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The Electric Sail and Its Uses

The electric sail is an intriguing propulsion concept that Pekka Janhunen at the Finnish Meteorological Institute has been championing for some years. It’s currently the subject of a NASA Phase II study and continues to draw attention despite the fact that we’re in the early stages of turning what looks like sound physical theory into engineering. What captures the imagination here is the same thing that is so attractive about solar sails — in both cases, we are talking about carrying no propellant, but instead relying on natural sources to do the work.

Here we have to be careful about terminology, because it’s all too easy to refer to solar photons as a kind of ‘wind,’ especially since the predominant metaphor is sailing. So let’s draw the lines sharply. There is indeed a ‘solar wind’ in today’s parlance, but it refers not to light but to the stream of particles, plasma and magnetic fields flowing out from the Sun into the heliosphere. An electric sail will ride this solar wind to achieve interplanetary velocities. A solar sail, on the other hand, will use solar photons, which carry no mass but do convey momentum.

Two entirely different concepts, even if both have resemblance to traditional nautical sails. Then we have the other terminological complication: A sail designed to be pushed not just by sunlight but rather by a laser or microwave beam is sometimes called a ‘light sail,’ which is how I have always referred to it, but it still uses photons for propulsion, even if they don’t come from the Sun. Maybe Manasvi Lingam and Avi Loeb have it right in their new paper to refer to photon-pushed sails of any kind as ‘light sails,’ distinguishing these from both electric and magnetic sails (magsails) that use the ‘solar wind’ as their driver. Thus:

Light sails — solar sails and those driven by beamed arrays — use electromagnetic radiation and the momentum transfer of photons. Electric sails use the particle stream of the solar wind.

The electric sail that Janhunen continues to study is the subject of Lingam and Loeb’s new paper, which has been submitted to Acta Astronautica. At the Florida Institute of Technology and Harvard University respectively, the two scientists have calculated performance possibilities for a spinning spacecraft that deploys a number of long wires to which an electrostatic charge has been induced. Solar wind protons (not photons!) reflect off these wires to produce thrust. The wires are kilometers long, and with that slight positive bias, the spacecraft carries an electron gun to manage the charge, retaining the bias against ambient solar wind electrons.

Image: The electric sail is a space propulsion concept that uses the momentum of the solar wind to produce thrust. Credit: Alexandre Szames.

Light sails, to use the Lingam and Loeb terminology, have been considered for interstellar missions for decades now (hats off to the early work of Robert Forward, Gregory Matloff and Geoff Landis, among others), but electric sails are new enough that we need information on how well an electric sail might do for this purpose. Could this technology get us to another star?

For a species like ours, anxious to see missions completed within a few human lifetimes, the answer is no. While a huge laser array like the one contemplated by Breakthrough Starshot could send a small light sail at relativistic speeds to another star, the electric sail cannot achieve the needed velocities.

A species with a different attitude toward time might fare better. The paper explains, for example, how electric sails could leverage the stellar winds of red dwarf stars, which are by far the most common kind of star in the galaxy. Because the interstellar medium itself can decelerate the sail, turning off the electron gun in deep space is essential. Careful maneuvering from star to star over millennia then allows relativistic speeds. From the paper:

…a series of repeated encounters with low-mass stars, and taking advantage of their winds, will enable the electric sail to achieve progressively higher speeds. We showed that sampling ∼ 104 stars could enable electric sails to achieve relativistic speeds of ∼ 0.2 c and that this mechanism would require ∼ 1 Myr. While this constitutes a long timescale by human standards, it is not particularly long in comparison to many astronomical and geological timescales. The ensuing relativistic spacecraft would be well-suited for tackling interstellar and even intergalactic exploration.

This is an eye-opener. We can’t rule out the possibility that species capable of operating in this time frame might deploy electric sails, but the time involved precludes their use as the primary propulsion method for interstellar missions by us. The authors note as well that because an electric sail will have a low cross-sectional area, its presence would be all but undetectable, whereas a light sail driven by a laser would demand huge amounts of energy and would be theoretically detectable at interstellar distances. So for a civilization hoping to explore in ‘stealth’ mode, an electric sail would have its advantages. These are not good SETI targets.

