Ali Hajimiri has spent a decade researching how to put solar panels in space and beam the energy down to Earth. Yet when the Caltech electrical engineering professor talks about his work, people always have three questions, usually in this order: Why not just put solar panels on Earth? Are you going fry birds in the sky? Are you building a Death Star?
Hajmiri jokes he plans to have the answers printed on a card. “I’m going to have it in my wallet to show people,” he said.
This year, Hajimiri and his team made a step towards making space-based solar a reality.
In January, they launched Maple, a 30-centimeter-long space solar prototype equipped with flexible, lightweight transmitters. The aim was to harvest energy from the sun and transfer it wirelessly in space, which they did, managing to light up a pair of LEDs.
But the “stretch goal” was to see if Maple could also beam down detectable energy to Earth. In May, the team decided to launch a “dry run” to see what would happen. On a rooftop on the Caltech campus in Pasadena, California, Hajimiri and the other scientists were able to pick up Maple’s signal.
The amount of energy they detected was tiny, too small to be useful, but they had succeeded in wirelessly beaming down power from space. “It was only after the fact that it dawned on us a little bit that, OK, well, this was something very special,” said Hajimiri.
Space-based solar may sound a wild, futuristic idea, but it is not new. As far back as 1941, it was described in a short story by science fiction writer Isaac Asimov. In the decades since, countries including the US, China and Japan have explored the idea — but for years it was written off. “The economics were just way out,” said Martin Soltau, CEO of the UK-based company Space Solar.
That may now be changing as the cost of launching satellites falls sharply, solar and robotics technology advances swiftly, and the need for abundant clean energy to replace planet-heating fossil fuels becomes more urgent.
There’s a “nexus of different technologies coming together right now just when we need it,” said Craig Underwood, emeritus professor of spacecraft engineering at the University of Surrey in the UK.
The problem is, these technologies would need to be deployed at a scale unlike anything ever done before.
What is space-based solar?
At its heart, space-based solar is a fairly straightforward concept. Humans could harness the enormous power of the sun in space, where it’s available constantly — unaffected by bad weather, cloud cover, nighttime or the seasons — and beam it to Earth.
There are different concepts, but it would work roughly like this: huge solar power satellites, each more than a mile long in diameter, would be sent into a very high orbit.
The satellite’s solar cells would capture the sun’s energy, convert it into microwaves and beam it down to Earth wirelessly via a very large transmitter, able to hit specific points on the ground with precision.
The microwaves, which can easily travel through clouds and bad weather, would be directed to a receiving antenna (or “rectenna”) on Earth made of mesh — “think of a sort of fishing net hung on bamboo poles,” Soltau said — where the microwaves would be converted back into electricity and fed into the grid.
The rectenna, approximately 6 kilometers (3.7 miles) in diameter, could be built on land or offshore. And because these mesh structures would be nearly transparent, the idea is the land underneath them could be used for solar panels, farms or other activities.
A single space solar satellite could deliver up to 2 gigawatts of power, roughly the same amount as two average nuclear power plants in the US.
An idea whose time has come?
Over the last decade, that has begun to change as companies such as SpaceX and Blue Origin started developing reusable rockets. Today’s launch costs at around $1,500 per kilogram are about 30 times less than in the Space Shuttle era of the early 1980s.
And while launching thousands of tons of material into space sounds like it would have a huge carbon footprint, space solar would likely have a footprint at least comparable to terrestrial solar per unit of energy, if not a smaller, because of its increased efficiency as sunlight is available nearly constantly, said Mamatha Maheshwarappa, payload systems lead at UK Space Agency.
Some experts go further. Underwood said the carbon footprint of space-based solar would be around half that of a terrestrial solar farm producing the same power, even with the rocket launch.
But that doesn’t mean space-based solar should replace terrestrial renewables, he added. The idea is that it could provide “baseload” power that can be called upon around the clock to fill in the gaps when the wind doesn’t blow and the sun doesn’t shine on Earth. Currently, baseload power tends to be provided by power plants running on fossil fuels or nuclear energy, which are able to operate with little interruption.
