NASA-supported diffractive solar sail will let you see the sun like never before

"A modern take on the decades-old vision of lightsails".
Deena Theresa
Diffractive solar sails, depicted in this conceptual illustration, could enable missions to hard-to-reach places, like orbits over the Sun’s poles.MacKenzi Martin/NASA

Thirty-six-year-old German astronomer, Johannes Kepler, was one among the many who watched a comet arch gently across the sky. The year was 1607, and no one knew that a comet had a 75-year orbital period.

The genius he was, Kepler wondered if sunlight was heating the comet - spreading its tail into a wide strip.

He had no way to prove his hypothesis - but it was a correct observation. The way the Sun's rays interacted with a celestial object led Kepler to believe that a space sail might capture sunlight the same way a boat sail catches the wind.

He wasted no time.

In a 1608 letter to Galileo Galilei, Kepler wrote that the humans might one day use the technology to reach for the stars:

“Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.”

Enter diffractive lightsailing

Centuries later, solar sails were successfully built and launched by NASA with their NanoSail-D spacecraft, the Planetary Society with their LightSail 1 spacecraft, and the Japanese Aerospace Exploration Agency (JAXA) with their IKAROS spacecraft.

However, existing reflective solar sail designs are very large and very thin, limited by the direction of the sunlight, forcing tradeoffs between power and navigation.

Now, diffractive lightsailing would extend solar sail capability beyond what's possible with missions in development today. This innovative idea was selected by NASA for Phase III study under the NASA Innovative Advanced Concepts (NIAC) program, as per a press release.

How do solar sails help a spacecraft?

A spacecraft gains most of its momentum when it is launched from earth, and then changes direction or increases its speed using chemical rockets that burn fuel that is carried on board.

It then maneuvers through space by reaching its maximum speed, or relies on gravity assists from other planets to get to their destinations.

When a solar sail enters the picture, the spacecraft can continue accelerating as long as it has light pushing on it. This would accelerate the spacecraft through its entire course, reaching speeds that would be impossible for chemical rockets to achieve.

In the aforementioned project, diffractive lightsails would use small gratings embedded in thin films to take advantage of a property of light called diffraction, which causes light to spread out when it passes through a narrow opening.

This would allow the spacecraft to make more efficient use of sunlight without sacrificing maneuverability.

Transforming space tech

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

The new Phase III award will give the research team $2 million over two years to continue technology development in preparation for a potential future demonstration mission. The project is led by Amber Dubill of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

The feasibility of the concept was previously studied under NIAC's Phase I and Phase II awards, led by Dr. Grover Swartzlander of Rochester Institute of Technology in New York, who continues as a co-investigator on the project. Les Johnson, lead for two of NASA's upcoming solar sail missions at NASA's Marshall Space Flight Center in Huntsville, Alabama, also is a co-investigator.

Under the earlier awards, the team had designed, created, and tested different types of diffractive sail materials; conducted experiments; and designed new navigation and control schemes for a potential diffractive lightsail mission orbiting the Sun's poles.

Close to the Sun

Under Phase III, the sail material will be optimized and ground tests will be performed in support of the conceptual solar mission.

Conventional spacecraft propulsion has its limits when it comes to achieving orbits passing over the Sun's north and south poles. Meanwhile, lightweight diffractive lightsails could easily place a constellation of science spacecraft in orbit around the Sun's poles and improve our space weather forecasting capabilities.

"Diffractive solar sailing is a modern take on the decades-old vision of lightsails. 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," said Dubill.

Phase III NIAC projects are close to becoming real projects, which means the possibility of a spacecraft moving around with solar sails to explore the Sun like never before, is around the corner.

"NIAC allows us to foster some of the most creative technology concepts in aerospace," said Mike LaPointe, acting program executive for the NIAC program at NASA Headquarters. "Our goal is to change the possible, and diffractive solar sailing promises to do just that for a number of exciting new mission applications."

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