On May 22, 2023, a receiver on the roof of the Gordon and Betty Moore Laboratory of Engineering at Caltech’s campus in Pasadena detected a microwave signal beamed from orbit. The signal came from MAPLE—Microwave Array for Power-transfer Low-orbit Experiment—a small array of flexible, lightweight microwave transmitters aboard the Space Solar Power Demonstrator satellite that had launched in January. It was not a useful amount of power. Ali Hajimiri, the Caltech professor who co-directs the project, called it a detection, not a transmission. But it appeared at the expected time, at the expected frequency, with the correct frequency shift predicted by its travel distance from low Earth orbit. It was the first time anyone had demonstrated wireless energy transfer in space using flexible lightweight structures, and the first time detectable power had been beamed from orbit to Earth’s surface.
The concept is straightforward to state and staggeringly difficult to execute: put solar panels in space, where the sun shines 24 hours a day with no atmosphere, no weather, no seasons, and no night, then convert the collected energy to microwaves and beam it to receiver stations on the ground. Solar potential in orbit is roughly eight times greater per square meter than the best locations on Earth’s surface. The energy supply is effectively unlimited and perpetually available. The idea has existed since 1968, when Peter Glaser first described it. He was granted a U.S. patent for the method in 1973. Half a century later, what Caltech demonstrated on that Pasadena rooftop is still closer to a proof of concept than a power plant.
Why it’s hard
The engineering challenges cascade in every direction simultaneously, and each one is large enough to be a career-ending problem for an ordinary technology program.
Size is the first constraint. A commercially relevant space-based solar power array would need to be enormous—on the order of square kilometers. Caltech’s eventual design envisions individual units that fold into packages about one cubic meter and unfurl into flat squares roughly 50 meters per side, with solar cells on one face and microwave transmitters on the other. A full power station would be a constellation of these modular units. Some reference designs for operational systems run to 3.5 square miles of collector area in geostationary orbit, 36,000 kilometers above the equator, with similarly massive rectenna arrays on the ground to capture the incoming microwave beam. For context, the International Space Station has a total pressurized volume roughly equivalent to a five-bedroom house. The structures being discussed for space-based solar power are orders of magnitude larger.
Mass is the second. Getting material to orbit costs money, and the cost scales with weight. Caltech’s approach specifically targets this: their modular design uses ultralight, flexible structures that avoid the need for robotic in-space assembly of rigid components. The SSPD-1 prototype weighed 50 kilograms total. But scaling from a 50-kilogram demonstrator to a multi-gigawatt power station involves mass quantities of solar cells, structural materials, power electronics, and transmission hardware that must survive launch vibration, thermal cycling, radiation exposure, and micrometeorite impacts for decades of operation. China has announced plans to launch a 200-tonne space-based solar power station by 2035. Two hundred tonnes is a starting point, not a ceiling.
Transmission efficiency is the third. Microwaves spread as they travel—diffraction is physics, not engineering, and you can’t negotiate with it. The size of the beam spot on the ground is a function of the transmitter size and the frequency of the microwaves. Caltech’s MAPLE used a very small transmitter, which spread the power over a very large area, which is why the rooftop detection captured only a tiny fraction of the transmitted energy. Scaling up the transmitter reduces the spread but requires the enormous structures described above. Atmospheric absorption, weather interference, and conversion losses at both ends—electricity to microwaves in orbit, microwaves back to electricity on the ground—all take their cut. End-to-end efficiency from sunlight in orbit to electricity in the grid is a number that determines whether space-based solar is cheaper or more expensive than terrestrial alternatives, and current estimates remain unfavorable against rapidly falling ground-based solar costs.
Cost is the fourth and arguably the defining constraint. Recent deep-dive analyses commissioned by NASA and the European Space Agency have thrown cold water on near-term affordability. The ESA’s 2021 assessment suggested space-based solar power might become viable in the 2040s, but only with sustained investment and strong public-private cooperation. Launch costs have dropped dramatically with reusable rockets—SpaceX has fundamentally changed the economics of getting mass to orbit—but the total system cost for a multi-gigawatt power station, including manufacturing, launch, deployment, operation, maintenance, and ground infrastructure, remains formidable. The question isn’t whether the physics works. The question is whether the economics work before terrestrial solar, wind, and battery storage make the whole concept unnecessary.
