The electric solar wind sail developed at the Finnish Meteorological Institute two years ago has moved rapidly from invention towards implementation. Electric sail propulsion might have a large impact on space research and space travel throughout the solar system.
The electric solar wind sail developed by Pekka Janhunen might revolutionize travelling in space. The electric sail uses the solar wind as its thrust source and therefore needs no fuel or propellant. The solar wind is a continuous plasma stream emanating from the Sun. Changes in the properties of the solar wind cause auroral brightening and magnetic storms, among other things.
The main parts of the device are long metallic tethers and a solar-powered electron gun which keeps the tethers positively charged. The solar wind exerts a small but continuous thrust on the tethers and the spacecraft.
“We haven't encountered major problems in any of the technical fields thus far. This has already enabled us to start planning the first test mission,” says Pekka Janhunen. An important subgoal was reached when the Electronics Research Laboratory of the Univ. of Helsinki managed to develop a method for constructing a multiline micrometeoroid-resistant tether out of very thin metal wires using ultrasonic welding. The newly developed technique allows the bonding together of thin metal wires in any geometry; thus, the method might also have spinoff applications outside the electric sail.
A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 20 μm) 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 (up to 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 50 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 implementation details are presently under study.
The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun. This is about 5,000 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 a 1 km2 effective solar wind sail area which is capable of extracting 1-2 millinewton force from the solar wind. For example, by equipping a small, 200 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. Small 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.
One 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.
The electric sail could enable faster and cheaper solar system exploration. It might also enable economic utilization of asteroid resources for, e.g. producing rocket fuel in orbit.
“The electric sail might lower the cost of all space activities and thereby, for example, help making large solar power satellites a viable option for clean electricity production. Solar power satellites orbiting in the permanent sunshine of space could transmit electric power to Earth by microwaves without interruptions. Continuous power would be a major benefit compared to, for example, ground-based solar power where storing the energy over night, cloudy weather and winter are tricky issues, especially here in the far North,” says Pekka Janhunen.
Component work for the electric sail was carried out at the Univ. of Helsinki and in Germany, Sweden, Russia and Italy. The electric sail was invented as a by-product of basic research done at the Finnish Meteorological Institute on the interaction of the solar wind with planets and their atmospheres. Work on the electric sail in Finland is currently funded by the Academy of Finland and private foundations.
The first international electric sail meeting will be arranged at ESA ESTEC in Noordwijk, The Netherlands, on May 19, 2008.
