The European Space Agency is exploring a unique way to dramatically cut carbon emissions by tapping sunlight closer to the source.
WHETHER YOU’RE COVERING deserts, ugly parking lots, canals, or even sunny lakes with solar panels, clouds will occasionally get in the way—and every day the sun must set. No problem, says the European Space Agency: Just put the solar arrays in space.
The agency recently announced a new exploratory program called Solaris, which aims to figure out if it is technologically and economically feasible to launch solar structures into orbit, use them to harness the sun’s power, and transmit energy to the ground.
If this concept comes to fruition, by sometime in the 2030s Solaris could begin providing always-on space-based solar power. Eventually, it could make up 10 to 15 percent of Europe’s energy use, playing a role in the European Union’s goal of achieving net-zero carbon emissions by 2050. “We’re thinking about the climate crisis and the need to find solutions. What more could space do to help mitigate climate change—not just monitor it from above, as we’ve been doing for the past few decades?” asks Sanjay Vijendran, who heads the initiative and plays a leading role in the agency’s Mars program as well.
The primary driver for Solaris, Vijendran says, is the need for continuous clean energy sources. Unlike fossil fuel and nuclear power, solar and wind are intermittent—even the sunniest solar farms sit idle the majority of the time. It won’t be possible to store massive amounts of energy from renewables until battery technologies improve. Yet according to Vijendran, space solar arrays could be more than 90 percent efficient. (The remaining 10 or so percent of the time, the Earth would be directly between the sun and the array, blocking the light.)
The program—unrelated to Stanisław Lem’s sci-fi novel with the same name—is considered a “preparatory” one, meaning the ESA has already completed a pilot study, but it’s not yet ready for full-scale development. It calls for designing an in-orbit demonstration of the technology, launching it in 2030, developing a small version of a space solar power plant in the mid-2030s, and then scaling it up dramatically. For now, ESA researchers will begin by investigating what it would take to robotically assemble the modules of a large solar array, for example while in geostationary orbit at an altitude of about 22,000 miles. This way, the structure would remain continuously above a particular point on the ground, regardless of the Earth’s rotation.
For the project to go forward, Vijendran and his team must determine by 2025 that it’s indeed possible to achieve space-based solar in a cost-efficient way. NASA and the Department of Energy explored the concept in the 1970s and '80s, but sidelined it because of the expense and technological challenges. Still, much has changed since then. Launch costs have dropped, mainly thanks to reusable rockets. Satellites have become cheaper to mass-produce. And the cost of photovoltaics, which convert sunlight into electricity, has fallen, making solar power in orbit more competitive with terrestrial energy sources.
There’s another hurdle, though: How do you get all that energy down to the electricity grid? One could use laser beams, but clouds would block them. Instead, Vijendran and his colleagues think converting the electricity to microwave radiation is the way to go. Those waves would seamlessly pass through the atmosphere without much energy loss. But because a microwave beam gets bigger over vast distances, and the transmitter would be so high up, that would mean building a rather large—and therefore costly—receiving station on the ground, probably one that’s more than a square kilometer. The array in orbit would be significant, too, with the whole thing possibly weighing thousands of tons—much larger than the International Space Station. “This would be the largest structure put in orbit by humanity,” Vijendran says.
But researchers are considering other designs too. For example, they could deploy three or more smaller arrays in a medium Earth orbit. Instead of functioning at a fixed point in the sky, as a single geosynchronous satellite would, they would form a relay. Each time one array rotated out of transmission range, another would take its place and continue to beam down energy. This could allow for nearly uniform, predictable solar power, gathered at multiple locations on the ground. It would also allow for smaller receivers, since the arrays would be closer to Earth, says Sergio Pellegrino, co-director of the California Institute of Technology’s Space Solar Power Project, which is complementary to Solaris.
For a technology demonstration, on January 3, Pellegrino and his team launched a modified Vigoride spacecraft built by the space transportation company Momentus. It includes three experiments: Alba, which tests different kinds of photovoltaic cells; Maple, which tests wireless microwave power transmitters; and Dolce, which tests the deployment of a lightweight structure. “You bundle up this whole thing and launch a whole set of them, and then create a constellation in space. By integrating all of the pieces, we project that it is possible to do this at a cost that is essentially the same as for electrical power now produced on Earth,” Pellegrino says. They estimate that this design could generate electricity at $0.10 per kilowatt-hour.
Researchers working on Caltech's Dolce device. COURTESY OF CALTECH
Other groups have made advances with space-based solar too, including the Space Energy Initiative. The London-based organization, a partnership between the UK government, researchers, and industry, got to work following a 2021 report that recommended proceeding with a study of space solar power. “We realized the government was going to find it difficult to pursue such an ambitious concept without seeing that industry, and particularly the energy sector, was strongly behind it,” says Martin Soltau, co-chair of the initiative.
Soltau and his colleagues are developing a satellite concept called CASSIOPeiA. Its design features collectors that always point at the sun, and it can accommodate an elliptical orbit, which can come closer to the Earth than a circular one. It’s possible to pull off such a configuration with four or five smaller satellites at a lower cost than a bigger complex higher up, he says. In addition, SEI is working on bolstering its financial support beyond the UK government: They’re currently in talks with potential international partners, including Saudi Arabia.
And other organizations are in the space solar mix too, including Northrop Grumman and the Air Force Research Laboratory, which are partnering to study its potential use for military purposes. Japan’s space agency has a solar program, and so does China’s, which plans to run tests using the country’s new Tiangong space station.
Deploying a bunch of these structures in orbit raises plenty of questions and possible concerns. Astronomers have drawn attention to the reflectivity of satellites that have begun transforming the night sky, like those in SpaceX’s sprawling Starlink network. These could potentially cause problems for astronomical imaging and change people’s views of constellations. But solar engineers say they intend for their arrays to absorb sunlight; if they end up reflecting anything, it would be a sign they’d been designed poorly.
And there could be some worries about the use of microwave beams; some countries have been studying directed energy lasers as possible weapons against spacecraft. While the low-intensity beams needed for space solar could not damage anything or anyone, the arrays would need a particular range of dedicated frequencies so that they don’t create spectrum interference with other satellites or radio telescopes. They might need their own orbital slots too, to manage space traffic and avoid collisions.
Still, if it works, and within a couple of decades solar arrays are orbiting and delivering gigawatts of energy to the ground, it could pay big dividends. It could supplement other forms of clean energy and be part of a solution to climate change—and it’s much closer to becoming a reality than industrializing fusion energy, for example. Pellegrino points out that the related technologies are mature enough to move it past the theory stage, and into building and testing hardware. “This is an area of tremendous opportunity and promise,” he says.