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Solar Sails: Deeper into the Aerographite Option

Aerographite is an ultra lightweight material made of carbon microtubes, just the sort of thing that seizes the imagination in terms of material for space sails powered by solar photons or laser beam. Such materials are much in my thinking these days and have been for some time, ever since I first read some of Robert Forward’s papers on using laser beaming to boost enormous sails to a substantial fraction of lightspeed. What kind of materials would be used, and how could the mass be kept low enough to allow significant payloads to be deployed?

These days, we think in terms of much smaller sails with miniaturized payloads of the sort advocated by Breakthrough Starshot. But of course advances in sail technology enable a wide range of concepts, and the place to start is with laboratory experiment — this is where we are with aerographite right now — moving into space demonstrators that can be low-cost and near-term. The kinds of missions conceivable with aerographite include fast access to the outer Solar System and, with the help of a close solar pass, interstellar trajectories to nearby stars.

What we are examining in this series of posts is a concept paper that asks for the first time whether aerographite can become a sail material, noting its low cost and our ability to begin testing not just on the ground but in space to find out whether it can carry a payload and survive the stresses of deep space journeys. Lead author René Heller (Max Planck Institute for Solar System Research, Göttingen), with co-authors Guillem Anglada-Escudé (Institut de Ciencies Espacials, Barcelona), Michael Hippke (Sonneberg Observatory, Germany) and Pierre Kervella (Observatoire de Paris), offer a pointer to areas of investigation we will have to address.

Image: An aerographite sample from the Technical University of Hamburg now under study as its potential as a sail material is examined. Sample courtesy of B. Fiedler/H. Beisch (TU Hamburg). Image by R. Heller (MPS Göttingen).

Exploring Aerographite’s Potential

When the teams at the Technical University of Hamburg and the University of Kiel developed aerographite, they dubbed it “the lightest known material.” It is a synthetic foam connected by carbon microtubes with a density of 180 g/m3. This intriguing material has useful properties indeed, though some of these, as we’re about to see, are problematic. For now, let’s note that aerographite is ultra-black at the 1 mm scale considered in the Hamburg team’s paper (citation in yesterday’s post), though as Guillem Anglada-Escudé explained, its opacity is unclear as we go below that scale, a matter that will have to be addressed.

Aerographite’s opacity is important because we are talking about a sail that functions not through reflectivity but absorptivity. The sail concepts we’ve bandied about in these pages have for the most part revolved around reflection, so let’s pause on this. I was unsatisfied with my own description of an absorptive sail in a rough draft of this post, so I asked René Heller for his own take. An opaque sail, he replied, absorbs any light that impinges upon it:

Think of a shower of photons that bumps into an aerographite sample. The sample is almost entirely black, and so most of the photons will not be reflected but they will transfer their kinetic energy (and their momentum) to the sample. As a consequence, the sample will be heated and “pushed” into the same direction as the incoming photon shower was moving before it got absorbed. This is due to conservation of momentum. Now if the sample were reflective then the incoming photons would bounce back from the sample in some angle. If the sample were 100% reflective and if it had a perfectly flat surface, and if the photons were coming in in a perpendicular angle with respect to the surface, then the sample would receive twice the amount of the momentum that it received in comparison to the absorptive case described above. In other words, reflection is twice as effective as absorption.

And yet absorption in the right material can be enabling. While we begin deeper investigations into aerographite’s properties, it’s worth noting that the 1 mm shell thickness in the putative sail explored in these calculations is already useful for some mission concepts. Solar photons alone can push an aerographite sail of this thickness beyond Solar System escape velocity.

If laboratory work confirms that the shell can be reduced below 1 mm while remaining opaque, we can start talking about on-board technology like scientific instruments and the needed communications devices. We can also start talking about much faster transit times. As mentioned yesterday, Heller et al. find the orbit of Mars reachable within 60 days assuming an aerographite shell of 0.5 mm. That gets you to Pluto in 4.3 years, nicely halving the New Horizons travel time. As you can see, we are getting into interstellar precursor range.

This is done through solar photons alone, without the considerable overhead of the massive laser array assumed by Breakthrough Starshot, with obvious (and enormous) cost savings. Starshot wins on speed, aiming for 20 percent of lightspeed and thus a transit time to Proxima Centauri of about 20 years. But put a 1 μm thin aerographite hollow sphere at 0.04 AU (this is Parker Solar Probe territory) for a close solar pass and you achieve 6900 km/second, which gets you to Proxima Centauri in 185 years, well below the 1,000 year threshold thus far determined for a graphene sail.

