Light-induced Propulsion of Graphene-on-grid Sails in Microgravity

ESA-backed researchers demonstrate the laser-propulsion of graphene sails in microgravity. In an article recently published in Acta Astronautica, they report a scalable design that minimizes the overall sail mass and hence increases their thrust upon light irradiation. In addition, they prove the new sail concept by accelerating prototypes in a free-fall facility with 1W-lasers, reaching up to 1 m/s2. This milestone paves the way for lightweight ultralarge sails and eventually may help us to reach other star systems in a human lifespan.

Above – Graphene light sail of 3mm in diameter with a mass of 0.25 mg ‘sets sail’ when pointed with a 1W laser. The prototype has a graphene micromembrane design that reduces the overall mass while keeping functional the complete area of the sail.
Credit Dr. Santiago Jose Cartamil-Bueno

“Graphene is part of the solution”, says Dr. Santiago J. Cartamil-Bueno, SCALE Nanotech’s director and leader of GrapheneSail team. “We demonstrate a novel sail design that reduces the overall sail mass by using perforated films. By covering the holes with CVD graphene, the full area of the sail is again available for optical performance at minimal mass cost. The fabrication is relatively simple and could be easily scaled up to squared kilometers, although the in-space deployment of such a giant sail will be a serious challenge”.

researchers gained access to the ZARM Drop Tower in Bremen (Germany), in order to test the graphene sails in space-like conditions. Here, experiments are performed in a free-fall capsule that ensures a high-quality microgravity environment (<10-6 g) for few seconds. When the sail prototypes of small sizes were weightlessly floating, they were irradiated by 1W lasers and started to move with accelerations up to 1 m/s2.


• Light sails based on graphene have a reduced mass density that boosts their thrust.
• Graphene sails were accelerated with a laser in vacuum and microgravity.
• Microgravity enables sail release with reduced clamping effects during take off.
• Delays on the light-induced displacement may be caused by material desorption.
• 2D sails pave the way for reaching other star systems in a human lifespan.

27 thoughts on “Light-induced Propulsion of Graphene-on-grid Sails in Microgravity”

  1. I wonder if it would build up with particles in space, being colder space far away. Then build up more and more to become a comet. That due to gravity and swirling and such of the solar system. Would just return back to earth in several thousands of years.

  2. “in-space deployment of such a giant sail will be a serious challenge” Perhaps an early step in ISRU will be to use the *resource* of not needing to fold for launch, or withstand the gs of launch, avail in the destination, Space. Merely assembling launched parts in orbit may so simplify the eventual product production that it pays to start it very soon. Then, as material from Moon or asteroids becomes avail, the assembly can start incorporating that stuff for simple structure, or fuel.

  3. the consequence of the math definitely puts a damper on things.

    Maths is always doing that. Total killjoy.

  4. I am not arguing the details, but offer a more *form* sort of observation. “WAY too thin an fragile to directly tie to the payload.” means that the forces are too great, that the sail is working TOO well, in a way. Sort of like the circular detonation rocket, an opportunity.

  5. In 2020 it may actually be cheaper to send the test sails into space.
    I wonder how much spacex charges per milligram?

    Of course, you need the laser and the measuring equipment too. But if you used the laser as a lidar to measure the microsail acceleration via doppler analysis of the reflected light…
    I would not be surprised if you could fit the entire experiment into a 1 kg cubesat that takes say… one PhD project to assemble, launch, analyze the results…

    Science and engineering advances much faster when the subject matter is small and cheap enough to break down into PhD and Masters project sized lumps.

  6. What’s the speed of a solar-sail interstellar probe? Nothing to write home about, I’d imagine. We need to find better candidates.

  7. Which is how the idea to capture this stuff on the way and use it for reaction mass comes about. Q: Is this true if not moving fast, just hanging in the Solar system on light pressure? Or is the .1 c the main problem? Are these atoms scooting fast anyway?

  8. Yes, I should have limited my proposal to larger, in Solar system context. Habs. Near term stuff. “puttering around the inner solar system”. Even the refurl process would be better off using the mass for propulsion or something, unless it is being saved for reuse before it is degraded.

  9. I do NOT share your enthusiasm on regeneration.  Better just to furl-it-back-in whilst underway. Like a spinnaker in a hurricane. Got to take the big sail out of harm’s way.  No other real choice.  

    You might want to consider also the “energy budget” of a sub-ton (all in!) spacecraft, and the idea of continuously regenerating anything on board. All regeneration equipment is yet-another-parasitic-mass, either lowering the vital payload, or decreasing ultimate velocity.  Or both.  

    When mission profiles extend past a hundred years or so, the attention-span of Humans kind of votes against enthusiastic embrace of millennial scale low-probability-of-success missions.  

    ⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  10. They do, and they don’t. (BTW, thank you for the ⊕1 on other comments. Manna appreciated!)

    The real issue is that — insofar as I can tell — the USEFUL physics constraints on the sailcloth and spacecraft are really, really close to requiring both magic and unobtainium.  

