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The Path toward an Aerographite Sail

I’ve focused on aerographite these past several days because sail materials are a significant determinant of the kind of missions we can fly both in the near-term and beyond. The emergence of a new ‘contender’ to join graphene as a leading candidate for deep space missions is worthy of note. Whether or not this ultra lightweight material produced by teams at the Technical University of Hamburg and the University of Kiel lives up to its promise will depend upon a thorough investigation of its properties as adapted for sails, one which has already begun.

Sail materials matter because we have already begun flying spacecraft with these technologies, so that as we climb the learning curve in terms of design and engineering, we need to be thinking about how to increase performance to allow ambitious missions, and perhaps even audacious ones like Breakthrough Starshot, though the authors of the first paper on aerographite for sails are skeptical about whether the material could withstand the demands of the Starshot laser push.

Aerographite seems to allow strikingly fast missions using solar photons alone — the paper discusses reaching Pluto orbit in less than 5 years, for example — but the authors of this paper are fully aware of the number and complexity of the issues that need to be addressed to make such a thing happen. The idea is to advance the concept, bat the ideas around, learn from laboratory experiment, and try this relatively inexpensive material out in space, starting probably with near-term projects on the International Space Station and going from there.

Image: This is Figure 1 from the aerographite discovery paper. Note: TEM = Transmission electron microscopy. The caption: Overview of different Aerographite morphologies by controlled derivations of synthesis. a) Photograph of macroscopic Aerographite. b–d) 3D interconnected structure of closed-shell graphitic Aerographite in different magnifications and TEM inset of wall. e–h) Hierarchical hollow framework configuration of Aerographite in different magnifications. i–l) Other variants of Aerographite. i) Aerographite network in low aspect bubble-like configuration. j-k) Aerographite with nano porous graphite filling. l) Hollow corrugated pipe design of Aerographite surface by detailed adoption of template shape. Credit: Mecklenburg et al.

Among the issues I didn’t have time to discuss yesterday is the sail’s absorption of photons from a high-energy beam, like Starshot’s projected ground-based array. An aerographite sail that absorbs solar photons may have trouble under an intense beam, considering that the material melts at 400 C — remember, we are talking about a black sail that works through absorbance rather than reflectivity. 400 C is a figure that the Hamburg team measured under atmospheric conditions — consider it a combustion temperature, as Alex Tolley reminds me, rather than a melting point — so we would still need to learn about the situation in space. This would be a matter of simple experiment but a crucial issue to resolve if we’re interested in beamed sails.

A larger problem emerges with the fact that aerographite is an electrical as well as a heat conductor. That means an aerographite sail will accumulate charge from solar UV radiation or possibly the solar wind, as the paper is quick to note. Launching an aerographite sail from low Earth orbit could result in deflection of the sail’s trajectory by Lorentz forces induced by the Earth’s magnetic field. Interplanetary and interstellar magnetic fields pose a challenge depending on the mission.

Thus we have a navigation issue to investigate, one that would likely need to be resolved, says lead author René Heller (Max Planck Institute for Solar System Research, Göttingen) by an active, autonomous on-board computer “…and some form of photon “wings” or “rudders” to trim the sail.” The effect of charge upon aerographite can be viewed in a video: https://www.youtube.com/watch?v=Oh8skH1oQDE&feature=emb_title.

Heller said in an email discussion that before we start talking about interstellar missions, we need to figure out how to move an aerographite sail in predictable ways:

At this point, we are introducing the basic equations and some sample trajectories to show that interstellar escape is possible in principle. But steering is very complicated. There are so many unknowns that would affect a sail trajectory such as interplanetary magnetic fields (leads to deflection if the sail gets electrically charged, e.g. by cosmic particle hits), the solar wind, interstellar magnetic fields, limited knowledge of the actual position and velocity (“proper motion”) of the target star etc. that I can’t see at this point how one could passively steer an aerographite hollow sphere or cone or parachute web – whatever – to a star at 4 light years. That’s why we entitled our paper an “interstellar precursor”. Even aiming at Mars would be hard with a passive sail, which is why we talk about reaching the orbit of Mars and the orbit of Pluto rather than Mars and Pluto themselves.

