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Solar Sailing: The Beauties of Diffraction

Knowing of Grover Swartzlander’s pioneering work on diffractive solar sails, I was not surprised to learn that Amber Dubill, who now takes the idea into a Phase III study for NIAC, worked under Swartzlander at the Rochester Institute of Technology. The Diffractive Solar Sailing project involves an infusion of $2 million over the next two years, with Dubill (JHU/APL) heading up a team that includes experts in traditional solar sailing as well as optics and metamaterials. A potential mission to place sails into a polar orbit around the Sun is one possible outcome.

[Addendum: The original article stated that the Phase III award was for $3 million. The correct amount is $2 million, as changed above].

But let’s fall back to that phrase ‘traditional solar sailing,’ which made me wince even as I wrote it. Solar sailing relies on the fact that while solar photons have no mass, they do impart momentum, enough to nudge a sail with a force that over time results in useful acceleration. Among those of us who follow interstellar concepts, such sails are well established in the catalog of propulsion possibilities, but to the general public, the idea retains its novelty. Sails fire the imagination: I’ve found that audiences love the idea of space missions with analogies to the magnificent clipper ships of old.

We know the method works, as missions like Japan’s IKAROS and NASA’s NanoSail-D2 as well as the Planetary Society’s LightSail 2 have all demonstrated. Various sail missions – NEA Scout and Solar Cruiser stand out here – are in planning to push the technology forward. These designs are all reflective and depend upon the direction of sunlight, with sail designs that are large and as thin as possible. What the new NIAC work will examine is not reflection but diffraction, which involves how light bends or spreads as it encounters obstacles. Thus a sail can be built with small gratings embedded within the thin film of its structure, and the case Swartzlander has been making for some time now is that such sails would be more efficient.

A diffractive sail can work with incoming light at a variety of angles using new metamaterials, in this case ‘metafilms,’ that are man-made structures with properties unlike those of naturally occurring materials. Sails made of them can be essentially transparent, meaning they will not absorb large amounts of heat from the Sun, which could compromise sail substrates.

Moreover, these optical films allow for lower-mass sails that are steered by electro-optic methods as opposed to bulky mechanical systems. They can maintain more efficient positions while facing the Sun, which also makes them ideal for the use of embedded photovoltaic cells and the collection of solar power. Reflective sails need to be tilted to achieve best performance, but the inability to fly face-on in relation to the Sun reduces the solar flux upon the sail.

The Phase III work for NIAC will take Dubill and team all the way from further analyzing the properties of diffractive sails into development of an actual mission concept involving multiple spacecraft that can collectively monitor solar activity, while also demonstrating and fine-tuning the sail strategy. The description of this work on the NIAC site explains the idea:

The innovative use of diffracted rather than reflected sunlight affords a higher efficiency sun-facing sail with multiplier effects: smaller sail, less complex guidance, navigation, and attitude control schemes, reduced power, and non-spinning bus. Further, propulsion enhancements are possible by the reduction of sailcraft mass via the combined use of passive and active (e.g., switchable) diffractive elements. We propose circumnavigating the sun with a constellation of diffractive solar sails to provide full 4π (e.g., high inclination) measurements of the solar corona and surface magnetic fields. Mission data will significantly advance heliophysics science, and moreover, lengthen space weather forecast times, safeguarding world and space economies from solar anomalies.

Delightfully, a sail like this would not present the shiny silver surface of the popular imagination but would instead create a holographic effect that Dubill’s team likens to the rainbow appearance of a CD held up to the Sun. And they need not be limited to solar power. Metamaterials are under active study by Breakthrough Starshot because they can be adapted for laser-based propulsion, which Starshot wants to use to reach nearby stars through a fleet of small sails and tiny payloads. The choice of sail materials that can survive the intense beam of a ground-based laser installation and the huge acceleration involved is crucial.

The diffractive sail concept has already been through several iterations at NIAC, with the testing of different types of sail materials. Grover Swartzlander received a Phase I grant in 2018, followed by a Phase II in 2019 to pursue the work, a needed infusion of funding given that before 2017, few papers on diffractive space sails existed in the literature. In a 2021 paper, Dubill and Swartzlander went into detail on the idea of a constellation of sails monitoring solar activity. From the paper:

We have proposed launching a constellation of satellites throughout the year to build up a full-coverage solar observatory system. For example a constellation of 12 satellites could be positioned at 0.32 AU and at various inclinations about the sun within 6 years: Eight at various orbits inclined by 60 and four distributed about the solar ecliptic. We know of no conventionally powered spacecraft that can readily achieve this type of orbit in such a short time frame. Based on our analysis, we find that diffractive solar sails provide a rapid and cost-effective multi-view option for investigating heliophysics.

Image: The new Diffractive Solar Sailing concept uses light diffraction to more efficiently take advantage of sunlight for propulsion without sacrificing maneuverability. Incidentally, this approach also produces an iridescent visual effect. Credit: RIT/​MacKenzi Martin.

Dubill thinks an early mission involving diffractive sails can quickly prove their value:

“While this technology can improve a multitude of mission architectures, it is poised to significantly impact the heliophysics community’s need for unique solar observation capabilities. 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.”

A useful backgrounder on diffractive sails and their potential use in missions to the Sun is Amber Dubill’s thesis at RIT, “Attitude Control for Circumnavigating the Sun with Diffractive Solar Sails” (2020), available through RIT Scholar Works. See also Dubill & Swartzlander, “Circumnavigating the sun with diffractive solar sails,” Acta Astronautica
Volume 187 (October 2021), pp. 190-195 (full text). Grover Swartzlander’s presentation “Diffractive Light Sails and Beam Riders,” is available on YouTube.

