A secretive military space plane will soon test the idea of using microwave beams to send solar power to Earth from space. The U.S. Air Force's X-37B space plane is expected to launch into orbit Saturday (May 16) with an experiment on board that tests the possibility.

The Photovoltaic Radiofrequency Antenna Module Flight Experiment (PRAM-FX) represents the first orbital test of a sci-fi technology first envisioned in the 19th century — solar satellite power. Build a big solar array in orbit, the idea goes, and it could collect enough sunlight (unfiltered by atmospheric effects or clouds,) to generate a powerful beam of microwaves. A collection station on Earth would then convert that beam into useful power. Launch any satellite into a high enough orbit and it will receive a near-constant stream of sunlight, with only brief passes through the Earth's shadow. A whole constellation of solar arrays might offer uninterrupted 24/7 power.

"The idea got a lot of attention, and sort of came into its own in the late 60s, early 70s, when there became an imperative to explore energy sources other than fossil fuels ," when fossil fuel supplies became unstable and prices skyrocketed, said Paul Jaffe, a civilian electronics engineer at the U.S. Naval Research Laboratory (NRL) and leader of the NRL's beamed energy research.

That research tapered off as fuel prices dropped, Jaffe said. But in 2007, the Department of Defense picked up the baton. A satellite beam is a much safer and more efficient way of getting power to an overseas military base than convoys of fuel trucks, he said. Those trucks, stuffed with combustible fuel, can be attacked and destroyed, risking the lives of their drivers and guards. But a microwave beam passes invisibly through the atmosphere unguarded. You can't shoot at it.

With time, the beams might also power military drones, like the ones now used for spying and killing overseas. Powered by a microwave beam, the drones could buzz endlessly overhead without ever having to land to refuel.

So far, PRAM-FX can't do any of that. But it offers the NRL team a first chance to test a key component of a solar power satellite in the environment where it would eventually function.

The experimental device sandwiches its electronics between a solar array and a backplate, according to Chris DePuma, an electronics engineer at the NRL also working on the project. The solar array collects energy from the sun, converts it to a DC electric current, and then uses that current to power a 2.45 gigahertz microwave "that theoretically in the future would be transmitted out of an antenna pointed toward a receiver site," DePuma told Live Science.

For PRAM-FX's purposes though, the microwave energy lands on a coaxial cable that "dumps it off" into an instrument used to record data, DePuma said. The NRL researchers will compare that output to the energy taken in using the solar array to figure out the efficiency of their setup.

"This will inform the feasibility and the economics of something like solar power satellites," Jaffe told Live Science.

This isn't the first time these researchers have tested the equipment. Experiments in vacuum chambers on Earth, using lamps to mimic the effects of the orbital sun, have offered clues as to how PRAM-FX will operate. But there's nothing quite like being up there, the researchers said.

The secretive platform

PRAM-FX will be one of several research payloads aboard the X-37B when it launches from Cape Canaveral, Florida on Saturday. That's unusual: In its previous five missions, the Air Force didn't mention X-37B carrying scientific payloads. In its cumulative seven years and 10 months in orbit, no details about the space plane's payloads or precise purpose were ever disclosed.

This time around though, a bit more information is on offer. According to a Space Force statement, the X-37B will carry a "service module" into space with the spaceplane's first payload of scientific experiments. It will deploy a satellite known as FalconSat-8 with some experiments aboard, while PRAM-FX and another experiment will remain attached to the X-37B.

A key advantage of affixing PRAM-FX to the X-37B, Jaffe said, is that his team can take advantage of the X-37B's communications systems, propulsion, and other resources. That saves the NRL team the trouble and expense of building in all the machinery necessary for a free-floating satellite to operate. And the X-37B's orbit will offer lots of different sun angles at which to test the equipment, DePuma said.

The uncrewed space plane operates a bit like a smaller, robotic Space Shuttle — launching atop an Atlas V rocket and staying in orbit for months on end. Its previous, fifth mission lasted 780 days before the machine glided back to Earth on Oct. 27, 2019.

NRL researchers considered other possibilities for getting PRAM-FX into space, including one of NASA's space station resupply missions, before landing on the X-37B.

"We did explore a number of different hosts as possibilities, and ultimately this offered the best combination of availability for flight and ability to integrate with — since our experiment isn't well suited to being its own satellite because of its [bulky] dimensions," Jaffe said.

This won't lead to a weapon, at least according to the Department of Defense scientists

If you've played the game SimCity, you might be familiar with a fictional scenario in which the beam from one such solar satellite gets diverted, setting fire to the surrounding area. It's also easy to imagine an orbital microwave beam being used as a weapon.

While it might not be technically impossible to engineer a disaster situation, Jaffe said, it's also not likely.

"Most people hear 'microwave' and picture that thing in their kitchen that cooks things," Jaffe said.

But microwave frequencies are also used in Wi-Fi and Bluetooth systems on your phone, he said, and they aren't inherently dangerous. And they aren't a terribly efficient way to set things on fire across great distances, because they have relatively low power densities.

"A way to think about power density is if you go out in the sun on a clear afternoon you're not going to burst into flames … but in that same sunlight that won't burst you into flames if you take a magnifying glass you can use it to set something on fire," Jaffe said. "Not because you're adding energy, but because you're concentrating the energy that falls on the magnifying glass such that it falls on a very small point."

That isn't a realistic scenario here, Jaffe said.

"For microwaves, it is very difficult to focus them in the same way that a magnifying glass focuses sunlight," Jaffe said. "That's why you need these really big antennas."

The bigger the antenna you have, the higher the power density you can create on Earth. But even huge antennas, more than a few miles long, would struggle to concentrate power to dangerous levels from the high orbits necessary. 

"A microwave-based solar satellite would be very difficult to weaponize if it could even be done at all," Jaffe said.

Still, if a full constellation of solar power satellites ever does get built, he said, it will be key to design them so that they don't exceed limits on microwave power already set by radiation safety regulators to prevent cancers and fires.

In the near term, Jaffe said, this technology is being developed for the military. But down the road, he said he hopes it will lead to a futuristic clean power source that could benefit everyone — and give the U.S. a new near-monopoly over global energy supply. 

Livescience

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