Returning to M-dwarf stars, the authors show that if stars are small enough (less than about 0.2 solar masses), the pressure of the stellar wind dominates over photon pressure, Speeds in the range of 500 kilometers per second seem feasible for electric sails near late-type M-dwarfs. Indeed, for F-, G- and K-class stars, electric sails fare better as propulsion systems in the vicinity of the home star than light sails.

So we are looking at a technology that, if it can be properly engineered, could play a role in shaping an interplanetary infrastructure, while yielding to faster methods for missions to other stars, unless we humans somehow attain an all but geological patience.

The paper is Lingam and Loeb, “Electric sails are potentially more effective than light sails near most stars,” in process at Acta Astronautica (preprint). For Pekka Janhunen’s concept of the electric sail as a fast interplanetary probe, see Electric Sails: Fast Probe to Uranus.

{ 36 comments… add one }
  • Antonio November 18, 2019, 16:11

    Mmm but, as you say, the speed of the solar wind is around 500 km/s, not 0.2 c. How does the sail achieve such high speed?

    • Paul Gilster November 18, 2019, 20:30

      Antonio, the authors are talking about a huge number of stellar flybys with electric sail augmentation to achieve speeds like these. That’s about the only way to do it, according to the paper. We’re talking about a million year process, involving perhaps 10,000 such flybys.

      • Antonio November 19, 2019, 3:08

        ¿So is basically gravitational assists?

        • Paul Gilster November 19, 2019, 7:00

          No, as I read the paper, the whatever gravitational assists such a probe would get would be augmented by another 500 km/s boost from the electric sail, which would be reactivated at each stellar pass. So you have a combination.

    • Project Studio November 18, 2019, 22:04

      When the sail reaches a speeds equivalent (or greater than) the speed of the stellar wind, might the electric sail then create a breaking effect rather than an acceleration? Is there a sailing technique (equivalent to upwind tacking) that allows the electric sail to keep accelerating when it is already going faster than the stellar wind? Might a hybrid electric/magnetic sail be more suitable to an multi-hop interstellar mission?

      • Paul Gilster November 19, 2019, 7:01

        I’m much in favor of exploring the hybrid idea.

      • Antonio November 19, 2019, 11:09

        Yeah, that’s what puzzles me, how it can be propelled by a wind slower than it.

  • Alex Tolley November 18, 2019, 19:12

    Fast light sails require beamed energy top reach the fractional c velocities. Can we do something similar with electric sails? Could we not use accelerators to create beams of relativistic protons to accelerate the sail? While beam divergence is an issue, how feasible is it to create a bean to rapidly accelerate the electric sail to the velocities suitable for interstellar flight comparable to beamed light sails in mission time?

    • Brett November 19, 2019, 4:56

      I don’t see why it would be impossible. It might even be easier to intensely focus the solar wind versus using massive laser arrays, although it all depends on megastructure construction costs for which we can only speculate.

  • Robin Datta November 18, 2019, 21:55

    If Homo sapiens could evolve a facility to consider, think and plan, not just for seven generations into the future as with some Native Americans/First Nations, but rather for 70,000 or even 700,000 generations (or more) into the future, then work could commence on setting up a basal level of “one-size-fits-noneall / all-purpose infrastructure in anticipation of a future buildout from that level, while acknowledging that it is also a realm of “known unknowns” and “unknown unknowns”, which overwhelm the knowns in very short order.

    But first of course, we would also have to shed those pre-eucaryotic imperatives that today, on a vastly grander scale, replay the resource depletion and environmental degradation abilities of yeasts in a vat of sugar-water. As an aside, that’s how beer is made: will some cosmic denizen deign to drink the beer we. might leave behind?

  • T Pliska November 18, 2019, 22:49

    How would you power your electron gun for 1 million years? Solar (I mean, stellar)?

  • Dmitry Novoseltsev November 19, 2019, 5:35

    We can use both technologies in the same device. https://cloud.mail.ru/public/5NZN/2Jc6Q3tgp

  • Michael Fidler November 19, 2019, 10:54

    Remember in the “Breakthrough Starshot: Early Testing of ‘Wafer-craft’ Design” the point I made about the Earth’s magnetic tail and the magnetic reconnection that takes place there?
    “Michael Fidler May 15, 2019, 22:23
    Maybe even an experiment with using the earth’s magnetic re-connection in our Magnetosphere.
    Dr. Torbert and colleagues found that the symmetrical reconnection events last only a few seconds, producing extremely high velocity electron jets — over 9,320 miles per second (15,000 km per second) — and intense electric fields and electron velocity distributions.

