A Finnish team has introduced a new wrinkle on the solar sail idea. Or more specifically, on the general principles of the magnetic sail, which would tap the propulsive power of the solar wind to push a ‘sail’ created as a field around the spacecraft itself. The so-called ‘electric sail’ would use fifty to one hundred 20-kilometer long charged tethers, their voltage maintained by a solar-powered electron gun aboard the vehicle. We’re talking about tethers made of wires that are thinner than a human hair, thin enough that each can be wound into a small reel.
But unwind the tethers and you get interesting results. The electric field of each wire now extends tens of meters into the solar wind flow. A single tether yields the equivalent effective area of a sail roughly a square kilometer in size. You can see the promise of deploying multiple tethers to reach high velocities. What’s more, this sail allows the spacecraft to ‘tack’ towards the Sun as well as sailing outward from it.
All told, the electric sail points toward faster transport within the Solar System. The team, whose work was published in late March, has been running supercomputer simulations to verify the concept. Their numbers are encouraging: taking an average figure for the solar wind (which can range from 300 to 800 km/s), their sail can generate speeds up to 100 km/s, which would get a probe to Pluto in less than four years and into the nearby interstellar medium in fifteen.
Image: In this phase the wires have been deployed and the electron gun has been started. The blue lines symbolise the electron beam of the gun. The spinup propulsion arms and associated fuel tank have been jettisoned to save mass. The solar wind acts on the wires, bending them slightly. The electric field around the wires is depicted by a dashed red line. Credit: P. Janhunen/Kumpula Space Centre.
Although we lose the solar wind as we move beyond the outer planets, the Finnish design does have interstellar implications, at least by extension. From the paper:
In interstellar space the plasma ?ow is rather slow. Thus the electric sail cannot be used for acceleration, but it can instead be used for braking the spacecraft. There are some concepts such as the laser or microwave sail which are designed to “shoot” a small probe at ultrahigh speed towards e.g. a remote solar system. In these concepts the power source is at Earth so that the accelerated probe needs no propulsive energy source. Stopping the probe at the remote target is very dif?cult, however, if one has to rely on power beamed from the starting point. The electric sail might then provide a feasible stopping mechanism for such mission concepts. In other words, one would shoot a probe to another solar system at ultrahigh speed using a massive and powerful laser or microwave source installed in near-Earth space, brake the probe before the target by the electric sail action in the interstellar plasma and ?nally explore the extrasolar planetary system with the help of the electric sail and the stellar wind. A similar idea was proposed by Zubrin and Andrews (1990) for their magnetic version of the solar wind sail.
Note the difference between the electric sail (and magsail concepts like Robert Winglee’s M2P2) and the standard solar sail. For one thing, the electric sail relies on a stream of charged particles (the solar wind) to push it, while a solar sail taps the momentum of solar photons. For another, a solar sail is a physical structure that faces problems of deployment that only get magnified as you go to larger and larger sail designs. An electric sail’s deployment problem comes down primarily to tether extension, a much less demanding proposition. And that puts sails front and center as we look to ramp up travel times to the outer planets.
The paper, available for download, is Janhunen et al., “Simulation study of solar wind push on a charged wire: basis of solar wind electric sail propulsion,” Annales Geophysicae 25 (March 29, 2007), pp. 755–767. And here’s a backgrounder, including an interesting animation, from the Kumpula Space Centre in Helsinki.
Now *that* is a neat idea!
Alfven (in a letter to Science in the 1970s) proposed another kind of plasma sail, in which a single linear conductor, perpendicular to the flow of a magnetized plasma, has a current induced in it by the magnetic field. Plasma contactors at the ends complete the circuit through the plasma.
Thanks for the info on that one, Paul. I’ll have to look Alfven up.
Amazing how thoughts spawn solutions… I get it now. E = mc^2. m = E/c^2.
E = h*nu. Therefore m = h*nu/c^2. Disregard (delete?) previous post.
djlactin, it took me a long time to figure out photons and momentum, too. Not exactly intuitive at first blush!
Articles like this cause me to become emotional over the death of NIAC. I hope this concept doesn’t wither away due to lack of research funding.
So which is potentially faster, this electric sail or the magnetic sail proposals I’ve heard about? I’m finishing up a novel set in the Asteroid Belt a century from now, and I’ve been assuming magnetic sails (including M2P2 types) would be pretty standard. Would electric sails be even better, or just about as good, or what?
And what’s this about a maximum speed the sail can generate? I thought in space it was all about acceleration — as long as you could produce thrust, you could accelerate indefinitely. Is there some kind of drag force that comes into play?
Christopher, the last time I checked on M2P2 (magsail), Robert Winglee was talking about 50 to 80 km/s, as opposed to the electric sail figure of 100 km/s, but at this stage of the game both seem like very competitive designs.
(Sometimes I can’t leave an idea alone…)
m = h*nu/c^2….
considering that momentum p = mc and that c = lambda*nu,
p = h/lambda
The momentum of a photon is simply planck’s constant / wavelength!
And what’s this about a maximum speed the sail can generate? I thought in space it was all about acceleration — as long as you could produce thrust, you could accelerate indefinitely. Is there some kind of drag force that comes into play?
As soon as you’re traveling faster than the solar wind (100 to 400 km/s, depending), the sail interacting with the solar wind can no longer accelerate you away from the sun.
Good piece of work, but I think Bob Forward described something very similar in astronautic acta about 20 years ago. All kinds of sails will work, but we can’t afford to get to low earth orbit. Very frustrating.
DGA
What a pleasure to have you here, Dana! I’ve mentioned it before, but a recent paper by Dr. Andrews should be required reading for those interested in our topics: It’s “Interstellar Propulsion Opportunities Using Near-Term Technologies” appearing in Acta Astronautica Vol. 55 (2004), pp. 443-451. His work with Robert Zubrin on magsails and Jordin Kare on the sailbeam concept should also be familiar; we have some items on each in the archive here.
And need I say how much I agree with the frustations of getting to low Earth orbit?
Dears,
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Thank you for your attention
Best regards
Would a “parachute” design which would include transverse wires be helpful? It would increase the cross-sectional area, may eliminate the need for axial rotation, and may inflate spontaneously.
Interesting idea, Doug. And I should note that some of the earliest solar sail design (i.e., sails driven by the momentum imparted by photons) used a parachute design. Thus Carl Wiley’s early sail design in Astounding:
https://www.centauri-dreams.org/?p=623