How to make a sail out of this material? Anglada-Escudé explained in an email that aerographite:

…has structural integrity (it keeps its shape and recovers over deformation), and it is “relatively” simple to make. Think of it as a foam. You make a template of porous ZnO [zinc oxide], carbon fills in the gaps through a deposition process forming carbon fibres/tubes/sheets, etc. You remove the template material, and you have a block of very light aerographite foam with the shape of the original ZnO template.

The authors discuss hollow spheres as the sail’s shape, but a range of possibilities exist. The spherical shape was chosen for the paper because it provides the simplest solution at this stage of the evaluation of aerographite, but a cone is likewise feasible, and would likely provide stability — I think of this in terms of Jim and Greg Benford’s work on ‘beamriding’ sails, obviously an issue with Breakthrough Starshot.

The today typical ‘flat’ sail is likewise a possibility. The authors have discussed the matter with the Hamburg group behind aerographite, with the latter seeing no problem in manufacturing the material in arbitrary shapes and sizes. Adds Anglada-Escudé:

A spherical shell with a wall of 1 mm is already unbound to the Solar System, but if we could work with a more classic flat (or quasi flat) sail in ‘parachute’ configuration, we could work with up to 4 mm in thickness (which is pretty macroscopic) and add payload easily. I would think that the idea is to start the conversation and let the community evolve it to the most useful application. This is now mission planning, not speculative design! Deploying it would be so cheap… it is almost painful to think about why we have not done it yet.

Or we can get more exotic still, as lead author Heller told me, envisioning separate sail components connected by carbon nanotubes:

The shell (or hollow sphere) design that we focus on is just better than a solid sphere or a cube but not necessarily the optimal shape. I personally think a web of dozens or hundreds of cm-sized cones or parachutes, all of which automatically orient themselves to the solar radiation individually will be a more practical application if it really comes to moving payload through the solar system. Such a web of aerographite parachutes would be more resilient to failure (of one or a few parts) and it would allow larger mass margins for the payload. I wouldn’t pay too much attention to single-shell concepts for real applications, although hollow spheres make the math very simple, which is why we used them for this introductory paper. A web will be better for “large” (gram to kilogram) payloads.

Directions for Research

Aerographite gives the appearance of an ideal sail material if it can be used to carry a payload. The authors produce benchmark scenarios showing that the weight of the payload is 1000 times the mass of the transport system, quite a contrast with chemical rockets — think New Horizons and its Atlas V — where the transport system is 1000 times heavier than the payload.

The authors calculate payload mass in terms of shell thickness, using benchmarks of 1 μm and 100 μm, with this result:

…a 1 m radius hollow aerographite sphere with a shell thickness of 1 μm (100 μm) would weigh 2.3 mg (230 mg) and have a margin of 2.4 g (2.2 g) to get interstellar. Upon release to the solar radiation in interplanetary space at 1 AU from the Sun, a payload mass of 1 g would yield a terminal speed of 51 km s−1 (41 km s−1), which is 3 times (2.4 times) the terminal speed of Voyager 1. A travel to the orbit of Pluto would take 3.9 yr (4.7 yr). An increase of the sail radius to 5 m would allow payload masses of 10 g to reach the orbit of Pluto in almost half the time.

These are tiny payloads, which is why the authors consider the aerographite sail in terms of scalability. The notion of a swarm of aerographite spheres connected by carbon nanofibers — “the additional weight of which would be small compared to the mass of the aerographite shell” — implies a combined thrust that would allow larger payloads on the one hand, or faster travel time for a given payload. Payloads could thus be brought into the kg domain if sail sizes in the 100 meter range can be achieved.

One advantage of this concept is that it would aid in tracking the spacecraft, which is itself an issue we need to address. But so is the matter of how a payload is positioned, either attached to the sail itself or, depending on sail shape and configuration, drawn behind it in parachute fashion. This is a problem faced by Breakthrough Starshot as well as it seeks the right sail material and optimum configuration for ‘beam-riding’ with its proposed laser array. The authors do not address the question of payload configuration and attachment in this paper.

