PASADENA — A clean energy dream once dismissed as "sheer fantasy" became reality on May 22, 2023. Scientists at the California Institute of Technology (Caltech) successfully transmitted solar energy collected in space back to Earth in the form of microwaves, which were received on a rooftop at Caltech's campus in Pasadena.

This breakthrough marks the first time humanity has completed a full-chain, wireless space-to-ground power transmission from orbit. The journal Science called it "bringing credibility to the field of space solar power."

From Microwave Ovens to a Space Power Station: A "Giant Leap" of 200 Milliwatts

The historic experiment was carried out by the "Microwave Array for Power-transfer Low-orbit Experiment" (MAPLE), part of the Space Solar Power Demonstrator (SSPD-1) mission. The device, about the size of a lunch tray, consists of an array of 32 flat antennas operating at an altitude of roughly 550 kilometers above Earth.

The experiment showed that MAPLE successfully transmitted 200 milliwatts of electricity in the form of microwaves. Although that amount of power is only equivalent to the energy of one or two small watch batteries — far less than even a smartphone camera's flash — the research team confirmed that the signal precisely illuminated an LED light on the ground receiving station.

"It took a few minutes for everyone to realize what had happened, and then we were all ecstatic," said Ali Hajimiri, co-director of the project and professor of electrical engineering at Caltech. He emphasized that the technology demonstration proved that precise steering of a microwave beam can be achieved electronically without the need for expensive mechanical components.

A Century-Scale Bet by a Philanthropist

Behind the SSPD-1 mission is what could be called a "century-scale bet" in philanthropy. In 2011, Donald Bren, chairman of the Caltech Board of Trustees and chairman of the Irvine Company, was inspired after reading a popular science article. Through his foundation, he went on to commit more than $100 million to the project.

"This is not only a scientist's dream, but a cause for all of humanity," Bren said in a statement praising the team's efforts. Northrop Grumman also provided $12.5 million in research funding.

Conquering the Challenges of "Flexibility" and "Shadows"

In addition to the successful MAPLE transmission, SSPD-1 is also tasked with two other key experiments aimed at addressing core challenges in building space solar power stations.

The first is DOLCE, a composite deployable structure. To collect enough energy in space, future power stations may require deployable structures hundreds of meters or even kilometers in scale. DOLCE is testing an ultralight, foldable sail-like structure — success would solve the problem of "stuffing a large structure into a rocket."

The second is ALBA, a photovoltaic module test. The space environment is filled with radiation and extreme temperature swings, and not all solar cells perform well there. ALBA is testing 32 different types of photovoltaic cells to identify those with the highest efficiency and greatest durability under extreme space conditions.

Meanwhile, research into optimizing the "support beams" for ground-based photovoltaic plants is also progressing in parallel. Multiple studies in 2023 have pointed out that if the backside of conventional bifacial photovoltaic modules is shaded by support beams, power generation gains are significantly reduced. Research by Japanese scholars and solutions released by companies like Jinko Power Technology have shown that by optimizing the support beam structure (e.g., adjusting beam spacing), backside gain loss can be cut by half or even eliminated.

Competition and the Future: A Global Race Has Begun

Caltech's success has ignited global enthusiasm for space solar power. A team at Kyoto University in Japan plans to conduct similar orbital tests by 2025, while U.S. startup Virtus Solis aims to launch a pilot plant in 2026 and achieve commercial power delivery by 2030.

Although the current transmission power is small and there is still a long road ahead to building the gigawatt-scale stations (requiring antenna arrays kilometers wide) needed to power a city, as Ali Hajimiri put it: "We have a long way to go, but that's exactly what makes this exciting."

The era of wireless energy may have quietly begun.