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{ 11 comments… add one }
  • Alex Tolley June 7, 2022, 14:30

    I didn’t realize that the force on the solar sail was always perpendicular to the plane of the sail. This makes sailing “crosswind” harder as the sail orientation becomes increasingly edge-on to the sun. This is rather different from the orientation of 45 degrees that is often depicted.

    The case for diffraction sails seems very convincing if true. They should have a better performance for some missions than reflective sails, such as the solar observation mission that the youtube video proposes.

    I look forward to seeing the proposed CubeSat mission take flight, hopefully, sooner rather than later. Just what is the possible timeline for the successful completion of the NIAC phase III report and the mission launch?

    • Ron S. June 8, 2022, 7:39

      In a vacuum there’s no rudder or hull to use to lever the craft on the medium. So all we have is sail dynamics alone. A rigid sail also performs differently than one that billows and luffs. In a way solar sail dynamics are simple to understand, and perhaps surprisingly so, because there are only a few variables at play.

      • Alex Tolley June 8, 2022, 12:18

        Perhaps I should have made myself clearer.

        For some reason, I had in my head that the force was in the direction of the reflected beam only. It is the wrong mental model based on a misinterpretation of the windmill blade rotation forces from the wind.

        I am aware that a solar sail orientation to reduce its orbital velocity around the sun needs to angle the sail so that the net force has a component that reduces the velocity allowing the sail to fall towards the sun. My false model was that a 45-degree sail orientation was providing all that retarding force in the direction of the orbit. In reality, there is a tradeoff between the orientation of the sail, the area that receives the solar photons, and the resultant force on the sail. I am guessing the optimum angle is closer to 45 degrees to offer the best compromise between the impinging photon flux and the net force component reducing the forward, orbital velocity of the sail. [I think that might be a simple BoE calculation.]

        The video made the force on a reflecting solar sail, and the diffraction sail very clear.

        • Ron S. June 8, 2022, 14:00

          I hadn’t intended to come across as critical. I understood your comment. I sought to add to it by mentioning that critical difference between a solar sail and sails we commonly encounter. With a medium to lever against we can have propulsion in a direction quite different to that of the force on the sail. That can lead one to falsely assume that the force on the sail is different from what it truly is, whether in space or on a sailboat.

          • Alex Tolley June 8, 2022, 15:28

            Thanks. I suspect that the fixed position of a windmill may have been part of the wrong mental model, even as I understood the drag issue that would tend to push the windmill backward.

            When I was at school, the sailing lessons used small sailboats with daggerboards. If it was raised with the sail up, one could see and feel the hull skipping sideways across the water with the wind, which stopped with the board down and then the boat would travel forward (mostly!).

    • Grover Swartzlander June 8, 2022, 9:09

      The NIAC Phase III runs from Oct 2022 – Sept 2024. In the short term a very small demonstration mission may be possible within 5 years. A full scale mission will require additional time to scale up the manufacturing and explore the packaging and deployment of a diffractive sail.

  • Alex Tolley June 8, 2022, 13:55

    The video was interesting for the use of different diffraction panels across the sail surface that offered the potential for steering and stability, especially with beamed energy. This seems an improvement over the approach IKAROS used, which in turn was, I think, superior to the mechanical use of small sails at the boom tips of earlier US designs.

    I would certainly like to read more about the use of metamaterials in sail design. The suggestion in the video is that electro-optic control of the diffraction would allow full control over the diffraction characteristics at any point [area?] on the sail surface to optimize stability, orientation and thrust. [The idea of using such sails as more robust alternatives to reaction wheels for pointing space-based telescopes was interesting, as they could extend the useable life of the instrument.]

    I think some metamaterials can flip states from reflection to diffraction. If that was possible for sails, then the sail in diffractive mode could make the transfer from Earth to the Sun for a sundiver maneuver, then switch to reflection once passed perihelion.

    On Earth, sailing ships lost their commercial advantage to steamships, especially once fuel bunkers were established to extend the range of such ships. This is not so applicable in space for rochets, which makes sails still one of the better solutions for the transport of payloads both within the system and for an interstellar flight using beamed assists. That such sails might look like rainbows to the observer just enhances the “romance” of such propulsion.

    • Harold Shaw June 8, 2022, 16:57

      I just finished Alistair Reynold’s Revenger trilogy. A great space opera with hard enough science. Best of all, the world is filled with solar sailing ships.

      A sail that applies a force perpendicular to its plain sounds like a keel. I must be missing something. Why couldn’t a ship with both sail types sail around a star gathering velocity?

      • Alex Tolley June 9, 2022, 9:59

        I need to write better. For a solar sail, the force acts on the sail similarly to a square-rigged sail running before the wind. (Is that perpendicular to the plane (surface) of the sail?)

        As Ron states above, there is no medium that allows a “keel” to react against. Within a magnetosphere there is the possibility of having an electromagnetic “keel”, but that isn’t the case in most of the solar system.

        Why couldn’t a ship with both sail types (…)

        This was why I raised the speculation that maybe there was the potential of metamaterials to do this for a sail. Whether the types of metamaterials are suitable for this IDK, but it would be interesting if the sail could change between reflecting and refracting depending on its position and direction of travel. [A little like the use of aerofoil sail shapes for sailing in a crosswind, to the parachute-like spinnakers for running downwind.]

  • Mike Serfas June 9, 2022, 20:06

    Advantages: a) can present its full surface to the sun but send the light sideways. b) might be tuned by some electronic change to the metamaterial without changing its actual orientation. c) can absorb less light than a mirror? (that one surprises me because mirrors can be *very* reflective)

    Disadvantages: a) We are going to get very tired of having space probes named after My Little Pony characters.

    • Alex Tolley June 9, 2022, 21:49

      As long as they name at least one Bifröst. I cannot think of a more appropriate name.

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