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Electric sail propulsion technology

Final Report Summary - ESAIL (Electric sail propulsion technology)


Executive Summary:

Efficient planetary exploration with high scientific return and especially sample return missions call for improved in-space propulsion technologies. The electric sail is a new European invention which has the potential to improve the state of the art by 2-3 orders of magnitude if using the lifetime-integrated total impulse versus propulsion system mass as the figure of merit. The electric sail is a propellantless method which uses the natural solar wind's momentum flux for producing spacecraft propulsion. In the ESAIL project we designed, built and tested prototypes of the key electric sail components (tethers, reels, spinup and guidance/navigation) needed to build a large electric sail ultimately capable of ~1 newton thrust and having ~100 kg propulsion system mass. Specifically, we produced a 1 km long sample of final-type E-sail tether, built and enviromental-tested a laboratory prototype of the so-called Remote Unit (an active nanosatellite-type device installed on the tip of each E-sail tether), built dynamical simulations of how the E-sail flies in real time-varying solar wind, analysed quantitatively how much an E-sail system of given thrust would weigh and made rigorous orbit calculations a large number of E-sail missions in the solar system.

Project Context and Objectives:

How to move a spacecraft in the solar system is a fundamental problem of space activities. Our present techniques (chemical rockets and ion engines) solve the problem only partially because they enable a total delta-v capability for the spacecraft which is not sufficient to perform some of the more ambitious missions. Also in many cases which are technically possible with the traditional techniques, the cost is nevertheless high because the propulsion system is heavy in comparison to the payload that must be moved.

The electric solar wind sail (electric sail, E-sail) is a novel propellantless propulsion method which is based on harnessing the solar wind for producing spacecraft propulsion. The main objectives of the ESAIL project were the following:

1) Produce 1 km piece of final-type E-sail tether, to prove that manufacturing kilometre length thin and micrometeoroid-resistant multi-wire tethers is possible by University of Helsinki's unique wire-to-wire ultrasonic bonding technique.
2) Demonstrate successful and reliable reeling in and reeling out of the tether, also after the reeled tether has been shaken in a vibration test bench to simulate launch vibrations.
3) Assess coating options for the E-sail tether. A coating is not absolutely necessary, but using a non-metallic coating would reduce the equilibrium temperature of the tethers in space and thus enable shorter solar distances for the mission. A coating would also likely decrease the probability of cold welding on the reel during launch vibrations (which is however small anyway) and improve optical visibility of the tethers which would be useful although not necessary for diagnostics.
4) Design and build a prototype "Remote Unit": a small autonomous device which hosts the auxiliary tether reels and small thrusters forinitiating and controlling the spin of the E-sail tether rig. Furthermore the Remote Unit must be lightweight, it must stay operational in a sufficiently wide solar distance range and it must tolerate the usual launch vibration and space vacuum and thermal environment conditions. Our prototype Remote Unit the solar distance range is 0.9-4 au which we consider a good achievement. Our Remote Unit and other hardware built in the ESAIL project does not contain any radioactive or otherwise dangerous or poisonous substances.
5) Produce and test a piece of prototype auxiliary tether. The auxiliary tether connects together the tips of the main tethers to guarantee dynamical stability of the E-sail tether rig despite solar wind variations.
6) Design, build and test a prototype main tether reel. The maintether reel that we built will be flight-tested in the Aalto-1 CubeSat mission after the project.
7) The Remote Unit is designed with two complementary propulsion options: miniaturised MEMS technology cold gas thruster and similarly miniaturised ionic liquid FEEP thruster. The thrusters are complementary in the sense that the cold gas thruster is at somewhat higher technical readiness level (TRL) while the ionic liquid FEEP thruster has much higher total impulse capability. Both types of thrusters developed in this project, in addition to their use in E-sail Remote Units, are directly applicable for generic attitude and orbit control tasks of satellites and other spacecraft. Because the thrusters are miniaturised, they are in fact enabling technology for small autonomous spacecraft (needed e.g. in economical in situ exploration of asteroids) and for affordable low mass tight formation flying satellite constellations (needed e.g. in more advanced Earth observation and telecommunication applications). The cold gas and ionic liquid FEEP thrusters can also be used as replacements for heavier traditional thrusters in almost any satellite application or spacecraft which needs micropropulsion in the relevant parametre range.
8) Develop software for dynamical simulation of the E-sail in realistic, time-dependent solar wind. The software acts as a "flight simulator" which was used extensively during the project for comparing flight properties of different geometric design options for the E-sail tether rig.
9) Develop quantitative concept for an E-sail spacecraft, including mass budget of its various subsystems. We published our mass budget analysis in a peer reviewed journal.
10) For a wide range of possible E-sail missions, search the mathematically optimal orbits and thrusting schedules for obtaining e.g. the mission flighttimes to different planets and asteroids, as function of E-sail thrust (tether rig size).
11) Public outreach: media interest towards the E-sail invention is high andwe had a large number of dissemination events and publicity during the project.

