Researchers have taken a small but necessary step towards realizing a long-standing dream: to collect solar energy in space and transmit it to Earth. A satellite launched in January directed power in a microwave beam at targets in space and even sent some of that power to a detector on Earth, the developer of the experiment, the California Institute of Technology, announced June 1 ( Caltech). “No one has ever done this before,” says space scientist Sanjay Vijendran of the European Space Agency (ESA). “They’re lending credibility to the argument by demonstrating this ability.”

Credibility has long been the challenge for space-based solar energy. To produce as much energy as a typical coal-fired or nuclear power plant, a satellite would need a collection area kilometers in diameter, which would require hundreds of launches and in-orbit assembly. NASA planned a demonstration mission during the 1970s energy crisis. But with the technology of the time, carried aloft by the Space Shuttle and assembled by astronauts, the mission would have cost a trillion dollars. Few took it seriously after that.

The space has changed since then. Solar cells and microwave beams are cheaper and more efficient. Robots capable of assembling structures will soon be in orbit and companies like SpaceX have cut the cost of launches. Recent studies commissioned by ESA and the UK government suggest that giant orbiting generators will soon be able to generate electricity at costs comparable to land-based nuclear power plants.

A few scattered research programs have pushed the field forward. Since the 1980s, researchers at Kyoto University have demonstrated short-distance power transmission to the edge of space using suborbital rockets. In 2020, a team from the United States Naval Research Laboratory sent a pizza box-sized “sandwich board” into orbit with solar cells on one side, a fill of electronics and microwave transmitters on the other side to demonstrate the conversion of sunlight into microwaves.

The Caltech mission, funded by the Donald Bren Foundation and Northrop Grumman Corporation, aimed to go one step further with lightweight, affordable, and flexible components. The microwave transmitter was an array of 32 flat antennas packed onto a surface area slightly larger than a dinner plate. By varying the timing of the signals sent to the different antennas, the researchers were able to steer the beam of the array. They pointed it at a pair of microwave receivers about a forearm’s length away and switched the beam from one receiver to another at will, lighting an LED on each.

The transmitted power was small, just 200 milliwatts, less than that of the light from a cell phone camera. But the team was still able to direct the beam at Earth and detect it with a receiver at Caltech. “It was a proof of concept,” says Caltech electrical engineer Ali Hajimiri. “Indicates what an overall system can do.”

The Caltech spacecraft still has two more experiments planned. One is now testing 32 different varieties of solar cells to see which survives the rigors of space best. The second is a folded piece of ultralight composite material that will unfold into a sail-like structure 2 meters in diameter. While the sail contains no solar cells, it is intended to test the kind of thin, flexible, large deployments needed for a future power plant.

Interest in solar space power seems to be gaining momentum. This year, ESA commissioned two studies on potential architectures for orbiting power plants. Vijendran says energy supply companies have joined the effort. Last month the Kyoto team announced that it will work with Japan’s space agency JAXA to test in-orbit power beaming.

Kyoto electrical engineer Naoki Shinohara says he was delighted to hear of Caltech’s success, “but at the same time disappointed that we, the Japanese, aimed to make the world’s first [wireless power transmission] satellite experiment in 2025”.

Startup Virtus Solis Technologies has also tested power beaming and plans to launch a pilot plant into orbit in 2026. CEO John Bucknell says the company plans to offer commercial power to customers by the end of the decade. “Space solar is the only clean, robust and scalable energy technology [with] a credible path to true zero carbon emissions.

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