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.