Structural reinforcement may turn out to be necessary given the material’s relatively low tensile strength. All of these matters need investigation, but the beauty of aerographite is that it is available for demonstrator work near Earth, as co-author Pierre Kervella adds:

…an interstellar spacecraft would likely be very different from the presented concept. The spherical shell could be a very valuable demonstrator in the vicinity of the Earth. Once we have convincingly shown that interstellar velocities are realistically within reach, it will likely change the research landscape and boost the innovations in ultra-light materials. Aerographite has such remarkable properties, but it was not developed at all for being a solar sail! There is certainly a large margin for improvement through a dedicated research effort.

A black sail presents interesting challenges when we want to track it from Earth. We have an infrared signature to work with that would be compromised by the absorption of atmospheric water vapor, making space-based observation the key. The sail’s effective temperature will drop as it recedes from the Sun, making the wavelength of its peak emission increase. The authors calculate that the James Webb Space Telescope could track such a sail, and demonstrate this by calculating its temperature in thermal equilibrium with absorbed sunlight.

The result: JWST observations of an aerographite sail of 1 m radius are possible out to 2 AU. A sail 10 m in radius can be observed to 3 AU. But we have other options as we move into the outer Solar System. A swarm configuration of sails as discussed yesterday could, the authors believe, be tracked in deep space by ALMA (the Atacama Large Millimeter/submillimeter Array) at distances of 1000 AU and beyond as the sail’s temperature drops to 10 K.

We’re hoping, of course, for a communicative spacecraft, one relying on ultra lightweight instrumentation. Ideally we would want to implement a laser on-board to remain in contact with Earth. From the paper:

Miniaturization of electronic components has made great progress in the last few decades, but we focus on mass margins above 1 g because we do not expect sub-gram margins to be relevant for the foreseeable future. Commercial lithium-ion batteries weighing a few grams and with power densities > 1 kW kg−1 (Duduta et al. 2018) as well as ultra-high-energy density supercapacitors with power densities of ∼32 kW kg−1 (Rani et al. 2019) are already available, allowing energy emission of a gram-sized power source of 32 W in theory.

And again:

A laser sending the proper time of the sail to Earth would allow distance and speed measurements through the relativistic Doppler effect. Measurements of gravitational perturbations (Christian & Loeb 2017; Witten 2020) under consideration of dust and gas drag as well as magnetic forces exerted from the interstellar medium (Hoang & Loeb 2020) could also be used to search for the suspected Planet Nine in the outskirts of the solar system. Its expected orbital semimajor axis is between about 380 AU and 980 AU (Batygin & Brown 2016; Brown & Batygin 2016).

An interesting thought! René Heller mentioned this as well in an email, talking about the prospect of hundreds or thousands of aerographite sails, each with a gram-sized on-board transmitter to allow tracking of distance and speed relative to Earth. A reconstruction of their individual trajectories could be used to look for gravitational perturbations leading to the putative Planet Nine. Remember, this is a low-cost material. The authors estimate that meter-sized aerographite spheres with a thickness in the μm range could be produced in large numbers for roughly $1000 US. Breakthrough Starshot is using a per-sail cost estimate of $100 US.

Let’s look long-range again and consider an interstellar implication. If aerographite did allow a mission to, say, Proxima Centauri with a travel time of less than 200 years, would there be any way to decelerate it upon arrival? Obviously we could brake against the star’s light, but the problem with Proxima is that its light is relatively weak, and deceleration would be negligible. Heller and Hippke have in the past considered using Centauri A and B as buffers for deceleration, with the residual kinetic energy absorbed by Proxima Centauri itself (see By ‘Photogravitational Assists’ to Proxima b).

But I’m getting way out in front in going this route. What we need now is something we can deploy in the near-term, and here it’s conceivable that aerographite may become valuable even for a laser-beaming project like Breakthrough Starshot. From co-author Guillem Anglada-Escudé (Institut de Ciencies Espacials, Barcelona):

…we are aiming at something we can deploy immediately, at a low budget. As you know, space is very slow in demonstrating technology. But most unknowns can be sorted out if we can make it fly instead of theorising about it for two decades. That’s the spirit. All knowledge on operating sails is of high value anyway. The micro-instrumentation that Starshot needs can be installed, tested and begin producing science with these sails on Solar System exploration for example.

A rousing prospect, that. An early hollow sphere aerographite demonstrator with a diameter in the range of a few meters might be brought into space as a piggyback add-on to an existing interplanetary mission, adding little mass given the lightweight nature of the material. Add to the low cost of aerographite itself the fact that deep space missions, conceivably interstellar ones, can be implemented using solar photons alone, without the need for a massive laser installation and all the issues ground-based laser beaming introduces, and the economic justification for pursuing this research becomes obvious.

