A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Therefore the solar wind ions "see" the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or "flying" the resulting spacecraft electrically.
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. For example, by equipping a 1000 kg spacecraft with 100 such wires, one may produce acceleration of about 1 mm/s^2. 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.
The main limitation of the electric sail is that since it uses the solar wind, it cannot produce much thrust inside a magnetosphere where there is no solar wind. Although the direction of the thrust is basically away from the Sun, the direction can be varied within some limits by inclining the sail. Tacking towards the Sun is therefore also possible.
A schematic view of the deployment phase of a spinning electric sail with auxiliary rockets. Only eight wires have been drawn for simplicity. The violet-blue surfaces are solar panels and the yellow lines are propulsive arms (with small rockets attached to the tips) which create the initial spacecraft spin.
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 dashed red line.
These figures are part of an 17.8 MB animation showing deployment and operation of a simple electric sail spacecraft whose spin is initiated by a pair of auxiliary rockets.
The method is economical because no auxiliary fuel or rockets are needed and the turning motor needs only small amount of power. With long tethers, small mechanical misaligments might cause the oppositively spinning tethers to collide which would be catastrophical. However, by running the antisunward tethers with a small to modest voltage during deployment the solar wind bends them slightly outward which should eliminate this danger.
The Siamese Twins animation (9.1 MB) shows the complete deployment sequence.
Model of electric sail hanging on display in Rikhardinkatu public library in Helsinkin October 2008
More information: Pekka Janhunen, firstname.lastname@fmi.fi