Project Results:

See attached file "achievements.pdf". For approximate resource usage, see attached file "approximate-resource-usage.pdf".

Potential Impact:

The E-sail is a device which produces a significant level of propellantless inexhaustible thrust from a system which is lightweight and in principle straightforward to build, and also safe and without poistonous, dangerous or radioactive components. The E-sail could have at least the following direct applications:

1) Enable spacecraft that can tour near-Earth objects and asteroids indefinitely in flyby and rendezvous mode. This is a dramatic improvement over present propulsion methods which allow only one or at most few targets to be explored by one mission before running out of propellant.
2) Enable getting a spacecraft to in principle any target in the solar systemk, with reasonably short traveltime and without increasing the launch mass.
3) Enable also two-way missions for many targets (although not for the outer solar system).
4) Enable missions that hover in an unnatural non-Keplerian orbit for specific tasks such as monitoring the solar wind with longer warning time or to have a permanent view to Earth's or other planet's or Sun's polar region.
5) Enable efficient and safe deorbiting of a satellite, for solving the increasingly acute problem of space debris in low Earth orbit.

Secondarily, the above-listed enabled technical capabilities could in turn enable the following novel kinds of larger application areas:

i) Economically feasible asteroid mining, because the E-sail solves the transportation problem. Asteroids could be mined e.g. for water, platinum group metals and iron and nickel structural materials. Water transported by E-sails to Mars orbit, for example would enable one to make the manned mission return propellant there, thus reducing the cost of manned Mars exploration by a large factor, also potentially enabling reusable vehicles that carry people and freight in both directions between the planets. Platinum group metals are valuable enough to be returned to Earth for direct selling. Iron and nickel from metallic asteroids could be used for large space constructions using e.g. remotely operated 3-D printing technology. In all the cases, the role of the E-sail is to transport the materials between asteroids and Earth or other planet.
ii) A traditional planetary mission requires a dedicated launch because the launcher upper stage typically gives the heliocentric kick towards the chosen planet. Because escape orbit capable launchers are all rather big and therefore expensive (the smallest one is currently Soyuz), this effectively means that a small planetary mission is not possible. When using the E-sail, this limitation is removed because any escape orbit is a possible starting orbit for any E-sail mission. Thus, several small E-sail probes could be launched with one escape-capable launcher, and the probes can be destined to different targets in the solar system. Thus, the E-sail is enabling technology for small, affordable deep space missions.

People and media have recognised these capabilities, broadly speaking. The media interest towards the E-sail is consistently high. We are interacting with the media often and the news spread far and wide. Some details are available in the Dissemination activities list included in this Final Report which contains 108 entries. The list is not complete because we are not practically able to keep track of all media attention that the E-sail project is receiving. As a recent example, our Estonian project partner Dr. Mart Noorma was selected the "citisen of the year 2013" in Estonia the launch of the E-sail testing satellite ESTCube-1 was voted as the "event of the year", and the prime minister of Estonia (Andrus Ansip) covered ESTCube-1 and the E-sail invention in his yearly speech to the Estonian Parliament in December 2013.

In short, the societal importance is that the E-sail could revolutionise space technology. Up to this point (TRL 4-5) the development has gone well and no potential show stopper are seen. The needed next step is testing and validation in space.

List of Websites:

ESAIL-specific website: http://www.electric-sailing.fi/fp7

General electric sail website: http://www.electric-sailing.fi

Finnish language general audience E-sail blog: http://www.electric-sailing.blogspot.fi

Contact: Pekka Janhunen, pekka.janhunen@fmi.fi +358 29 539 4635

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