Moving a small sample of aerographite with light in the laboratory is the next step in the research, a set of experiments now being devised. Breakthrough Starshot should be keeping an eye on this research.

The paper is Heller, Anglada-Escudé, Hippke & Kervella, “Low-cost precursor of an interstellar mission,” Astronomy & Astrophysics 7 July 2020 (abstract / preprint). The aerographite discovery paper is Mecklenburg et al., “Aerographite: Ultra Lightweight, Flexible Nanowall, Carbon Microtube Material with Outstanding Mechanical Performance,” Advanced Materials Vol. 24, Issue 26 (12 June 2012). Abstract.

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{ 19 comments… add one }
  • Thomas Mazanec July 30, 2020, 13:09

    These would be good for solar gravitational focus missions.

  • Andrew Palfreyman July 30, 2020, 14:56

    High absorbance and low melting point are precisely the characteristics one does NOT want from a photon sail.

  • Eniac July 30, 2020, 15:30

    One very serious problem with absorptive sails is that no transverse force can be generated, i.e. your sail cannot ‘tack’ like reflective sails can. In fact, I cannot think of a good way to steer such sails at all, absent a second form of propulsion.

    • Bryan Kelly August 3, 2020, 8:46

      For a capability like that, you’d need to both absorb and emit, effectively turning a sail moving away from the photon source
      into a pump moving toward the source, which in my opinion, will be a far more efficient mode of space propulsion.

      Materials already exist for this, e.g., the Vantablack nanotube configuration and hydrogenated graphene, known as graphane, display the kind of absorbance and radiant flux properties needed to achieve it.

      • Eniac August 4, 2020, 15:23

        True, but simply using a reflective material would be much better. Twice the thrust, excellent tacking ability.

  • charlie July 30, 2020, 16:02

    “An aerographite sail that absorbs solar photons may have trouble under an intense beam, considering that the material melts at 400 C — remember, we are talking about a black sail that works through absorbance rather than reflectivity. 400 C is a figure that the Hamburg team measured under atmospheric conditions, so we would still need to learn about the situation in space.”

    400°! Whoa ! That’s a low melting point substance even under atmospheric conditions! I am a bit surprised since usually graphite is considered a high temperature substance which can resist quite a bit of heat no matter what the source.

    To me moving close to the sun seems about equivalent to having yourself bombarded by high-intensity laser so I’m surprised that there’d be any particular difference between the two in terms of the reaction of the material to the radiation. Doesn’t it seem like a laser source and a intense sunlight are roughly equivalent?

    • Paul Gilster July 31, 2020, 12:26

      The figure of 400 C seems low to me as well, though I have it from email communication with the authors. I’m double-checking with them about it — could have been a typo. Let me see what they say.

      • Eniac July 31, 2020, 22:19

        Aerographite is molecularly the same as graphite. Graphite does not melt, like diamond it sublimates st 3600 C, which seems close enough to 4000 to perhaps explain a typo? Alternatively, lets not forget carbon is combustible. 400 C might be the temperature at which the material burns in air, which would not be an issue in space.

        • Paul Gilster August 1, 2020, 6:25

          Apparently not a typo, as a check with one of the paper’s authors confirms. The 400 C is what the Hamburg group came up with, adding that performance in a vacuum might be different. So I think you’re right, Eniac, to notice that combustibility of carbon, not a problem in space.

          • Alex Tolley August 1, 2020, 12:13

            So the 400C is really a combustion temperature, not a melting point. A quick google indicates that diamond combusts at about 850C.

            As the Parker Solar probe experiences 1200C at perihelion and the solar sail authors suggested that was a possible starting point for an interstellar mission, it is the melting/subliming point of graphite that is relevant in space, not the combustion temperature.

  • Michael Fidler July 31, 2020, 2:55

    Static charge can be converted into usable energy, the simplest is to charge a battery or capacitor. This makes me wonder if a part or smaller sphere in the larger main could be used as a super capacitor to power the electronics and transmitter.

    Another option is “Straintronics” the ability to engineer piezoelectricity into graphene! This could be used to convert the sphere into a disc or have it sprout wings.

    Straintronics: Engineers create piezoelectric graphene.