    Well two weeks later NASA’s Goddard Space Flight Center, issued this interesting article:
    May 29, 2019
    Three Ways to Travel at (Nearly) the Speed of Light.
    Number 2 on the list of 3:
    2) Magnetic Explosions
    (illustration of magnetic reconnection)
    Huge, invisible explosions are constantly occurring in the space around Earth. These explosions are the result of twisted magnetic fields that snap and realign, shooting particles across space.
    Credits: NASA’s Goddard Space Flight Center
    Download related video from NASA Goddard’s Scientific Visualization Studio
    Magnetic fields are everywhere in space, encircling Earth and spanning the solar system. They even guide charged particles moving through space, which spiral around the fields.
    When these magnetic fields run into each other, they can become tangled. When the tension between the crossed lines becomes too great, the lines explosively snap and realign in a process known as magnetic reconnection. The rapid change in a region’s magnetic field creates electric fields, which causes all the attendant charged particles to be flung away at high speeds. Scientists suspect magnetic reconnection is one way that particles — for example, the solar wind, which is the constant stream of charged particles from the Sun — is accelerated to relativistic speeds.
    Those speedy particles also create a variety of side-effects near planets. Magnetic reconnection occurs close to us at points where the Sun’s magnetic field pushes against Earth’s magnetosphere — its protective magnetic environment. When magnetic reconnection occurs on the side of Earth facing away from the Sun, the particles can be hurled into Earth’s upper atmosphere where they spark the auroras. Magnetic reconnection is also thought to be responsible around other planets like Jupiter and Saturn, though in slightly different ways.
    NASA’s Magnetospheric Multiscale spacecraft were designed and built to focus on understanding all aspects of magnetic reconnection. Using four identical spacecraft, the mission flies around Earth to catch magnetic reconnection in action. The results of the analyzed data can help scientists understand particle acceleration at relativistic speeds around Earth and across the universe.”

    So we have a electron beam that is closer then the moon and and the Electric Sail fits the bill perfectly to sail on it.
    Now the only thing we need to worry about is if the spacecraft goes thru a quantum entangled phase conjunctive mirror warp in the magnetic reconnection and ends up on the Planet of the Apes!!!

  • Alex Tolley November 19, 2019, 11:41

    It seems to me that Antonio has a valid point. Fig 4 of the paper shows the maximum velocity of 1300 km/s from very low mass stars, a value far higher than the solar wind velocity. The acceleration is always given as +ve, allowing for the multi-star flybys to kick the velocity up to fractions of c. This strikes me as indicative that the equations are incorrect. An electric sail moving faster than the local wind will be decelerated, just as the authors note will happen in the ISM, requiring the sail to be shut down in cruise mode. I suspect that dropping terms is the culprit, but I would want to examine the equations much more carefully.

    To travel faster upwind than the wind (UWFTTW) you need another mechanism.

    Interestingly, Greason’s talk at the 2019 TVIW suggested a mechanism to do this for a Plasma Magnet drive. The analogy is an anemometer placed on a wheeled cart. It will spin when the wind is blowing on it and would move downwind. However, if the rotational energy is used to expel mass in the downwind direction, the device could move upwind. His idea is to use 2 Plasma Magnets joined by a tether and turned on an off to act like the anemometer. When the tether aligns with the ISM “wind”, the matter is accelerated down the tether as a propellant. He seemed to expect mass to be carried along with the drive, but conceivably the ISM could be used as mass. This becomes more like a Bussard ramjet, so I am doubtful this use of the ISM could work. Greason says he is submitting a paper on this so I hope we get a chance to comment on it on CD. I particularly want to see how the pivot point in the tether would work, as well as the mass calculations for propellant and the impact on performance inside the heliosphere and out into the ISM. Sometimes devices can be counterintuitive, as the story of “downwind faster than the wind” shows.