Nor are we through with significant issues. How would the aerographite sail behave in space, given its tendency to accumulate electrical charge? Navigation issues immediately arise that we need to talk about tomorrow, and we also have issues regarding aerographite’s tensile strength.

Let’s also talk in the next post about the kind of missions we might want to deploy, assuming a successful research project produces flight-testable sails. Interesting options open up long before we start talking about true interstellar flight, perhaps piggybacking on other missions as we learn both about the material in space as well as the formidable issues of tracking an utterly black sail. More on all this next time.

The paper is Heller, Anglada-Escudé, Hippke & Kervella, “Low-cost precursor of an interstellar mission,” Astronomy & Astrophysics 7 July 2020 (abstract / preprint).

{ 35 comments… add one }
  • torque_xtr July 29, 2020, 9:04

    I’m realizing aerographite is wonderful material! But I thought immediately of couple of things here:

    It is likely a very efficient thermal insulator, especially in vacuum. This would create temperature difference between the surfaces of the sail, and illuminated side would radiate more heat than the forward-facing one. Since it scales with T^4, only ~30% difference in temperatures will mimic reflective behavior quite efficiently. Of course, it has to be opaque also to it’s own blackbody radiation as well for this.
    On the other hand, if the absorption law holds for aerographite thin films, it might be more efficient to make it partially transmissive, in terms of thrust per weight. Doubling the thickness of 50% transmissive sail will double weight but increase opacity and thrust only to 75% of totally absorptive sail.

    • Eniac July 29, 2020, 22:28

      This last one is an important observation. It means that with sails, thinner is always better, even if the material becomes transparent.

      As for geometry, a thin flat sheet is always best. Also, mathematically simplest. It is not clear why the authors here bother with other geometries. I hope it doesn’t indicate a calculation error.

      If you look at a micrograph of the material, you see the spaces between tubes are on the 10 micron scale. That means that 10 micron is the minimum thickness, and at that thickness the majority of the light will pass right through those spaces and not hit the tubes. In my estimation, you need 1 mm or so before absorption becomes close to 1. Just enough for solar escape, according to the math presented, but not enough for the interstellar dream mission.

      We should also consider that a 50 nm beryllium foil is both lighter and cheaper (you can order it today) than a 1 mm aerographite sheet. Plus, it is reflective so gets that factor of two.

      • Alex Tolley July 30, 2020, 7:24

        While you can order ultrathin beryllium foil from here it would need to be bonded to some lightweight, high tensile material to be useful as a sail. Today, graphene is not a possibility. Perhaps a carbon fiber or Kevlar grid?
        If the support material has to be a sheet, even a perforated one, then you are back to where you started with aluminized Mylar.

        It may be that in practise, aerographite just will not work as a sail material once all the support material mass is included, or its features require 1mm thickness for light absorption, or they prove difficult to handle. For now all we can do is some experiments to delineate the material’s capabilities as a sail.

        • Paul Gilster July 30, 2020, 9:19

          Exactly so. By the way, Alex, you asked about Les Johnson and his interest in graphene. Les follows Centauri Dreams, but he’s busy enough that I’m sending him links to the aerographite articles here, just to make sure he sees them soon. We’ll soon know what his thoughts on the material are.

        • Michael Fidler July 30, 2020, 9:52

          Any of you remember this:

          Project Echo.

          Echo 1

          “The 30.5-meter (100 ft) diameter balloon was made of 0.5-mil-thick (12.7 μm) biaxially oriented PET film, metalized at a thickness of 0.2 micrometers (0.00787 mils) (a type of film commonly known by the trade name Mylar), and had a total mass of 180 kilograms (397 lb). ”

          Echo 2
          “Echo 2 was a 41.1-meter-diameter (135 ft) balloon satellite, the last launched by Project Echo. A revised inflation system was used for the balloon, to improve its smoothness and sphericity. Echo 2’s skin was rigidizable, unlike that of Echo 1A. Therefore, the balloon was capable of maintaining its shape without a constant internal pressure; a long-term supply of inflation gas was not needed, and it could easily survive strikes from micrometeoroids. The balloon was constructed from “a 0.35 mil (9 µm) thick mylar film sandwiched between two layers of 0.18 mil (4.5 µm) thick aluminum foil and bonded together.”[6] It was inflated to a pressure that caused the metal layers of the laminate to slightly plastically deform, while the polymer was still in the elastic range. This resulted in a rigid and very smooth spherical shell.”