What actually happened with Caltech’s mission
The SSPD-1 mission, which ran through most of 2023, tested three technologies. DOLCE demonstrated the deployment of a 1.8-meter-by-1.8-meter ultralight structure—the packaging and unfolding mechanism for future modular spacecraft. It deployed successfully despite two anomalies that gave the team, in Sergio Pellegrino’s words, “many new insights” into the structural challenges of ultralight deployable systems. ALBA tested 32 different types of photovoltaic cells over 240 days in orbit, including three entirely new classes of ultralight solar cells custom-fabricated at Caltech that had never been tested in space. And MAPLE demonstrated the wireless power transmission that made headlines.
The mission ended in November 2023 when the testbed stopped communications with Earth. The team’s assessment was measured: MAPLE proved the basic concept works—flexible lightweight structures can survive launch, deploy in orbit, and transmit a detectable beam to a ground station. It also revealed weaknesses. Hajimiri’s team pushed MAPLE to its limits deliberately, exposing failure modes that will shape the next generation of hardware. The mission accelerated what would normally be years of in-space testing into months, giving Caltech a feedback cycle that is unusually fast for space technology development.
Who else is working on it
Caltech isn’t alone. Japan’s JAXA demonstrated wireless microwave power transmission of 1.8 kilowatts over 50 meters on the ground in 2015. Mitsubishi Heavy Industries transmitted 10 kilowatts over 500 meters the same year. China has been building a testing base in Chongqing’s Bishan District and has announced progressively ambitious plans for space-based solar power stations, with CAST vice-president Li Ming stating China expects to be the first nation to build a working station with practical value. Researchers at King’s College London estimated in 2025 that space-based solar could provide Europe the majority of its renewable energy needs by 2050.
Aetherflux, a venture-funded startup that raised $50 million, proposed a constellation of small low Earth orbit satellites using infrared lasers instead of microwaves—a different transmission approach that allows smaller ground stations (5 to 10 meters in diameter versus the square-kilometer rectennas required for microwave systems). The company received partial support from the U.S. Department of Defense’s Operational Energy Capability Improvement Fund. Then, in December 2025, Aetherflux pivoted to space-based data centers, which tells you something about the current commercial viability of orbital power beaming.
The military angle deserves honest acknowledgment. Space-based solar power has dual-use implications that have shaped the field since the 1980s. The same technology that beams clean energy to a rectenna can, in principle, deliver directed energy to other targets. The 1980s Strategic Defense Initiative drew directly from early space solar power concepts. The 2025 Golden Dome missile defense program continues to fund orbital power generation and directed-energy transmission. The technology doesn’t just solve a clean energy problem. It creates a weapons capability. Both of these facts are part of the engineering reality.
Where it stands
Space-based solar power is the kind of technology that is simultaneously inevitable and permanently premature. The physics is unambiguous: space has more solar energy than Earth’s surface, and microwaves can transmit it through the atmosphere. The engineering is real: Caltech beamed detectable power from orbit to Pasadena. The economics are punishing: every analysis that accounts for full system costs, launch logistics, maintenance, and the pace of terrestrial renewable development concludes that the timelines are measured in decades, not years.
The honest framing is that space-based solar power is a hedge—a technology worth developing because there are plausible futures in which terrestrial renewables plus storage aren’t sufficient to meet global energy demand, and because the enabling technologies (lightweight structures, wireless power transmission, low-cost launch) have applications beyond solar power even if the full system never reaches commercial viability. It is not the next solar panel. It’s the generation after that, waiting for the cost curves to cross.
Caltech detected a signal on a rooftop in Pasadena. That’s a first. The distance between a detected signal and a powered civilization is approximately the same distance as the distance between the first telegraph and the internet—not a difference in kind, but a difference in scale that might take longer than the optimists think and less time than the skeptics assume.
We cover space-based solar power alongside fusion energy, quantum computing, and the full landscape of civilization-scale moonshot technologies across our Moonshot 2169 course—including why the best solar panel location on Earth isn’t on Earth.