Enter the Electric Sail

Some years back at the Aosta interstellar conference I had the pleasure of being on a bus making its way at night through the Italian alps with Pekka Janhunen sitting immediately in front of me. Janhunen (Finnish Meteorological Institute) is the developer of the electric sail concept soon to be tested by the ESTCube-1 satellite, which launched last night aboard a Vega rocket from the Kourou spaceport in French Guiana. Our group had been talking about interstellar issues all day at the conference and now, headed back to the hotel following a memorable dinner at high elevation, I was curious whether an electric sail had interstellar applications.

The immediate answer seemed to be no, given that the highest velocities Janhunen had been talking about for the idea were about 100 kilometers per second, much faster than Voyager 1’s 17 kilometers per second, but a long way short of what we would like to see on an interstellar flight. But the ever thoughtful Pekka pointed out to me that as a means of deceleration, electric sails might have a future, braking against the stellar wind from a destination star. Deceleration being a huge problem for any interstellar probe, the idea has stuck with me ever since.

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

What an electric sail would do is to ride the stream of charged particles flowing out from the Sun, and fast missions to the outer system could thus be implemented if we get the system into full gear. The ESTCube-1 satellite, the work of Estonian students testing out Janhunen’s ideas, uses a long wire that maintains a steady electric potential as its means of interacting with the solar wind. Janhunen, in an article in IEEE Spectrum, calls ESTCube-1 “…the first attempted experiment to measure the Coulomb drag experienced by a charged wire or tether in moving plasma.”

ESTCube-1’s tether is a 50 micrometer wide, 10 meter long wire made out of four strands of aluminum that will gradually be deployed from the satellite in a process that could take as much as a week. Once deployed, the tether will be charged and variations in the satellite’s rotation rate will, if all goes well, reveal the interactions between it and atmospheric ions. But future electric sails will soon be deploying longer wires. A follow-up to ESTCube-1 called Aalto-1 is designed around a 100-meter tether. This one is also a student project, built at Aalto University in Finland and designed in part to test charged tethers as a means of deorbiting small satellites.

Assuming the concept passes its initial muster, we can look forward to upsized missions using tethers up to 20 kilometers long, deploying as many as a hundred of these from a single spacecraft. This is the design that, in computer simulations, yields potential speeds of 100 kilometers per second, fast enough to get a payload into the nearby interstellar medium in about fifteen years. With a spacecraft like this, keeping the sail’s wires in a 20 kV positive potential allows the sail to ride the solar wind ions while making issues of deployment relatively simple.

A sail like this is surprisingly efficient. From a page on the concept maintained by Pekka Janhunen:

The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun. This is about 5000 times weaker than the solar radiation pressure. Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft’s electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind.

As with any sail, the effect is small but cumulative and yields serious velocities over time:

…by equipping a 1000 kg spacecraft with 100 such wires, one may produce acceleration of about 1 mm/s2. After acting for one year, this acceleration would produce a significant final speed of 30 km/s. Smaller payloads could be moved quite fast in space using the electric sail, a Pluto flyby could occur in less than five years, for example. Alternatively, one might choose to move medium size payloads at ordinary 5-10 km/s speed, but with lowered propulsion costs because the mass that has to launched from Earth is small in the electric sail.

ESTCube-1 will help us measure the forces exerted on its single tether by the ionospheric ram flow acting on the satellite, a flow that substitutes for the solar wind in the case of this small CubeSat mission. The Aalto-1 test will occur next year if all goes well, and we will then have to see how the electric sail stands up compared to its solar sail competition. More on electric sail concepts tomorrow, when I want to look at important questions of stability.

A key paper on electric sails is Janhunen and Sandroos, “Simulation study of solar wind push on a charged wire: solar wind electric sail propulsion,” Annales Geophysicae 25, (2007), pp. 755-767.