          • Paul Gilster July 30, 2020, 10:33

            Yes, an early demonstration, as it turned out, of the effects of solar photon pressure. Though it was never intended as such.

        • Michael Fidler July 30, 2020, 10:27

          Apparently there have been a number of light spherical satellites placed in orbit around earth. It would be interesting to see what happened to them when solar maximum was blasting them. PasComSat, Dragsphere 1 and PAGEOS are three interesting examples that are not very well know. PasComSat was a U.S Air Force project and Dragsphere was a U.S. Navy project and I would imagine there where many smaller secret military projects. PasComSat was made of a soft aluminum wire grid embedded in a special plastic designed to dissolve in space under the sun’s strong ultraviolet rays. It it had a reflective power five times greater than that of a solid sphere!
          I’m just wondering if any of the long term results from these lightweight globes might give some information that would help with aerographite development?

          • James Jason Wentworth August 3, 2020, 10:07

            PasComSat’s RF reflectivity has long made me wonder about such possibilities for an inflation-deployed (with UV-subliming plastic), microwave-pushed wire grid sail (a Starwisp-like probe that could actually work). Even the latest Breakthrough Starshot concept calls for 3 m – 4 m diameter laser-pushed, inflatable lightsails (the spherical sails would be self-centering in the great “bundled” beam from the launching laser array), so spherical wire grid microwave sail probes might also work well.

        • Eniac July 30, 2020, 14:51

          Right, I expect both types of material will have to be supported over longer distances so that large sheets can be formed that are resistant to tearing. I am thinking some sort of carbon fiber chicken wire, but there are obviously many different ways of doing that.

  • Mike Serfas July 29, 2020, 12:21

    The Breakthrough Starshot array shouldn’t be an either-or choice. To my eyes it looks like you’re designing a Breakthrough Starshot probe more or less within the conceptual specification, but with a larger than expected volume. The absorptive quality of the aerographite changes the game a bit, since there is no way for the probe to steer against reflected light intentionally or otherwise, but you at least have choices of where to put lasers and when to fire them. Also, these cosmic dustbunnies should be immensely susceptible to electrostatic effects – scattering charge over such a large area and small mass should provide a “high specific capacitance” (I haven’t gone over https://www.sciencedirect.com/science/article/abs/pii/S2211285517301350 , sorry). If you are able to manipulate whether positive or negative charge builds up during collisions with solar wind particles, you might be able to steer within any electromagnetic fields the probe encounters. Has there been any work on that aspect of propulsion?

  • Alex Tolley July 29, 2020, 15:25

    If the sail picks up +ve charge (due to the UV knocking out electrons?) doesn’t this mean the sail becomes more like an electric sail, where the charged sail element has a field that extends beyond the physical sail dimensions? If so, a non-rotating (it has low tensile strength) sail of long aerographite strips (or squares at the intersections of a carbon fiber grid) might prove a simple electric sail not requiring an electron emitter to maintain its surface charge.

    One advantage of the “flat” configuration using small asymmetric components like cones, is that by controlling their orientation asymmetrically across a grid, the sail could be navigated like a reflective sail to move outwards or inwards in the solar system.

    • Eniac July 30, 2020, 15:52

      I don’t think any absorptive sail can be steered at all, simply because there is no outgoing reflected stream of photons to provide transverse momentum. Heat emissions would be pretty much isotropic, so also not be of use. If we could control the charge, we could use interplanetary/interstellar fields to exert some control, but it would be at least as difficult to navigate as balloons on Earth. More difficult, because magnetic fields in space are complicated and very hard to know or predict.

  • Alex Tolley July 29, 2020, 15:34

    IIRC, Les Johnson has doped graphene sails on his roadmap for advanced solar sailing. Has he been contacted to weigh in on this material?

    It seems to me that aerographite is going to be much easier to manufacture at scale than doped graphene, and has teh advantage, apparently, of not requiring continuous sheets, but rather has teh stiffness to be added to a grid whose overall shape can be maintained in various ways, e.g. parachure.

    We seem to be seeing a flowering of possible sail material technologies, offering various options to tailor the sail configuration, navigational advantages, and performance. We’ve come a long way since Drexler’s aluminium foil sails, and even aluminized mylar seems to be a transition material as we look for high performance.

