Electric Space Sail to Get Its First Test

A small contingent of Estonian students is counting down the hours at the Kourou spaceport in French Guiana, where a Vega rocket is set to carry the country’s very first satellite into orbit a little after 2 A.M. GMT on 4 May.

Dubbed ESTCube-1, the satellite will be the first test of a concept for an electric sail made out metal tethers. Unlike an ordinary solar sail, which uses the radiation pressure created when photons collide with a spacecraft to physically push it along, an electric sail would propel a spacecraft by keeping a steady electric potential on long wires, or tethers. These tethers would move the spacecraft by electromagnetic interactions with the solar wind, the steady stream of charged particles emanating from the sun.

Proponents of the idea say such tethers could one day carry spacecraft around the solar system. Closer to home, a tether could be charged up and used to deorbit a satellite at the end of its life, reducing the amount of space junk.   

The scheme depends on making very narrow and very long tethers. The exact specifications depend, of course, on how big a spacecraft you want to move and how fast you want to get it to its destination. But as a guide, proponents of the electric sail say that, over the course of a year, a modestly-sized 1000-kg craft with 100 tethers could accelerate up to a decent clip of 30 km/s. That's about twice the current speed of the New Horizons spacecraft, currently en route to Pluto. 

To pick up enough charge, electric sail tethers will need to be long—perhaps as much as 20 km. But each wire can be just a few dozens micrometers thick, which would keep its overall weight to just a few hundred grams.

The first step is proving this approach can work with a single tether. ESTCube-1 will be the “the first attempted experiment to measure the Coulomb drag experienced by a charged wire or tether in moving plasma,” says Pekka Janhunen of the Finnish Meteorological Institute in Helsinki, who proposed the electric sail.

The 1-kg nanosatellite, which is based on the standard 10-by-10-by-10-cm CubeSat design, will ride into space along with two much bigger satellites, the European Space Agency’s Proba-V, which will map global vegetation cover, and a Vietnamese Earth observation satellite called VNREDSat-1.

Once in orbit, ESTCube-1 will slowly reel out a 50-micrometer-wide, 10-meter-long aluminum tether. This process will likely happen very slowly, says Mart Noorma, a professor at the University of Tartu and the academic advisor to the team: "It could take a week.”

When the deployment is completed, the team will test a MEMS-based electron gun, which will be used to charge the tether. Then the team will try to measure the interaction between the tether and atmospheric ions by looking for slight deviations in rotation rate.

As the European Space Agency notes [pdf], tethers “have historically had a mixed record in space—about half have snapped or failed to deploy.” Noorma says that was a prime consideration in designing the tether, which is constructed from four individual strands of aluminum. These were bonded together so that there is some space between the wires. This creates some redundancy that should in theory reduce the chance that a micrometeroid impact will completely sever the tether.

The same tether technology will get another test next year, with a follow-up experiment called Aalto-1. That mission will test a much longer (100-meter) tether.

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