The power would be “very portable,” said Peter Garretson, a senior fellow in defense studies at the American Foreign Policy Council. It could be beamed from space to the top of Europe, for example, and then to the bottom of Africa.
Many advocates point to the potential it could offer developing countries with deep energy needs but a lack of infrastructure. All they would need is a rectenna. “It will provide real democratization of abundant affordable energy,” Soltau said.
Space-based solar could also help power remote Arctic towns and villages that lie in almost complete darkness for months each year, and could beam power to support communities experiencing outages during climate disasters or conflict.
The challenges
There is still a huge gulf between concept and commercialization.
We know how to build a satellite, and we know how to build a solar array, the UK Space Agency’s Maheshwarappa said. “What we don’t know is how to build something this big in space.”
Scientists also need to figure out how to use AI and robotics to construct and maintain these structures in space. “The enabling technologies are still in a very low technology readiness,” Maheshwarappa said.
Then there’s regulating this new energy system, to ensure the satellites are built sustainably, there’s no debris risk, and they have an end-of-life plan, as well as to determine where rectenna sites should be located.
Public buy-in could be another huge obstacle, Maheshwarappa said. There can be an instinctive fear when it comes to beaming power from space.
But such fears are unfounded, according to some experts. The energy density at the center of the rectenna would be about a quarter of the midday sun. “It is no different than standing in front of a heat lamp,” Hajimiri said.
And to build a satellite capable of doing harm to people, it would have to be many times bigger than the concepts currently being developed, Hajimiri said. “Anyone who tries to start building that, everyone else would know.”
That doesn’t mean questions shouldn’t be asked, he said. The idea is “to benefit humanity, and if it doesn’t, there’s no point.”
For some, however, the whole concept of space-based solar is misplaced.
For Lovins, promises that the system would be a great source of baseload power don’t hold up either. There are techniques to match energy demand to supply, rather than the other way around, without consumers even noticing. Having a huge power source that is producing all the time is “undesirably inflexible,” he said.
“Why spend money on something that has no chance of a business case if you succeeded, whose need will have been met before you could build it and whose most optimistic future cost estimates are the same as the current price of terrestrial solar power plus batteries?” he asked.
The future
But governments and companies around the world believe there is huge promise in space-based solar to help meet burgeoning demand for abundant, clean energy and tackle the climate crisis.
A development program able to demonstrate proof of concept is about five or six years away, Soltau said. It will then take another five or six years to industrialize and scale up the gigawatt-scale system to be fully operational.
Strong government support will be key, he said. “It’s an ambitious thing to create a brand new energy technology.”
In the US, the Air Force Research Laboratory has plans to launch a small demonstrator called Arachne in 2025, and the US Naval Research Laboratory launched a module in May 2020 aboard an orbital test vehicle, to test solar hardware in space conditions.
The China Academy of Space Technology, a spacecraft designer and manufacturer, is aiming to send a solar satellite into low orbit in 2028 and into high orbit by 2030, according to a 2022 South China Morning News report.
There’s been a burst of activity from the UK government. It commissioned an independent study which reported in 2021 that space-based solar was technically feasible, highlighting designs such as the UK-led CASSIOPeiA, a satellite 1.7 kilometers (1 mile) in diameter that aims to deliver 2 gigawatts of power. In June this year, the government announced nearly $5.5 million in funding to universities and tech companies “to drive forward innovation” in the space-based solar sector.
And Europe has its Solaris program, to establish the technical and political viability of space-based solar, in preparation for a possible decision in 2025 to launch a full development program.
“Obviously, before you build something, everything is speculation,” said Garretson, “but there are strong reasons to think that this might actually be economically possible and viable.”
Back in California, Hajimiri and his team have spent the last six months stress testing their prototype to extract data to feed into the next generation of design.
Hajimiri’s ultimate vision is series of lightweight, flexible sails, that can be rolled up, launched and unfurled in space, with billions of elements working in perfect synchronization to send energy where it is needed
He views their project as “part of this long chain of people who build upon each other’s work and help each other,” he said. “So we are taking an important step, perhaps, but it is not the last step.”