    • Michael Fidler July 29, 2020, 19:56

      Alex, do you have any other info on IIRC, Les Johnson, like web site or research papers about doped graphene? I would be very interested to hear of any other material available about this subject. Thanks!

  • charlie July 29, 2020, 17:21

    ” If the sample were 100% reflective and if it had a perfectly flat surface, and if the photons were coming in in a perpendicular angle with respect to the surface, then the sample would receive twice the amount of the momentum that it received in comparison to the absorptive case described above. ”

    ” … the sample would receive twice the amount of the momentum that it received in comparison to the absorptive case … ”
    ” … twice the amount of the momentum … ”

    Sounds like you’re almost getting something for nothing doesn’t it? I of course I know that isn’t true; but isn’t it probably because of the fact that the reflected photons that come back from the surface have a lesser wavelength then they had before they encountered the reflective surface, and therefore they have lesser energy?

  • charlie July 29, 2020, 17:38

    Another thought occurred to me here; could the density of this material be well dropped dramatically below the 180 grams per cubic meter that is now being touted as the low end? What I would envision is to take the material and ‘puff it up’ so to speak with some type of volatile gas that would escape in a vacuum environment producing a more ‘fluffy’ (if you will) substance than you normally would see otherwise.

    Assumption wise, the material would be considerably less dense and have a high degree of porosity then what we would normally see in the prepared material, but we are operating in a vacuum and therefore strength and perhaps some degree of flexibility could be sacrificed for having a less dense but still strong material. Any thoughts on the matter out there?

    • Eniac July 30, 2020, 16:00

      Making it fluffier also makes it more transparent to light. Volume density is in fact a bad material characteristic to optimize for in sails. We need to optimize the product of areal density (i.e. kg/m^2) and opacity (percentage of light absorbed or reflected). Metal nanofoils have a clear advantage over aerographite in this regard. Furthermore, metal foils are reflective, which adds a factor of two and provides steerability. The latter is, of course, very important.

  • charlie July 29, 2020, 17:44

    more exotic still,
    aerographite , close solar pass , massive laser array assumed by Breakthrough Starshot,

    • Alex Tolley July 29, 2020, 22:54

      The BS lasers are so powerfull that the BS sail needs to be almost a perfect mirror. An absorptive sail would just instantly vaporize. However, a much less powerful laser running for longer might give it a boost. If the sail can tolerate the solar radiation at o.o4 AU, that implies that it can withstand a 625x increase in radiation over its surface. Less than a 1 MW laser could achieve this. That sort of power should be achievable relatively easily from Earth, although it would have to run continuously for some time (or a bank would switch individual lasers on/off in sequence).

      However, I am wondering whether a cheaper, microwave beam might not be more appropriate for a carbon allotrope sail. Isn’t that what the Benfords tested?

      • Eniac July 30, 2020, 16:11

        Less than a 1 MW laser could achieve this

        Assuming the sail is far from both the sun and the laser (i.e. >10AU away), such a laser is very difficult to provide. The sun provides 3*10^26 W in all directions. In order to beat that using a 1 MW laser, you would have to make up what you lack in power by the tightness of the beam. You have to have a beam divergence of 10^-10. At 300 nm wavelength, that requires an aperture of 3 km, at the diffraction limit. In order for it to make sense, you’d have to beat the sun by a lot, so you are talking either 10 MW or 10 km aperture. Lasers with this type of divergence are of dream-level technical readiness, at this point.

      • Mike Serfas August 4, 2020, 17:21

        I think this may hold up … here’s what I’m thinking. Breakthrough Starshot proposal is for probes that weigh ‘a few grams’, AFAIK. If it were possible to make a 1 m radius sphere for 2.3 mg (which I do doubt) then you could make 10000 m^2 or more of surface area to absorb the beam, compared to the 16 m^2 proposed for Starshot probes. But taking the proposed array of 100 GW / 10000 m^2 = 10 MW/m^2 which is still 6000 times brighter than sunlight under perfect conditions on Earth. I had expected the aerographite filaments could endure Edison light bulb temperatures, and the 4th root of 6000 (Stefan-Boltzmann law) is less than 9, meaning the sphere should be no more than 9 times hotter than an ordinary black object orbiting in space around Earth. However … the third article says it melts at 400 degrees!