    • Jeff Greason December 3, 2019, 19:20

      The paper referred to is in the May 2019 JBIS. An electronic copy is available at https://tauzero.aero/wp-content/uploads/JBIS-May-2019-Greason.pdf — however, be warned, it is math intensive. And yes, the principle would work fine with a translating electric field as well. The e-sail and the plasma magnet are essentially dual concepts, one with electric fields and one with magnetic fields. They have different domains of applicability and performance parameters but broadly similar operating principles as they’re both a way of deflecting a plasma wind where the deflection area scales inversely with dynamic pressure

  • Alex Tolley November 19, 2019, 12:29

    I am going to make a bold statement. I think the authors have made a major error in calculating the finaly velocity of the electric sail.

    The reason is this. While they start with the velocity of the solar wind to estimate the pressure on the sail, this is a static value. They take no account of how this will change as the sail velocity increases and the pressure concomitantly reduces. Thus, as the authors note, the resulting final velocities of the sail are 3.5x those calculated by Janhunen. (equation 20]. This large discrepancy should have been a red flag to reexamine their equations. (If I was a reviewer, I would have requested more information on this).

    Since I am not a physicist, someone with better math chops than I should look at the equations and see what they think.

    • Paul Gilster November 19, 2019, 15:42

      Alex, one thing to keep in mind as you consider these calculations is that the interaction doesn’t occur because of protons hitting the wires of the craft. Instead, it’s an interaction of the voltage sheath around the wire. I learned from Les Johnson that at 1 AU, the effective interaction region for the protons around the wire goes 66 meters out, assuming 20 kV, so you have in effect a virtual sail. As the plasma density drops, the size of the voltage sheath increases, so the effective sail area gets larger because the plasma that keeps it compressed is getting less dense. So the effective area of the sail can grow from the original 66 meters out to 180 meters as you move away from the Sun. How this would affect Loeb’s calculations I don’t know and would need someone with much more mathematical skill than I have to say. But we’d have to factor this in.

      • Harold Shaw November 19, 2019, 15:54

        Your math skills are likely superior to mine so this question may be irrelevant. If the size of the virtual sale increases as plasma density decreases wouldn’t the two effects cancel out. The sail is larger but there is also less pressure applied per square meter.

        • Paul Gilster November 19, 2019, 17:01

          Harold, you may well be right. Someone needs to do the math on that.

        • Paul Gilster November 19, 2019, 17:05

          Good point, Harold, and if the effects cancel, then this is irrelevant.

        • Alex Tolley November 19, 2019, 20:08

          IIRC, constant acceleration is what Janhunen claimed in the original paper IIRC. The sail area increases as the proton density declines, providing a near-constant thrust irrespective of the distance from the sun. In the current paper, however, the acceleration decreases with the distance from the star. [eqn 15]. [The Plasma magnet experiences near-constant acceleration wherever it is in the system]. Eqns 20, 21 give the final velocity. If we replace M* with the mass of our sun, then the terminal velocity is about 424 km/s, about the same velocity as the solar wind. This is 3.5x that calculated by Janhunen. Because the terminal velocity is now dependent on just the mass of the sun and its solar wind flux, a small mass star apparently allows teh sail to outrun its wind velocity. Figure 3 shows teh terminal velocities of electric vs light sails against stellar mass. Absurdly, as the stellar mass approaches 0, the velocity of the electric sail reaches (exceeds?) 2000 km/s, 5x the wind velocity, whilst the light sail is 0 km/s [technically imaginary]. In section 3.2, the authors assume that the sail will receive a new acceleration kick by turning on near the perihelion of the star, at about the same distance from the star as it was initially launched. This acceleration occurs even though the sail is now traveling faster than the star’s solar wind. I would claim this makes no sense at all. The star’s solar wind would act as a retarding force, much as the author’s note that the ISM would be if the sail was not turned off. But clearly the proton flux would be much denser and retard the sail far more quickly.

          Re: Crosswind velocity as the sail orbits the star.
          I don’t see the relevance to the issue of the sail outrunning the solar wind. Even if the sail is effectively running cross-wind, if its velocity is faster than the solar wind it would still be running into the solar proton stream ahead of it and deflecting the protons ahead of it, effectively slowing down. The only difference is teh angle of the sail and the non-normal angle of deflection.

