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Sunday, July 3, 2022

NASA Moves Ahead With Wild Solar Sail Concept

Artist’s conception of the diffractive lightsail. The rainbow-like patterns would be similar to how CDs exhibit similar patterns when held under a light.

Artist’s conception of the diffractive lightsail. The rainbow-like patterns would be similar to how CDs exhibit similar patterns when held under a light.
Illustration: MacKenzi Martin

A project to develop diffractive solar sails has advanced to the third and final phase of NASA’s advanced concepts program. The team behind the project now has two years to further develop this unconventional means of space propulsion.

In addition to the two-year extension, the diffractive lightsails project, led by Amber Dubill of the Johns Hopkins University Applied Physics Laboratory, was awarded an additional $2 million, NASA announced today. The phase 3 funding was granted through the space agency’s Innovative Advanced Concepts (NIAC) program. With the extra time and cash, Dubill and her colleagues will now work toward a demonstration mission.

“As we venture farther out into the cosmos than ever before, we’ll need innovative, cutting-edge technologies to drive our missions,” NASA administrator Bill Nelson said in the statement. “The NASA Innovative Advanced Concepts program helps to unlock visionary ideas—like novel solar sails—and bring them closer to reality.”

The diffractive solar sails project graduated to NIAC phase 2 status in 2019. Rochester Institute of Technology engineer Grover Swartzlander led the first two NIAC phases of the project and will now continue as a co-investigator.

“I am beyond excited to be joining the imaginative and creative group of people that make up the NIAC fellows. I have actually participated in different NIACs since 2017 in various capacities, but being able to lead one is an honor that I could have only dreamed of,” Dubill wrote to me in an email. “I’ve been mentored by my [co-investigators] for years and it means a lot that they trusted me to take the lead on this one and continue developing this research at an institution that encourages this kind of innovation.”

Solar sails work by using sunlight to propel vehicles through space, similar to how wind pushes sailboats along the water. Instead of using reflective sails like the one developed by the Planetary Society, the system being proposed would use diffractive sails. A desirable attribute of diffraction is that it causes light to spread out when it travels through a small opening. Here’s how Swartzlander described the concept back in 2019:

We’re embarking on a new age of space travel that makes use of solar radiation pressure on large, thin sail membranes. The conventional idea for the last 100 years has been to use a reflective sail such as a metal coating on a thin polymer and you unfurl that in space, but you can get a force based on the law of diffraction as well. In comparison to a reflective sail, we think a diffractive sail could be more efficient and could withstand the heat of the Sun better. These sails are transparent so they’re not going to absorb a lot of heat from the Sun, and we won’t have the heat management problem as you do with a metallic surface.

Shortcomings of the conventional reflective design include sails that are big and thin. They’re also limited by the direction of sunlight, which serves to constrain either power or navigation, as you can’t have both. Diffractive lightsails, by comparison, employ tiny grates on the sail material to diffract light in all directions. As NASA says, this will allow spacecraft to “make more efficient use of sunlight without sacrificing maneuverability.” The design being proposed by Dubill could result in smaller and nimbler sails. And as a fun side effect, the sails would be rainbow-patterned, similar to how CDs look when held to light.

Under NIAC phase 1 and 2, the team designed, created, and tested various diffractive sail materials. The team also ran tests and developed navigation and control schemes specific to a future solar mission. Indeed, diffractive sails could enable a constellation of satellites in orbit around the Sun’s polar regions. Zipping over the Sun’s north and south poles, the solar satellites, with a perpetual source of propulsion, would perform unprecedented scientific observations.

“Diffractive solar sailing is a modern take on the decades old vision of lightsails,” Dubill explained in the NASA statement. “While this technology can improve a multitude of mission architectures, it is poised to highly impact the heliophysics community’s need for unique solar observation capabilities.”

Dubill described two major objectives for the next two years: developing the diffractive sail itself and developing the solar polar orbiter constellation mission with diffractive sailcraft.

“We plan to design, optimize and manufacture diffractive sail material samples which we want to characterize with optical testing and subject to space environmental testing,” Dubill told me. “In parallel, we plan to evolve the vision of a solar polar orbiter sailcraft mission by establishing optimal trajectories and attitude control of the sail to achieve the solar observations of a payload suite dictated by our heliophysicists.”

Should all go according to plan, the concept could result in an actual space-based mission and the proposed solar satellites.

“Through expanding the diffractive sail design and developing the overall sailcraft concept, the goal is to lay the groundwork for a future demonstration mission using diffractive lightsail technology.”

This post was updated to include Amber Dubill’s emailed comments.

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