  • Mike Serfas July 29, 2020, 20:07

    I was just daydreaming of a somewhat amusing failure mode for the probe: what if the aerographite absorbs photons of one circular polarization state more than the other? I did some back of the envelope calculation… Sunlight delivers an energy flux of 1388 W/m^2, so for a 1 meter radius probe (cross-sectional area pi meters), 4360 J/s of light strikes it. Sunlight is at an average of 550 nm. E = h nu = hc/lambda, so the energy of one photon is h(3e8 m s^-1/5.5e-7m) = 0.54 e15 h / s where h is the Planck constant = 6.63e-34 J s. Therefore the rate of photons striking the probe is (4360 J/s)/(0.54e15 h / s) = 8e-12 J / h = 1.2e+22/s.

    Now each photon carries angular momentum hbar, so if only one chirality were absorbed, a torque (angular momentum per second) of 1/2*(h/2pi radians)*(8e-12 J/h) = 6.4e-13 J/radian [=N m] would be applied to the probe. Given the probe is a hollow sphere 1e-6 m thick with surface area 4 pi m^2, the total volume of aerographite is 12.6e-6 m^3 = 12.6 cm^3; its density is 0.18 mg/cm^3 so this weighs 2.26 mg. The moment of inertia is 2/3*(2.26 mg)*(1 m)^2 = 1.5e-6 kg m^2. Dividing the torque by this yields 4.3e-7 rad/s^2 of angular acceleration.

    At this low a density, aerogel has tensile strength 1kPa = 1000 N/m^2. A 1 micron-wide band running around the equator of the sphere can resist 1e-9 N of force, and has mass (6.3e-12 m^3)(0.18 kg/m^3) = 1.1e-12 kg. Taking T = mw^2R/2pi and solving for w, w = sqrt(2pi radians*1e-9 N/(1.1e-12 kg*1 m) = sqrt(5.7e-3/s^2) = 0.075 rad/s. In this brutal approximation, the probe fails after (0.075 rad/s)/(4.3e-7 rad/s^2) = 1.7e+5 s = 2 days.

    Now I don’t know enough about aerographite production to know if any chirality is plausible, and it would surely be far less than absolute, and it could be ruled out quickly by measuring the circular dichroism of the material, but it’s curious that such a thing conceptually could happen. I have to admit though that halfway through the calculation I lost my suspension of disbelief where the 1 micron shell is concerned. 2 mg of material is not much! If I took a pencil and colored in six square maters of paper darkly enough to notice, I’d expect to need to do quite a bit of sharpening. Would rubbing a mechanical pencil lead on paper and measuring the volume/mass consumed be adequate to settle the question of how thick the probe shell actually needs to be?

    • Eniac July 29, 2020, 22:05

      Interesting thought. I believe chiral nanotubes is a thing, and aerographite is just a thicket of nanotubes, so it should be possible. Hard to make, though, I would expect.

  • Michael Fidler July 30, 2020, 10:33

    Apparently there have been a number of light spherical satellites placed in orbit around earth. It would be interesting to see what happened to them when solar maximum was blasting them. PasComSat, Dragsphere 1 and PAGEOS are three interesting examples that are not very well know. PasComSat was a U.S Air Force project and Dragsphere was a U.S. Navy project and I would imagine there where many smaller secret military projects. PasComSat was made of a soft aluminum wire grid embedded in a special plastic designed to dissolve in space under the sun’s strong ultraviolet rays. It it had a reflective power five times greater than that of a solid sphere!
    I’m just wondering if any of the long term results from these lightweight globes might give some information that would help with aerographite development?

    • Eniac July 30, 2020, 16:21

      Interesting idea, but the solar cycle won’t have an impact on it. Sunlight does not increase at solar maximum, and the pressure of the solar wind is negligible compared to sunlight radiation pressure.

      That PasComSat wire mesh would have to be meant for microwave radiation, it would not be reflective for light.

      • Michael Fidler July 31, 2020, 10:11

        But when solar storms are at their peak they pack quit a wallop both outside and inside of our magnetic field. I do not think that reflective light is going to be the main force for these Aerographite sails but the electro/magnetic fields and charged particles from the solar storms. What is going to have a much larger influence is the rotation of a static charged sphere; a rotating charged ball generates a magnetic field, note that every charge on the ball will move in a circle, so there is in fact a current, and that current will generate a magnetic field. So now we have a magnetic sail or magsail and when not spinning an electric sail to add to the photon sail. This is where a close approach to the sun will push the craft to higher velocities then even the full laser barrage will propel the beamriding sails of BS.

