Solar power has long been coveted as an energy source for vehicles around the world—and now, NASA is one step closer to harnessing it to explore the cosmos. The Diffractive Solar Sailing Project, led by Amber Dubill of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, would enable spacecraft, like probes and satellites, to travel great distances just using sunlight. This type of lightsail would be the first of its kind.
The project was selected for the third and final phase of NASA’s Innovative Advanced Concepts (NIAC) program, which helps develop promising ideas for scientific, government, and commercial use. The team will receive $2 million to support an additional two years of development to demonstrate their technology’s effectiveness ahead of a potential mission. It’s the fifth project to ever reach Phase III stage since the program started in 2012.
Solar sails use the pressure of sunlight to propel themselves through space—much like sailboats pushed by wind—removing the need for rockets and fuel to push a craft forward. But diffractive lightsails like the one Dunbar’s team created go a step further than the conventional design of reflective lightsails. Reflective lightsails need to collect and redirect solar rays, which means they have to be coated in a metal-like film and must always face the direction of the sun. This dependency limits navigation, as there’s a constant tradeoff between energy capture and easy maneuverability. What’s more, the design of reflective sails makes them large, thin, and unstable. The necessary equipment for stabilizing and orienting the sails ultimately slows the spacecraft down.
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Diffractive sails are different. When light is diffracted through narrow openings rather than reflected over wide planes, it spreads out in different directions. On the diffractive sails, the team takes advantage of this property of light by using small gratings embedded onto the surface that can scatter light to where it’s needed, even if the sail is at a suboptimal angle or not directly facing the sun. This, in turn, allows the spacecraft to navigate more nimbly and efficiently. With this design, solar sails can be smaller, use less power, and operate at lower costs—all without sacrificing power.
Dubill likens the concept to actual boat sails. If you’re trying to steer into the wind with the equivalent of a reflective sail, you’d have to move it back and forth to go in the intended direction. If you have something more like a diffractive sail, you could use the force of the wind to blow you forward, while also hurtling straight into it.
“[This design] is the novel part. It’s more efficient and gets around previous lightsail issues,” Dubill says, adding that, in a small study they conducted, the team found that the technical effort to replace reflective lightsails with diffractive lightsails was “well worth it” and that “the benefits far outweigh the cost.”
Under Dubill’s direction, the team will improve the metallic material of their solar-ray collector and perform ground tests throughout the Phase III period. They are laying the groundwork, she says, to ultimately send a constellation of lightweight diffractive lightsails holding scientific instruments to orbit around the sun’s poles. While the NASA and European Solar Agency Solar Orbiter recently took high-resolution images of the sun, direct images of the poles have never been captured.
“There’s a lot about the sun that we don’t know. This technology can play a big role in monitoring the complexities of solar weather,” Dubill says. “[Our team] has been working on this project for so long; it’s exciting to see it have this opportunity in the future of flight missions.”