    “They modeled graphene doped with lithium, hydrogen, potassium and fluorine, as well as combinations of hydrogen and fluorine and lithium and fluorine on either side of the lattice. Doping just one side of the graphene, or doping both sides with different atoms, is key to the process as it breaks graphene’s perfect physical symmetry, which otherwise cancels the piezoelectric effect. The results surprised both engineers.
    We thought the piezoelectric effect would be present, but relatively small. Yet, we were able to achieve piezoelectric levels comparable to traditional three-dimensional materials,” said Reed. “It was pretty significant.”

    Designer piezoelectricity.
    “The researchers were further able to fine tune the effect by pattern doping the graphene — selectively placing atoms in specific sections and not others.
    We call it designer piezoelectricity because it allows us to strategically control where, when and how much the graphene is deformed by an applied electrical field with promising implications for engineering.”

    https://engineering.stanford.edu/magazine/article/straintronics-engineers-create-piezoelectric-graphene

    Paul, Proxima A and B should be Centauri A and B?

    • Paul Gilster July 31, 2020, 5:30

      Thanks, Mike. I hate it when I do that — yes, Centauri A and B. I’ve corrected the typo.

  • AlexTru July 31, 2020, 3:25

    The described concept is not serious, I can accept is as joke only .
    Highly absorbance of material, and no any relation to Yarkovsky effect.
    Proposition to use lasers and no any relation to one simple detail – how they are going to be pointed to right direction?
    High absorbance of material and relatively low material destruction temperature…
    Not serious, fairy tales… at least on this stage with this approach.

  • Mike Serfas July 31, 2020, 7:56

    I imagine these spheres might be maneuverable enough to permit their development as a weapon in Earth orbit, or in more diplomatic terms, a tool for removal of space junk. Bulk aerographite can be compressed to a small fraction of its original size. If filaments collapse the spheres temporarily on demand or a shape-memory alloy can compress skeins of the material, the thrust could be regulated over a wide range. Perhaps the accumulation of charge can be regulated by controlling how nanotubes doped with N or S are exposed (making the sphere behave like more fur or like amber when rubbed by the solar wind). Additional charge regulation might be done by passing through Van Allen belts, and the interaction with the Earth’s magnetic field would need to be plotted out. Ground-based lasers might also provide occasional boosts.

    I don’t understand the mechanics of collision with a series of very low density carbon filaments at speeds perhaps a bit greater than escape velocity, but it should involve a small change in momentum for the amount of energy transferred. Perhaps a train of diffuse spheres could melt a victim satellite, even vaporize it as if targeted by a particle beam, leaving comparatively little dangerous debris? Alternatively the spheres might match course with a satellite and starve it of light, or slowly alter its orbit with their minuscule force. I’d expect any space force would want to reserve a few tons of these probes in orbit…

    • Mike Serfas August 2, 2020, 9:59

      One thing I left out above was atmospheric drag: these spheres should undergo “aerobraking” at altitudes where drag on other spacecraft is negligible, but with access to free thrust this may be an advantage. Navigation would be complex since the drag depends on both probe velocity and pressure, and the pressure of the upper atmosphere depends on daily changes in temperature, weather, even the pull of the moon … I perceive an Ancient Mariner quality to the sailing of these probes. Also if they impacted a victim satellite at low speeds, they might tangle it like a fishing lure from a summer lake and pull it down to Davy Jones.

  • ljk July 31, 2020, 10:11

    I need someone to convince me why a laser stationed somewhere in the Sol system powering an interstellar probe is better than a vessel that carries its own propulsion so as not to be subject to the vagaries of human behavior and other issues that could stop the laser from working at any time.

    I also find this whole concept of a huge laser as the key to pushing a ship to another star more of a ploy to kick the can down the road, to use a phrase. Yes, let’s keep working on a technology that will not only take decades and cost trillions of dollars or more, but may never be built in the end because most of the organizations that could afford it will not spend their money on a robot probe to the stars.

    If anything they would only support and build such a laser if it were used as a weapon first, and that would only make other nations balk at such a plan, or try to come up with their own laser and then the focus would be akin to stockpiling nuclear weapons rather than as instruments for exploration and science. The history of rocketry is a perfect example here and I see little about the human race that is going to change just because the new technology is a big and powerful laser. Most people in power have not and do not care about science, only remaining in their thrones.

