Will a NASA-assisted diffractive solar sail take us to the Sun?

Guess we'll have to watch and wait.
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

"Think of a sailboat in space. If you've one and you're trying to sail into the wind, you've to tack back and forth. But with a diffractive sail, you would be able to sail directly into the wind, getting the most efficient thrust. And be able to explore the Sun like never before," Amber Dubill of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, tells IE about her project. 

Dubill's Diffractive Solar Sailing project was selected for Phase III study under the NASA Innovative Advanced Concepts (NIAC) program. Phase III concepts are the closest to becoming working projects, so it's just a matter of time before a spacecraft could be flying around, using super-fast solar sails to check out the Sun's poles - a hitherto impossible mission.

"We started working on the diffractive solar sailing project when I was at the Rochester Institute of Technology in New York. I started working with Dr. Grover Swartzlander, the past PI, on Phase I and Phase II of the project. Now we're at Phase III - and I've kind of taken the lead on this one to continue it all through John Hopkins," says Dubill. 

How does a solar sail work?

A spacecraft gains most of its momentum when launched from Earth. It then changes direction or increases its speed using chemical rockets that burn fuel carried on board.

After reaching its maximum speed, it maneuvers through space or relies on gravity assistance from other planets to reach its destinations.

When a solar sail is involved, the spacecraft can continue accelerating as long as it has light pushing on it. This allows the spacecraft to accelerate throughout its entire journey, reaching speeds that would be impossible with traditional propulsion.

In an interview with IE, Dubill tells us about the futuristic technology that could take us to the Sun.

Interesting Engineering: What does the award mean to you?

Amber Dubill: I've worked on three NIAC phases - two phase ones (the other being PHLOTE) and one phase two. I've been working on this since 2018. My co-investigators, Dr. Swartzlander and Les Johnson, have been great in mentoring me, and it means a lot - as someone a bit younger than your typical NIAC fellow. I used to think that it would be cool to lead a NIAC project of my own. 

So it's something that I couldn't have dreamed of. And I'm excited to be joining the other NIAC fellows at the symposium - being able to talk and show the work that we've been doing, will be pretty awesome.

IE: What got you interested in solar sails?

Traditional propulsion isn't going to get us where we need to go in the solar system. Take the ESA (European Space Agency). The solar orbiter they launched last year is flying 30 degrees above the ecliptic. You can't get much further with traditional propulsion. If you want to be able to go crazy places like the Sun, where our designated mission is, or even further out, you need solar sails. They're low thrust, but they have a very high Isp (specific impulse). Solar cells specifically are a cool problem because they're almost massless in the form of propulsion. Like, you're never going to run out of sunlight. And if you don't have to carry that fuel with you (because the light is almost massless), you can do so many cooler, efficient mission trajectories, the much more efficient mission trajectories we can do.

So, the cool thing about solar sailing for me is that it's just such a unique, solvable problem. When you add the diffractive on top of it, it's just a unique, modern take that I haven't seen before. It helps you get around a lot of the issues that reflective solar cells have. But there are challenges - the propulsion, attitude control, and the deployment side. 

IE: What are these issues?

With a reflective solar sail, the light comes in, hits the sail, and bounces off. That way, you control that momentum transfer, which is how you get your thrust. With a diffractive sail, you don't have to do that - instead, you can tailor the optics (made from small microstructures) embedded within the sail in such a way that the diffraction of incoming light causes a momentum transfer as well. The grating of the sail can be designed in a manner that it can go in the direction you want. That's where the sailboat analogy comes from.

IE: Tell us about the work in Phase I and II.

Phase one was coming up with the idea of diffractive solar sailing and proving its feasibility theoretically. So Dr. Swartzlander did a lot of work with the optics behind the diffraction gratings [A grating is an optical element that disperses light composed of lots of different wavelengths into light components by wavelength]. I came up with a simple control scheme, and the overall mission of the solar polar orbiter concept, which we identified as the most beneficial and impactful type of mission - based on heliophysics.

Because, as you get closer to the Sun, your efficiency of the radiation pressure increases. Missions that go closer to the Sun are poised to have a higher impact from this technology. So, we explored and expanded on the solar polar orbiter concept. Environmental testing on specific readings that we had designed was conducted, and an overall mission study in comparison with existing mission architecture was carried out. In Phase II, we continued radiation pressure testing on the torsional oscillator in different capacities.

IE: What about Phase III?

In Phase III, we hope to continue the concept of the solar polar orbiter but into a solar polar constellation. So you'd have 4 pi steradian [square radian] and the coverage of the Sun. So you could do solar weather monitoring - a complete solar model. And just have a continuous observation from unique vantage points. In Phase III, bringing the project to APL [Applied Physics Laboratory] links us to a well-established heliophysics community. We want to have a justification for an entire instrument suite and understand the unique viewpoints in our mission studies. And then, on top of that, APL also can manufacture/improve the gratings that we design and then help us develop a roadmap for increasing the TRL [technology readiness level].

Phase III is about transitioning out of the NIAC program. And then we also have plans with NASA Marshall [Marshall Space Flight Center] to test the gratings that we plan to manufacture at APL, with RIT's expertise in optical design. And we want to go through an entire space weather test - an environment testing campaign as we want to make sure that the gratings don't degrade over time and the optical efficiency hopefully stays relatively constant.

IE: Does the diffractive solar sail have any downsides?

Manufacturing the diffractive solar sail is a bit harder than a reflective sail - which is one of the things we hope to explore in Phase III [which involves the APL's manufacturing group]. Diffractive sails are newer, so there is inherently slightly more risk. Traditional propulsion is widely used and understood - so it is easier to model. 

IE: How close is the project to reality?

The point of the NIAC Phase III is to raise the TRL of the technology. We hope that by testing on the gratings, doing an improved design, and exploring the solar polar constellation mission architecture, we're laying the groundwork for the next phase of this technology, after NIAC, for our mission proposal or to be picked up by another mission proposal. That's the hope and timeline of this technology. Unlike the NIAC Phase I and II projects, where they're talking about 50 years, this would be out in different capacities in five or 10 years.

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