          I don’t think there is anything magical about this. The sail works by deflection the protons. As soon as the sails velocity reaches the proton velocity, the sail is no longer experiencing any protons pushing on it. If it exceeds the proton velocity, it would be pushing the protons from behind and therefore experiencing a retarding force.

          A sail entering a star system at a velocity exceeding the local solar wind speed would always be pushing up against the proton stream whatever its position. This would retard the sail, not accelerate it.

          As I said, I think the equation is incorrect. The authors assume a constant STATIC pressure and INSTANTANEOUS acceleration wherever the sail is in the system. That is correct. But as the sail’s velocity increases, that pressure, and acceleration fall. AFAICS, none of the equations take the sail velocity into account when calculating the pressure on the sail. So their equations show an acceleration inside the system that is primarily due to proton pressure minus the star’s gravity, which implies that it could always add velocity when visiting another star system and turning on the sail at perihelion. I think this is their error.

          As this paper is being published in Acta Astronautica it has been peer-reviewed. I find it difficult to believe that I have caught a fundamental error that the reviewers missed, which is why I would like someone to check the equations and explain my error interpreting the equations. AFAICS, if the sail could accelerate and pick up velocity at each visited star, barring drag forces, it could reach the same velocities as a light sail even though the propelling proton stream is traveling well below that of the photons propelling a light sail.

          [Note that a lightsail sees a redshift in the photons accelerating the sail as the velocity increases, reducing the photon pressure. As the relative velocity of the electric sail and the solar wind converge a similar effect should occur, reducing the pressure to zero as the velocities match.]

          • Bruce D. Mayfield November 21, 2019, 12:24

            Well wait a just a minute. Paul’s post stated that it has been submitted for publication. Perhaps it hasn’t passed peer review as yet, and maybe it never will? Your objections make more sense to me than the proposal Alex.

      • Charlie November 20, 2019, 15:48
    • Ron S. November 19, 2019, 16:00

      If I understand the trajectory correctly the electric sail craft is not on a radial path outward from the star. In that case the velocity and quantity of charged particles will decline to zero once the craft reaches the wind velocity and acceleration would also go to zero.

      However the craft has a tangential velocity so that it is traveling across the wind. This is due to the launch from a planet which has a orbital (tangential) velocity. As the craft accelerates the speed can increase beyond the wind velocity because they are not in the same direction. The limit remains finite since you as you proceed the craft will ultimately outrun the wind.

      I have not done this calculation or read a paper that describes this in detail so I don’t know that velocity. But my intuition says it can’t be more than the sum of the launch planet’s orbital velocity and the wind velocity. But that’s isn’t much more than the wind velocity unless you start from an orbit very close to the star.

      Reading Paul’s comment I don’t see how that gets you to a higher velocity. Probably I’m missing something.

      • Paul Gilster November 19, 2019, 17:04

        You may well be right, Ron. I threw that out there in hopes someone could tell me if it made a difference in the outcome, but it may well not.

    • Mike Serfas November 20, 2019, 11:54

      The Jahunen and Sandroos paper that equation 7 comes from is at https://www.ann-geophys.net/25/755/2007/angeo-25-755-2007.pdf – it seems to examine simply the downwind pressure in a simulation of a wire in a perpendicular solar wind of fixed velocity.

  • Mike Serfas November 19, 2019, 14:33

    This network of kilometers-long wires seems like it should be exquisitely sensitive for detecting the “sounds” of space, but the closest I see to an article about it in ArXiv is https://arxiv.org/ftp/arxiv/papers/0901/0901.0047.pdf , which seems only worried about the material. How much could you detect with one of these at the heliopause?

  • James M Essig November 19, 2019, 14:48

    I have a cool book to be published next month on interstellar sails. I normally write on relativistic rockets, but the electric sails presented in this thread have caused me to think anew about interstellar sails. The wonderful things about light-sails and electric sails is that they kind of “live off the land” extracting potential energy embodied in light and plasma.

  • Neil Stahl November 19, 2019, 20:42

    I share the concern about how it could be accelerated faster than the solar wind that’s driving it. However I remembered the star S5-HVS1 that’s currently zipping out of the galaxy, presumably propelled by our central black hole using the “Hills mechanism”, whatever that is.

