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Harnessing the sun’s energy tens of thousands of miles away from Earth to ultimately feed it into your local power distribution grid is a mighty engineering task that, if successful, would allow humanity to have access to uninterrupted clean power. 

Let’s break down step-by-step how space-based solar power would work. 

“The concept of space solar power is extremely simple and elegant,” said former NASA physicist John Mankins who’s been working on the topic for decades and even wrote a book about it. 

“On Earth, the sun shines only during the daytime. In the winter, it shines quite little and in the summer you can have overcast weather for days and weeks,” he said. “But in space, the sun is always shining.” 

From Earth to space 

Accessing those constant sun rays would involve sending a large satellite into space to about 22,000 miles (36,000 kilometers) above Earth — a distance known as the geostationary orbit where the satellite moves at the same speed as Earth, appearing fixed from the ground — and then attaching tens of thousands of solar panels to it. 

The panels would capture power from the sun that would be converted into microwaves. Those microwaves would then be beamed back down to a receiving antenna — or ‘rectenna’ — on Earth and reconverted into electricity.

Artistic rendering of Space Solar’s concept, CASSIOPeiA, beaming energy back to Earth in the form of microwaves. Image shared courtesy of Space Solar.

A variety of designs could work. A solar-power satellite might have wings of mirrors to redirect sunlight onto photovoltaic panels, like Mankins’ proposal, or two huge mirrors that similarly concentrate sunlight onto photovoltaic panels, like a proposal from Space Solar, a company backed by the government of the United Kingdom. 

Whatever the design, these satellites would be huge.  

Space Solar’s proposal, known as CASSIOPeiA, would be one mile (1.7 kilometers) in diameter, dwarfing the International Space Station (ISS), currently the largest structure in space at about 356 feet (110 meters). It would have 60,000 layers of power modules the size of coffee tables to collect enough sunlight to power about one million homes on Earth, or roughly two gigawatts of energy. 

A space power plant that large would require about 100 launches of materials from Earth and would have to be assembled in orbit using robots controlled remotely, which has never been done before. 

“In-space assembly is going to be absolutely the next technology that unlocks the huge economic revolution in space,” said Martin Soltau, co-CEO of Space Solar. 

Solar panels designed for use in space would be made of materials that can withstand harsh conditions and be more efficient to harness the entire solar spectrum (the range of electromagnetic radiation emitted by the sun). Solar panels on Earth capture mostly visible sunlight. Silicon, the workhorse of Earth-bound solar, would also work in space but it would be less efficient than materials like gallium, a light silver metal used in many semiconductors.  

More efficient terrestrial solar panels also exist but they are expensive and can’t compete economically with the standard silicon ones.

John Mankins’ space solar satellite proposal, known as SPS-ALPHA Mark-IV. Concept and illustration provided courtesy of John C. Mankins.

About 90% of the energy collected by a solar satellite would be lost in the conversion to electricity on Earth. That sounds high, but around 80% of the energy from terrestrial solar installations is lost when converting sunlight into electricity. 

The key difference is that sunshine in space “is available all the time,” said Mankins. 

In fact, according to Soltau, a solar panel in space would generate 13-times more energy than a terrestrial solar panel in Northern Europe and 6- to 7-times more than a panel in sunny regions like the Middle East. 

From space to Earth 

Soltau’s company showed earlier this year, with help from scientists at Queen’s University in Belfast, that it’s possible to have a satellite constantly collecting rays from the sun while also beaming the energy back to a fixed point on Earth. Soltau called it a “world first.” 

That fixed point on Earth would be rectenna similar to a giant flat soccer net stretching over about eight miles (13 kilometers) like a wire mesh arrangement. It would be 80% transparent, could be installed on land or over water, and could even go over farmland if installed five to 10 meters (16 to 32 feet) above ground, Mankins said.

The rectenna would be made up of hundreds of millions of receptors, similar to the ones in our mobile phones that receive radio signals, said Sanjay Vijendran, who leads the space solar program at the European Space Agency (ESA), on a recent podcast. 

It could be hard to find space for these rectennas in densely populated parts of the world, like Europe, Vijendran said. He also expects public resistance to such large structures, although his agency is making efforts to educate the public now to reduce opposition down the road. 

A life cycle assessment by the University of Strathclyde in Scotland in 2022 found space-based solar would have a carbon footprint half that of terrestrial solar, mainly because the amount of clean energy delivered during a satellite’s lifetime, which the researchers put at 30 years, would compensate for the carbon-intensity of setting it up and running it. Terrestrial panels last about 25 to 30 years. 

There are still a lot of uncertainties, like how to operate such a huge structure in space, said David Ferguson, head of net zero innovation at EDF U.K., a subsidiary of the Paris-based energy company EDF Group. Ferguson describes himself as an optimistic skeptic; he believes space-based solar will happen but will probably take longer than people think. 

“You have to build this billion-euro thing in space and a billion-euro thing on land at the same time, and they have to be ready at the same time,” said David Ferguson. “On paper, it looks amazing. But it’s not ready today.”