CNN  — 

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.

Originally a space solar skeptic, Hajimiri’s interest was piqued when he started looking more closely at the idea. “On average, you get about eight times more power in space” compared with solar on Earth, he told CNN. The beam won’t kill animals either. And as for the Death Star? The beam won’t be powerful enough to be weaponized, he added.

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.

Because of the colossal size of these structures, they would be made up of hundreds of thousands of much smaller, mass-manufactured modules, “like lego bricks,” Soltau told CNN, which would be assembled in space by autonomous robotic assembly machines.

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.

With the International Space Station and the tallest building in the world, the Burj Khalifa, shown for scale, this illustration demonstrates how massive the CASSEioPia array would be.

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.

Illustration - Space-Based Solar Power involves transforming solar power into electricity via photovoltaic cells in geostationary orbit around Earth. The power is then transmitted wirelessly in the form of microwaves at 2.45 GHz to dedicated receiver stations on Earth, called 'rectennas', which convert the energy back into electricity and feed it into the local 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?

There’s “nothing science fiction” about space-based solar, Underwood, the UK professor, told CNN. The technology is mature, he said. “The big stumbling block has been simply the sheer cost of putting a power station into orbit.”

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.”

She gives the example of the Burj Khalifa in Dubai, the talles