    Maybe if it can make its way to the galactic center and navigate really skillfully it can get going really fast. I don’t suppose that’s what the authors had in mind though.

    • Charlie November 20, 2019, 15:42

      The Hills mechanism[1] is a phenomenon that occurs when a binary star system is disrupted by a supermassive black hole.

      Tidal forces from the black hole cause one of the stars to be captured by it, and fall into an orbit around it. The other star is jettisoned away from the black hole at very high speeds. The phenomenon, proposed by astronomer Jack Hills in 1988 and confirmed in 2019, when an example of such a jettisoned star was observed.[2] The star S5-HVS1 is currently traveling out of the Milky Way, away from the galactic core at a speed ten times the typical speed of stars in our galaxy.[3]

  • Alex Tolley November 21, 2019, 20:13

    Because sails rely on momentum trasfer, I see 2 ways that in principle, a craft could travel faster than the solar wind.

    1. The mass of the sail is much lover than the mass of the impinging solar wind. The momentun transfer would have to be near instantaneous, like the collision of 2 billiard balls. If the sail’s mass was much lowever than a “slug” of solar wind, the sail’s resulting velocity would be faster than the solar wind. In this system, t4eh sail would turn on very briefly to capture the momentun of teh solar wind. Thereafter, the sail must be turned off to reduce drag from the solar wind colliding with teh sail ahead of it.

    2. Increase teh velocity of teh solar wind. In this case, I envision the “sail” being a proton collector. The protons are then accelerated in an engine perhaps like an ion engine. The advantage of such a device is that the solar wind protons can be collected irrespective of whether they are coming from behind or in front of the craft. As with other types of craft traveling in a fluid medium, the drag effects of the solar wind in front of the craft will limit its terminal velocity.

    • Antonio November 22, 2019, 11:33

      I don’t see how option 1 can make the sail go faster than the wind, since it will be pushing the protons in front of it. It would be like a leaf floating in the middle of a bison stampede.

      Option 2 seems like Zubrin’s dipole drive in disguise.

      • Alex Tolley November 22, 2019, 18:39

        If you read my option 1 carefully you will note that the sail is turned off immediately after receiving its momentum transfer kick. This means If the velocity exceeds the solar wind, the sail will no longer act as a drag due to hitting the wind ahead of it. It is only an “ in principle” thought experiment. The solar wind is not hitting the sail like a massive slug, nor is there any likelihood a sail would have the low mass needed.

        Option 2 could be any device that can accelerate the solar wind. Zubrin suggested one approach. There was another that used alternating electric fields to create pulses of accelerated ions. While option 2 is a potentially viable approach, we have no experimental evidence of such a device working. If the craft had a (nuclear) powered ion accelerator and a large magnetic field to capture and concentrate the ions over a large area, the craft could, in principle, use the solar wind as a fluid to move through, rather than move with. It would be analogous to a jet engine, rather than a rocket.

        • Antonio November 23, 2019, 5:32

          About your first paragraph, it sounds like Maxwell’s demon and of course will not work ever.

  • Alex Tolley November 21, 2019, 20:13

    One observation I took from the author’s paper is that for very low mass stars, the energy of the solar wind increases relative to the photons. This suggests to me that just maybe, a Dyson swarm around a dim, long-lived M-dwarf might not block the sunlight, but rather the soilar wind. The habitats might use electric or magnetic fields to harvest the energy in the solar wind. This suggests another possible det4ection approach. If we could detect the size of the heliosphere around a star, such a Dyson swarm would reduce the size of the heliosphere compared to its expected size. Therefore this effect, rather than a dimmining of the star would indicate a K2 civilization.

  • AlexTru November 23, 2019, 3:42

    All physical explanations that are standing after this electric sail idea have significant deviation from real science laws. Too simplified models, that do not take in account that described system is not monopole, but always dipole. So electric field of the whole system will not be perpendicular to wires , but have direction along the wire , i.e. pointed to direction of negative electrode (electron gun?)…
    Suppose that neutrally charged sheet of some material located perpendicularly to solar wind flow (i.e. traditional sail) will work better as sail than described “electric” system…

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