On Saturday, the US Air Force is expected to launch its secret space plane, X-37B, for a long-duration mission in low Earth orbit. The robotic orbiter looks like a smaller version of the space shuttle and has spent nearly eight of the past 10 years in space conducting classified experiments for the military. Almost nothing is known about what X-37B does up there, but ahead of its sixth launch the Air Force gave some rare details about its cargo.
In addition to its usual suite of secret military tech, the X-37B will also host a few unclassified experiments during its upcoming sojourn in space. NASA is sending up two experiments to study the effects of radiation on seeds, and the US Air Force Academy is using the space plane to deploy a small research satellite. But the real star of the show is a small solar panel developed by the physicists at the Naval Research Lab that will be used to conduct the first orbital experiment with space-based solar power.
“This is a major step forward,” says Paul Jaffe, an electronics engineer at the Naval Research Lab and lead researcher on the project. “This is the first time that any component geared towards a solar-powered satellite system has ever been tested in orbit.”
Space-based solar power is all about getting solar power to Earth no matter the weather or the time of day. The basic idea is to convert the sun’s energy into microwaves and beam it down. Unlike terrestrial solar panels, satellites in a sufficiently high orbit might only experience darkness for a few minutes per day. If this energy could be captured, it could provide an inexhaustible source of power no matter where you are on the planet.
It’s an idea that was cooked up by the science fiction writer Isaac Asimov in the 1940s; since then, beamed power experiments have been successfully tested several times on Earth. But the experiment on X-37B will be the first time the core technologies behind microwave solar power will be tested in orbit.
“The science of microwave power beaming is fully understood; it is the engineering challenges of scaling known technology to a size never before seen on orbit that we need to progress,” says Ian Cash, the director of the International Electric Company Limited, which is developing a space solar platform called CASSIOPeiA. “But every endeavour must start with a first step.”
The experiment built by Jaffe and his colleagues at NRL is what he calls a “sandwich” module. It’s a three-tiered system for converting sunlight into electricity and then converting the electricity into microwaves. Usually, the conversion system is sandwiched between a high-performance solar panel and the antenna that is used to transmit the energy. But for this mission, Jaffe and his colleagues won’t be radiating the energy from space to Earth, because the radio signal would interfere with other experiments on the space plane. Instead, the sandwich module will send the radio signals through a cable so researchers at NRL can study the power output from the system.
The entire NRL experiment could fit in a pizza box and won’t produce enough energy to power a light bulb. But Jaffe says the experiment is a critical step toward a free-flying space-based power satellite. “There’s been a lot of work doing studies and analyses, and a lot less work on actual prototyping,” Jaffe says. “This isn’t necessarily the most refined version of what could be accomplished, but the main goal was to get up to space with a proof of concept.”
Jaffe has been working on space-based solar power for more than a decade at NRL and first unveiled his sandwich module prototype in 2014. This design was meant to solve a number of challenges that have plagued space-based solar power research for years. One of the biggest problems with the concept is that the solar panels in orbit have to be massive to collect enough sunlight to be useful for applications on Earth. Even if these structures could be built in principle, they would be incredibly expensive and challenging to launch.
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“It would be too large and cumbersome to launch a completed system,” says Chris DePuma, an NRL electronics engineer and the program manager for the experiment. “The sandwich module is a way to reduce mass and modularize the system so it could be assembled in orbit.” But before robots start building giant solar farms in space, there are a number of fundamental issues with the panels themselves that need to be addressed.
Jaffe says one of the hardest challenges has been thermal management. In space, the solar panel facing the sun may reach temperatures of up to 300 degrees Fahrenheit, while the electronics facing away from it must operate at just a few degrees above absolute zero. These electronics are just a few inches away from each other, so Jaffe and his colleagues had to figure out how to accommodate both extremes. Jaffe says this mainly involved swapping out materials and redesigning parts of the module so that the solar panel was isolated from the electronics, which operate better at lower temperatures. The upcoming X-37B mission will put this space-hardened version of the sandwich module to the test.
Flying the test on the Air Force’s secret space plane came with some compromises. If this kind of experiment was implemented on a satellite, it would be placed in an orbit where sunlight would almost always be available. But the X-37B will be flying in low Earth orbit, which means that it will pass through the planet’s shadow roughly every 90 minutes. Still, DePuma says the benefits of flying on the space plane are worth the trade-offs. “We got to focus a lot more on our experiment, rather than having to design a propulsion system and all the other things a satellite has,” he says. “It will just collect our data and send it to us in periodic distributions.”
If everything goes well, Jaffe says the next step would be to develop an experimental space solar power satellite and actually send energy from orbit to Earth. He acknowledged that this would require convincing the Department of Defense that the time and money are worth the effort. But the military is clearly interested in the technology. Last October, the Air Force Research Lab announced a $100 million program to develop hardware for a solar power satellite.
Jaffe sees space-based solar power initially enabling some unique use cases, like drones that never have to land or around-the-clock power for remote military bases. But Cash sees even bigger things in store for the technology. “Space solar power solves the biggest challenge of scaling existing terrestrial renewables, that of storage,” he says. “With the dramatic cost reductions offered by reusable space launch, space solar power could well become the cheapest source of continuous carbon-free power.”
Jaffe likes to compare the space-based solar power concept to GPS. If you told someone a few decades ago that a network of satellites loaded with atomic clocks would become the linchpin of modern society, they’d have thought you were nuts. But today, GPS guides everything from ride-share services to nuclear warheads. In fact, many of its most salient applications weren’t even imagined when the first GPS satellites were launched. Jaffe believes the same may turn out to be true for space-based solar power. Beaming solar energy from space to Earth sounds extravagant, esoteric, and borderline impossible—until it isn’t.
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