

"My opinion is that 15 years from now, it'll supply most of the power in the United States," said David Shepard, a veteran Silicon Valley entrepreneur from Ramona (San Diego County), who with Caldeira and other researchers in Australia and Canada is helping Roberts plan the helicopter-like version of a wind machine.

Others, like Caldeira, are more cautious. "In the 19th century, it took 25 years for oil to replace 1 percent of the coal market," Caldeira said. "The energy infrastructure tends to evolve slowly."

Engineering aside, there will be other hiccups to work out. For instance, there's uncertainty about how much the machines or their cables would threaten birds.

"These wind turbines will fly far above most birds and would fill only a tiny fraction of the sky," Caldeira said. "Nevertheless, it may be important to find ways to warn off birds."

Another concern might be whether such technology would pose a danger to airplanes. For his part, Shepard isn't worried. He points out that U.S. authorities have maintained a fleet of tethered balloons as part of drug-traffic-tracking operations along the U.S.-Mexican border. The Tethered Aerostat Radar System, which monitors aircraft, typically floats at an altitude of 15,000 feet and planes have never collided with them.

There is a remarkable variety of designs for high-flying wind machines, some of which resemble blimps or futuristic helicopters. Others look like Alexander Calder-style mobile sculptures. An early, 240-kilowatt prototype of a wind machine could weigh 1,140 pounds and have four rotors, each of which might be 35 feet wide from tip to tip and would spin up to five times per second.

At the moment, though, only small lab prototypes used in field tests have seen the light of day.

Rafe Pomerance, president of the nonprofit Climate Policy Center in Washington, thinks the jet stream-energy idea has merit. He held a private teleconference with Shepard and his colleagues on April 30 to find out more about it.

Afterward, Pomerance, a member of the U.S. negotiating team for the Kyoto treaty on global warming and a deputy assistant secretary of state for environment in the Clinton administration, told The Chronicle that high-altitude wind power should be investigated. He said he will be looking into whether his center should do anything to find research funds from federal agencies or private investors for Shepard's team.

"We need to be investing in multiple options because global warming requires massive transformation of the global energy system," he said.

Bob Thresher, director of the U.S. National Wind Technology Center, a division of the Department of Energy, offered a more restrained view of the scientists' plans.

"There's a tremendous advantage in going up (toward the jet stream) because there's much more energetic winds," he said. But if high-altitude wind generators are to succeed, "you have to be able do it very cheaply because the cost of (ground-based) wind energy has come down so dramatically, it's becoming competitive with conventional sources."

In the March 1 issue of IEEE Transactions on Energy Conversion, a journal for electric power professionals, Roberts and six other researchers, including Caldeira and Shepard, described their plans for a prototype, 240-kilowatt flying electric generator costing "something in the ballpark of $5 million," according to Caldeira.

Some schemes are more modest.

In December, the Canadian government's Sustainable Development Technology Corp. awarded a $950,000 grant to a startup firm, Magenn Power of Ottawa, to develop its proposed MARS -- or Magenn Air Rotor System -- wind generator. Held aloft by a helium balloon, it would fly only from 600 to 1,000 feet high and tap into the brisker winds closer to the ground, where trees and topography tend to muffle breezes. Backers hope it can generate 10 kilowatts, enough to support a village of 250 people with limited electricity needs.

"In India alone there are a million villages without power," said Mac Brown, Magenn's chief executive officer. "Our target market is that village which might have 50 or 60 or 70 huts. All they want is one or two lights in a hut, an electric water pump, and a TV and VCR for the village school. And they want a refrigerator for medicine, for when the doctor stops in once a month."

Magenn just opened an office in San Francisco and is sending its representative, Tom Tansy, to Silicon Valley, where he hopes to find investors.

Is a high-altitude wind machine too good to be true? "It might be," Caldeira acknowledged. "But the way to find out is by trying. High-altitude winds are the largest concentrated source of renewable energy available on Earth. In the middle of the jet stream, the amount of power available per unit area can be 100 times more concentrated than the energy of sunlight on the surface of the Earth.

"So the idea that we're not tapping into it -- or at least investigating it -- seems crazy to me. All the energy we need is flying by, 5 miles over our heads."

TO TRY AND CATCH THE WIND

The total energy contained in wind is 100 times the power used by everyone on the planet. If we could tap into just 1 percent of that energy, scientists say it still wouldnt produce a major adverse effect on the environment. -- LADDER MILL: A loop of kites attached to a cable would generate power as it moves continuously, pushed by high-altitude winds. -- ROTOR KITE: A helicopter-like wind machine would use four or more rotors to generate electricity, up to 240 kilowatts. -- ROTATING KITE: A wind generator held aloft by a helium balloon would fly 600 to 1,000 feet high, tapping into the brisker winds closer to the ground. It could generate about 10 kilowatts, enough to support a Third World village of 250 people. -- TURNTABLE KITE: Wind-blown kites would drive a rotating turntable just as falling water turns a turbine in a hydroelectric plant.

Torrent of raw energy

The jet stream is made up of several large currents of high-speed air that rush eastward through the upper atmosphere. Six miles high, the winds have exceeded 300 mph.