        • Eniac July 31, 2020, 21:53

          Solar wind pressure is almost four orders of magnitude weaker than sunlight radiation pressure.

          • Michael Fidler August 1, 2020, 10:26

            Eniac, please read the following article:

            CfA Scientists and Team Take a Look Inside the Central Engine of a Solar Flare for the First Time.


            Researchers offer unprecedented look into ‘central engine’ powering a solar flare.

            Shocking Measurements
            “The team’s measurements and matching simulation results revealed that the flare’s current sheet features an electric field that produces a shocking 4,000 volts per meter. Such a strong electric field is present over a 40,000-kilometer region, greater than the length of three Earths placed together side by side.
            The analysis also showed a huge amount of magnetic energy being pumped into the current sheet at an estimated rate of 10-100 billion trillion (1022-1023) joules per second — that is, the amount of energy being processed at the flare’s engine, within each second, is equivalent to the total energy released by the explosion of about a hundred thousand of the most powerful hydrogen bombs (50-megaton-class) at the same time.”


            Measurement of magnetic field and relativistic electrons along a solar flare current sheet.


            The Russian Tsar Hydrogen bomb was the most powerful weapon ever developed and had a yield of 50 megatons of TNT. Solar flares are producing each second the equivalent to the total energy released by the explosion of about a hundred thousand Tsar bombs. Now I think you see what this would do to a light electric charged sphere and why it would propel it to relativistic speeds! So take your sunlight radiation pressure and stick it where the sun does not shine…

            • Alex Tolley August 1, 2020, 13:01

              The problem with a solar flare is that even more than the solar wind, it is highly variable and the position of the flare cannot be usefully determined in advance. It is rather like sailing on Earth and hoping to be in the right place for a major blow somewhere on the planet. It may be powerful, but how do you effectively harness it from wherever you are. The photon sail has a very reliable flux wherever it is in orbit which makes harnessing and navigating a photon sail far easier.

              • Michael Fidler August 1, 2020, 17:41

                Yes, but do you want to wait 185 years to reach Proxima? There are indications that solar flares may be predictable and as in the article understanding the way they work will lead to a possible method to use them. The recent images showing thousands of small flares with magnetic recconection that are called campfires on the sun may work even better.
                There is a much closer and easier way to take advantage of the high acceleration of a charged sphere and that is in the geomagnetic tail of the earth. As I have mentioned before the magnetic recconection that takes place there could be the perfect place to try aerographite ability to be propelled to relativistic speeds. This would solve the problems of trying to get it near enough to the sun to be propelled. I hope that René Heller group will look into using aerographite in the powerful magnetic reconnection that takes place in the earths geomagnetic tail.

                • Alex Tolley August 1, 2020, 21:51

                  The magnetotail seems like the best bet for positioning a craft. However, I cannot make anything of the various formulae and current flows. Can you produce some numbers for the Earth’s magnetotail reconnection showing:

                  1. particle flow velocity and density of the current.
                  2. How to position a sail/craft to capture that flow (electric or magnetic sail?)
                  3. Some hoped for acceleration and terminal velocity for the sail based on some reasonable sail parameters.

                  Back of envelope calculations would be sufficient just to determine some order of magnitude performance for comparison.


                  • Michael Fidler August 2, 2020, 11:46

                    I thought we had a conversation before about this subject and found it in the comments no CD:

                    Ultrahigh Acceleration Neutral Particle Beam-Driven Sails.
                    by PAUL GILSTER on JANUARY 3, 2019

                    “Michael C. Fidler January 6, 2019, 10:39
                    I have an idea that may work with magsails, plasma-magnet-drives or the dipole drive that would use magnetic field reconnection to accelerate them. Now at first I was thinking using solar flares but having to wait around near the sun plus the high probability of frying the electronics does not sound good. But we have a very nearby source that may due the trick: Magnetic Reconnection in the
                    Earth’s Magnetotail. Take a look at these articles and see if this might work:
                    Magnetic Reconnection in the Earth’s Magnetotail.