    We have had plausible plans to get vessels to other star systems for decades now, but they are being ignored for reasons that seem to have more to do with politics than technology and physics. Such as this one:

    http://large.stanford.edu/courses/2012/ph241/klein2/docs/19890007533_1989007533.pdf

    If for whatever reason people really don’t want to become an interstellar species, I would prefer they just say so rather than play this game where it looks like they are all gung-ho about visiting Alpha Centauri but all we end up with are a bunch of white papers and a chorus of “maybe someday.” I am tired of it, just as I am tired of having to hear about how we are going to send humans to Mars “someday”. Pick a date already and stick to it, or don’t do it at all.

    • Alex Tolley July 31, 2020, 13:27

      Picking firm dates and goals is a common refrain of those I see on space blogs, e.g. The Space Review. There is a lot of merit in this, as history has shown us. Nasa has become a jobs program and port barrel item. I have read that the latest stimulus package includes yet more military spending, but nothing to get the Artemis Moon landing funded. Mars seems as far off as fusion power. Many of us recall that 1986 was the original possible target date. It is farther off now than it was then.

      The space laser arrays, particularly Lubin’s D-Star series are military use weapons that were sanitized for public consumption by originally indicating they were for planetary defense by vaporizing asteroids. Breakthrough Starshot then coopted the idea. I agree that building massive laser arrays, even on earth [in Chile?] is still a huge undertaking, unlikely to happen without DoD funding. Yet it is undeniable that there is so far no other technology that can deliver a vehicle, even a 1g-sized one at 0.2C. However, by the time we get such a craft, a slower one may well have reached Proxima before it.
      However we get to Proxima, especially with a 1MT sized probe, we cannot escape the energy requirements to get there (and decelerate?). Solar sails without an extra push even with sundiver maneuvers cannot get there fast enough unless the sail material is very exotic, or this aerographite material will work in practice at the scale needed. Reducing the mass size of the probe is clearly an advantage, although what we can do with a 1g probe on a fast flyby could be problematic – like claiming a tiny paper dart is a useful airplane.

      OTOH, I wouldn’t get too despondent. Other nations seem willing to accelerate exploration for nationalistic reasons, and private industry is attempting to accelerate the development of space hardware and services too. Maybe the USA is like post-WWII Britain, trying to keep its technology lead, but failing as the larger economic power overtook it. Unless you are a Brit, you cannot understand how dispiriting this was.

  • Alex Tolley July 31, 2020, 14:25

    Picking firm dates and goals is a common refrain of those I see on space blogs, e.g. The Space Review. There is a lot of merit in this, as history has shown us. Nasa has become a jobs program and port barrel item. I have read that the latest stimulus package includes yet more military spending, but nothing to get the Artemis Moon landing funded. Mars seems as far off as fusion power. Many of us recall that 1986 was the original possible target date. It is farther off now than it was then.

    The space laser arrays, particularly Lubin’s D-Star series are military use weapons that were sanitized for public consumption by originally indicating they were for planetary defense by vaporizing asteroids. Breakthrough Starshot then coopted the idea. I agree that building massive laser arrays, even on earth [in Chile?] is still a huge undertaking, unlikely to happen without DoD funding. Yet it is undeniable that there is so far no other technology that can deliver a vehicle, even a 1g-sized one at 0.2C. However, by the time we get such a craft, a slower one may well have reached Proxima before it.
    However we get to Proxima, especially with a 1MT sized probe, we cannot escape the energy requirements to get there (and decelerate?). Solar sails without an extra push even with sundiver maneuvers cannot get there fast enough unless the sail material is very exotic, or this aerographite material will work in practice at the scale needed. Reducing the mass size of the probe is clearly an advantage, although what we can do with a 1g probe on a fast flyby could be problematic – like claiming a tiny paper dart is a useful airplane.

    OTOH, I wouldn’t get too despondent. Other nations seem willing to accelerate exploration for nationalistic reasons, and private industry is attempting to accelerate the development of space hardware and services too. Maybe the USA is like post-WWII Britain, trying to keep its technology lead, but failing as the larger economic power overtook it. Unless you are a Brit, you cannot understand how dispiriting this was.

  • Michael Million August 3, 2020, 2:47

    Dump the Graphene and use tungsten, it has a much higher melting point and a much lower vapour pressure and is more reflective and although ten times denser its vapour pressure is ten thousand times better so can get really close to the sun.

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