OMEWHERE, Nikola Tesla is smiling.

More than a century ago, Tesla - as famous for his discovery of alternating current as for his claim of inventing a giant death ray - dazzled onlookers by sending bolts of electricity crackling 30 feet through the air. To him this was proof that one day information and electricity would be sent across the skies instead of through copper cable.

	Figure 1. (11/6/03) Source: website:

Since then, Tesla's intellectual descendents have fantasized about, and dabbled in, the possibility of reliably transmitting power without wires. After decades of on-again, off-again experimentation, this Tesla-inspired dream is now showing signs of becoming real, at least in a modest way.

In September, in a hangar in Huntsville, Ala., NASA engineers flew a small propeller-driven model plane powered from the ground by a beam of laser light. The Army, meanwhile, is looking to finance research into laser-charged drone aircraft. And Boeing engineers have already built a tiny lunar rover that runs on laser-transmitted energy.

All of those projects rely on a roundabout way to transmit power through the air. First, a laser converts electricity into photons of light and beams it to the plane or other vehicle. Then photovoltaic (light-collecting) cells on the receiving end convert the photons back into electricity.

Although photovoltaic cells can capture sunlight on their own and turn it into electricity, they do not do so particularly efficiently, said David Bushman, a NASA scientist. They use only thin slices of sunlight's wide spectrum, so they waste 80 percent or more of the energy that they collect.

Laser light, however, shines in very specific frequencies, not broad bands, and when tuned right can give cells efficient energy doses.

In the recent NASA test, four years in the making, Mr. Bushman and his NASA colleague Robert Burdine used lasers in the infrared range. By shining a beam on the photovoltaic cells mounted on the 11-ounce plane, they were able to power the craft from 60 feet away.

That would have been extremely tough to do in the not-so-distant past. But after years of improvements, lasers are more powerful than ever, so the scientists were able to use a beam that provided a sufficient number of photons, even from 60 feet. In coming tests, Mr. Burdine and Mr. Bushman plan to stretch that gap to over a mile.

"The reason we didn't do this 20 years ago is because laser technology wasn't there," Mr. Bushman said.

In time, lasers might grow powerful enough to help keep solar-powered unmanned aerial vehicles aloft for months at a clip. Aerospace visionaries believe that these drones could one day serve as communications arrays - cell towers in the sky, more or less.

But right now, "there's no solar-powered plane out there that can store enough energy to get it through the night," said John Del Frate, a NASA solar aircraft project manager.

Lasers could fill in the gap, recharging the planes' batteries with beams of focused light.

"There'd be no need to pull them out of the sky or into the shop to refuel," said Keith Braun, an engineer with the Army's Advanced Energy Armaments Systems Center at Picatinny Arsenal in New Jersey. His group is looking to finance power-beaming experiments in the coming year.

Lasers are already being used to send and receive data, Mr. Braun noted. So the same beam that powers up the drone could also transfer instructions and data.

The same principle could work in space. Solar energy collectors sent to the moon could send power by laser to robotic rovers exploring lunar craters, for example.

That concept was roughly demonstrated last year on Mount Haleakala in Hawaii by Richard Fork, a professor of engineering at the University of Alabama in Huntsville, and Mark Henley, a Boeing engineer. They used a small 50-watt laser to provide power for an 18-inch rover 60 feet away, sending the beam through a series of relay mirrors.

For his next trial, Mr. Henley wants to put the rover on a nearby island, 40 miles from Mount Haleakala. That will be the farthest that power has ever been sent wirelessly, he said, adding that it could happen in a year "if the government funding and the political will are there."

That has long been a central issue for experiments in power beaming, ever since a Raytheon scientist, William Brown, first sent a crude helicopter aloft with microwaves in 1964. NASA and the Departments of Defense and Energy have all since intermittently committed money to wireless power transmission.

The outsize ambitions of power beaming advocates may be one reason that investment in such research has not been steady. From powering a lunar base to providing energy for an ion-engine spacecraft to creating a wireless power grid in the sky, no project has been too mammoth to contemplate.

In the late 1970's, and again in the late 90's, NASA took a look at building a series of satellites that would collect solar power and beam it down to earth. But while "the science is relatively easy" for such a project, said John Mankins, the chief technologist for NASA's space flight division, "the engineering is really, really hard, orders and orders bigger and more challenging than anything anyone's done before."

Mr. Burdine and Mr. Bushman's next power transmission trials, scheduled for this month, are at the opposite end of the scale. First, they will fly their model plane outdoors at the Army's Redstone Arsenal in Huntsville, providing energy from more than a mile away. Then, in an echo of William Brown's pioneering efforts, they will power a 16-inch triangle-shaped helicopter-like craft with a one-kilowatt laser beam inside the vast, 360-foot-high Dynamic Test Stand at the Marshall Space Flight Center in Huntsville.

That should give the plane enough power to fly up and down the length of the structure. (This test must be conducted in a tightly proscribed space, Mr. Burdine said; the Air Force does not allow most lasers of significant power to be fired into the sky for fear that the beams might damage spy satellites.)

The test stand has been the scene of some of NASA's most spectacular trials. The massive hydraulics inside the steel structure violently shook the Saturn V rocket, the Skylab space station and the first space shuttle, for example, to make sure they could withstand the rigors of escaping the atmosphere.

Mr. Bushman and Mr. Burdine's test won't be nearly as dramatic. But they are hoping that their early flights will be the start of something just as big.