                    Just an idea, might be a lot of energy shooting away from earth and with the ability of a magnetic field to surf on it, all you need is the control of the direction…

                    Michael C. Fidler January 6, 2019, 10:45
                    Another idea, 2010: The Year We Made Contact, the best part was the spacecraft kevlar balloons using the atmosphere of Jupiter to slow down. So would this also work with magnetic sails?

                    Alex Tolley January 6, 2019, 13:19
                    There is work to demonstrate the viability of magnetic fields for aerobraking. The benefits are much lower mass than physical aeroshells. paper.

                    The Leonov’s ballutes in 2010 looked cool, but I don’t know whether they would have worked as envisaged in the movie version. [Amazing to think the much touted CG of the aerobraking was state of the art when the movie was made. Now they look very primitive. And those outdated hopelessly laptops and computers… ]

                    Michael C. Fidler January 8, 2019, 6:54
                    Here is something a little more up to date that is showing super-Alfvenic electron jets reaching 20,000 km/s or o.o66c during magnetic reconnection.

                    Satellites reveal the microphysics of magnetic reconnection.
                    Laminar electron dynamics drive reconnection on the side of Earth away from the Sun.


                    Physics > Space Physics
                    Electron-Scale Dynamics of the Diffusion Region during Symmetric Magnetic Reconnection in Space.

                    “Magnetic reconnection is an energy conversion process important in many astrophysical contexts including the Earth’s magnetosphere, where the process can be investigated in-situ. Here we present the first encounter of a reconnection site by NASA’s Magnetospheric Multiscale (MMS) spacecraft in the magnetotail, where reconnection involves symmetric inflow conditions. The unprecedented electron-scale plasma measurements revealed (1) super-Alfvenic electron jets reaching 20,000 km/s, (2) electron meandering motion and acceleration by the electric field, producing multiple crescent-shaped structures, (3) spatial dimensions of the electron diffusion region implying a reconnection rate of 0.1-0.2. The well-structured multiple layers of electron populations indicate that, despite the presence of turbulence near the reconnection site, the key electron dynamics appears to be largely laminar.”


                    Well, it looks like an electron jet is already up in space nearby the earth and it doesn’t cost much to get a ride so what drives would work in this type of environment – magsails, plasma-magnet-drives or the dipole drives?

                    Michael C. Fidler January 8, 2019, 7:39
                    What about Jupiter’s magnetic reconnection, it is the giant in the solar system, so could it’s magnetic reconnection be that much more powerful? Further away, but the dynamics may be different and speeds higher and maybe high speed ion jets?”


                    Surprised by the 2010 connection! Two ways that Aerographite could be used in this situation. First as an electrically charged sphere that could act as a giant electron. In this way it may be able to surf on the super-Alfvenic electron jets reaching 20,000 km/s, which is close to 7 percent the speed of light. The other method may be the rotation of a static charged sphere; a rotating charged ball generates a magnetic field, note that every charge on the ball will move in a circle, so there is in fact a current, and that current will generate a magnetic field. So now we have a magnetic sail or magsail.

                    This in both cases negates the need for a large ungainly sail and the form of a sphere would have the highest effect to interact with the Alfvenic electron jets/waves. The other point I would like to make is these measurements were taken when the sun was winding down from the 2012 peak in sunspot and flaring activity and this is what powers the reconnections in the earths magnetic-tail. The speeds may be much higher at solar maximum because of the intense activity of the sun, hopefuly giving speeds as high as 20 percent of the speed of light. As to the best method to do this is to send a cube-sat with several thousand compressed aerographite spheres and have the cube-sat monitor the area where magnetic reconnection has been taking place. When it picks up the radiations that recconection has started, it drops the spheres above that point and they are drawn into the current via magnetic field lines and plasma intake. They should be ejected both toward and away from earth on the Alfvenic electron jets/waves. This is similar as to the balls that have been released into tornadoes to see the patterns and structures in the storm.

                    Electron-Scale Dynamics of the Diffusion Region during Symmetric Magnetic Reconnection in Space.

  • Scott Guerin July 30, 2020, 21:12

    Sci-fi: How about a layered sail which when close in to a sun is powered by photons hitting it. Then beyond, the effective photon range the layers provide a differential Casimir effect that “pulls” the sail ever faster. To decelerate, it turns around.

  • Michael Fidler August 2, 2020, 7:34

    Ok, let me work on that and I’ll see what I can come up with!

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