    Graphene is lovely stuff, but is transparent. Not good for reflective sails. Needs layers of OTHER materials, coatings, patterned for single-wavelength laser beams (minimizing mass-to-area ratio).  All doable. But the backing stuff needs to be held by a tougher matrix.  

    Think 19th century hot-air balloons. You know, the thin canvas bags enclosed by a lightweight ‘fishnet’ mesh and ropes to suspend the gondola. Why that format?

    Because the gossamer bag’s cloth wasn’t strong enough to directly hold the mass of the payload. The ‘fishnet-and-lanyards’ efficiently distributed the payload mass to smallish sections of gasbag film.  

    On a micro-scale, the same needed for the interference-dichroic mirror film supported on monomolecular films of graphene.  WAY too thin an fragile to directly tie to the payload. Needs (probably) multiple cascades of ‘fishnet and lanyards’. Nanoscale to microscale. Micro to centimeter scale. Centimeter to meters, and meters to hectometers. And there to dozens of kilometers.  

    After cascading such, how really does one expect to constrain the mass of the sailcloth, still allowing a significant payload?  Man… magic and unobtainium.

    ⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  11. See my replies herein, to various other commenters. While I know full-well that you are quite enthusiastic about sailcloth powered interstellar and interplanetary flight, there are … issues. Not insignificant issues, either. My comments aren’t really meant to temper your enthusiasm, but the consequence of the math definitely puts a damper on things.

  12. Yah. 
    Except (again, as always) the unhelpful math. 

    Googling (or 40 years back, having a university astrophysics larnin’), one finds or remembers that the interstellar medium has a mean density of 1 atom/cm³. 10⁶ atom/m³. 10⁶ ÷ 6.022×10²³ = 1.6×10⁻¹⁸ mol/m³. 

    Keep that handy.  

    Our sailcloth is what, 100 mg/m²?  Not quite unobtainium, but still … mean molecular mass of what, 25 g/mol?  OK. 100 mg = 0.1 g/m². 0.1 ÷ 25 = 0.004 mol/m².   

    Our velocity is what, 10% c?  30,000,000 m/s. Each m² of sailcloth then ‘runs into’ 300,000,000 × 10% × 60 × 60 × 24 × 10⁶ atom/m³ = 2.6×10¹⁸ atom impacts per day, per m², at 10% c. That’s gotta hurt.  

    2.6×10¹⁸ atom ÷ 6.022×10²³ atom/mol = 4.3×10⁻⁶ mol/m² per day. But our sailcloth is only 0.004 mol/m², so … 0.00108 of the sailcloth atom impacts per day. 930 days to 100% equal impacts-to-sailcloth atoms. 

    Kind of puts it in perspective, doesn’t it?  Impacting the ISM would be very significant ‘drag’ on the free-flight of the spacecraft. At least if sailcloth mass dominates the whole craft mass. 

    ⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  13. Maybe it is because I’m getting old (AKA ‘impatient’), but so far, to date, I remain vexedly skeptical of solving the triple problem space of “light sails to nearby stars”.  

    № 1 — broadband vs laser flux
    № 2 — sum-of-power vs. mass to attain %c speed
    № 3 — laser-pumped wavelength scaling & sail heating

    № 1, we want ambient sunlight, without conversion. “get close to star, and then unfurl the sail! Whee!!!”

    Making super-lightweight yet significantly broadband sailcloth: We’re talking mg/m², AND reflectivity > 75% AND absorption < 2%. … AND not to collapse when at full flux.  

    However, with laser, it is much easier to achieve very low areal density. But then the problem is pushed to needing the pumping laser to vary its output wavelength by perhaps 100×, as the starcraft flies away. Being a laser-savvy goat, I can tell you, that is a TALL order. 

    № 2 — how little USEFUL mass is realistic? From there, what MINIMUM velocity to Any-Centauri is viable as a science criterion? From there, over what distance will the sail be illuminated? … from those, how much power to beam? And also, how big a ‘mirror’ to collimate the beam over that distance?

    № 3 — is kind of related to № 2, because it sets limits on how much power can be beamed.  

    Anyway, enough.
    I remain skeptical.

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  14. kind of … except without ‘the application environment’ the criteria-set is kind of open ended.  

    For instance, if reflecting a laser beam which nominally has one particular wavelength (the simplest-to-produce case), having an interference mirror of exactly that wavelength minimizes absorption, maximizes reflectance and potentially minimizes mass. Only one wavelength.  

    The ‘problem’ then becomes Doppler shift as the interference sail gains speed. In its frame of reference, the laser wavelength is stretched, longer and longer. There quickly becomes a mismatch between its sail reflectance sweet spot and the beam. The only real solution is to project a laser beam with ever shorter wavelength, to compensate appropriately. 

    That’s 1 case. 

    The 2nd is “reflect starlight” to whatever degree possible, absorbing little. This would be for a sun- (or star-) sail leaving Sol, or approaching Any-Centauri.  

    Now the grating needs to be quite broad-band, to work well. Good luck to that, in context of also saving mass.  

    It is a soap-covered-water-balloon problem. Squeeze anywhere, and the rest tries hard to escape.  

    ⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  15. yep: VERY likely that evaporative or ion-charging effects dominate. Especially without extraordinary equipment in play. Which costs a bundle, and is finicky no matter who is using it.

  16. wasn’t there recent research stating basically that lightsails depend more on the quality of their diffraction grating surface rather than the used materials per se…

  17. From the abstract of the actual (paywalled) article

    The measured thrust is one order of magnitude larger than the theoretical calculations for radiation pressure alone. This calls for further theoretical studies and attracts interest on graphene as light-sail material.

    So clearly they did the math too and were equally puzzled.

    I’d be suspicious of something short term and short range like
    — laser is evaporating off adsorbed gases from the graphene surface, making a simple laser thermal rocket (but only for a fraction of a second).
    — laser is knocking off electrons producing a charge that electrostatically pushes the microsails

    Something like that.

  18. Something is mathematically not right.  

    For a “perfect reflector”, the force from reflected light is

    F = 2 P / c or
    F = 2 × watts / speed of light. 

    Also, basic physics says:

    F = ma or, 
    F = kg • m/s²

    Both ‘F’ are newtons, so:

    2P/c = ma … with P (watts), m (kg), and c (300,000,000 m/s), find ‘a’:
    2 × 1 W / ( 300,000,000 × 0.250 mg × 0.001 g/mg × 0.001 kg/g ) = a
    a = 0.027 m/s²

    Only 37.5x off from the article’s 1 m/s². However, being off by 37.5x … is a bit HUGE.

    So… I wonder which is off.  
    The milligrams of the whiff of graphene, or the power?

    Here’s a theory: the actual acceleration(s) were MUCH lower, but still easily observable. The researchers, using the measured accelerations, then back calculated what the acceleration might have been had the backing-and-all-the-rest not been part of the equation. Subtracting out the test frame.  

    Calculate a free-floating whiff of the nano-pored stuff would deliver 1 m/s² per 1 W of material. Not at ¼ milligram, but a much lower mass.  Maybe 6 micrograms.  

    Alternately, perhaps the experimenters used the little whiff as a reflector, and whilst pointing 1 W of laser light at it, reflecting it back to a spherical mirror, which would re-focus it onto the target … and back, and forth, a bit less eery time. Wouldn’t take much to multiply the cavity laser field by 30× to 40×.  

    It makes weak data clear.

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  19. Light sails could make it economical to transport valuable commodities (water, oxygen, iron, regolith, etc.) from the orbits of large asteroids in the asteroid belt to cis-lunar space.

    Light sails could also make it economical to transport valuable commodities from low Mars orbit (carbon dioxide, nitrogen, hydrogen, chlorine, salt originally from the surface of Mars), Mercury (hydrogen and carbonaceous materials originally from the poles of Mercury), and Venus (carbon dioxide and nitrogen from the upper atmosphere of Venus using PROFAC technology) to cis-lunar space.

    The idea of using light sails to transport telescopes into orbit around the triple star system of Alpha Centauri within a human life time would also be exciting since there are humans alive today who might live to see that.

  20. Once the acceleration phase is complete (ie. once they are out past say Neptune) then the sail itself has finished its job so who cares if it gets shredded?

    Unless you are heading for another star and are planning to use the sail to slow down again. In that case, yes, you would prefer the sail stayed functional.

  21. Right now sun powered light sails are the only viable option for an interstellar probe. Get it as close to the sun as possible, unfurl it, and let is rip. Use a small sail to tack it sunwards. And then open the big one. Practice in the solar system. When you are ready build the big one for Proxima Centuria.

  22. I would think they would be continuously re done or expanded instead of allowed to fall apart and be lost matter, or even junk. Even 10% holes is not too bad.

  23. Not convinced that light/energy sails have much value in long-term usage – especially at high-fractions of c for multi-year extra-solar system journeys or heavy usage puttering around the inner solar system – unless we are talking 1s or 10s of inches. Though not tested much, high velocity particles and other accretion items will likely shred these adorable, gossamer things well before they can reliably hit a multi-LY target.

  24. Indeed. These items are meant to be made in space.

    Kilometre wide light sails are too frail to be launched from Earth and are the kind of technology enabling visits to the Solar gravity lens in a few years instead of several decades, and later can be used for making the first interstellar probes taking less than a century to go to Proxima.

    But not only that, they could serve as thrusters for mining drones and anything that doesn’t need to accelerate and arrive fast, but requiring to have autonomy and very little fuel usage.

  25. “The fabrication is relatively simple and could be easily scaled up to squared kilometers, although the in-space deployment of such a giant sail will be a serious challenge” Or, make them there!

  26. Lasers schmasers, we don’t have huge space based lasers and will not for some time.
    What can this do with the huge light source we already have available?
    Acceleration of solar sail = 2 P.A/(c.m)
    P = solar light energy w/sq.m
    A = area = pi 0.003^2/4 = 0.000007 sq.m
    c = 300 000 000 m/s
    m = mass = 0.25 mg

    Acc = 1.9 E-10 P

    So the acceleration at 1 AU from the sun, P = 1300, acceleration = 2 cm/s per day.
    Might want to pack a lunch.

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