Cosmos 1 - world's first solar sail space craft 

June 21, 2005 Cosmos 1 - the world's first solar sail spacecraft data helps analysing and develop solar sail technology. Its mission was  controlled from the Lavochkin Association in Moscow.  The Solar Sail in Orbit. NPO Lavochkin, Planetary Society (c) Courtesy

The electric solar wind sail, or electric sail for short, is a propulsion invention made in 2006 at the Kumpula Space Centre, a collaboration between Finnish Meteorological Institute and Department of Physics of the University of Helsinki.



The European Union has now selected the Finnish Meteorological Institute to lead an international space effort  ESAIL whose goal is to build the largest and fastest man-made device.

The electric sail is a new space propulsion concept, which uses the solar wind momentum for producing thrust. The electric sail is somewhat similar to the better-known solar radiation pressure sail which is often called simply the solar sail. (Courtesy Antonin Halas)

A full-scale electric sail consists of a number (50-100) of long (20 km), thin (25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts), used to keep the spacecraft and wires in a high (typically 20kV) positive potential.

The wires' electric field extends a few tens of meters into the surrounding solar wind plasma. The solar wind ions 'see' the wires as rather thick, 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 on average is about 2nPa at Earth distance from the Sun, about 5000 times weaker than the solar radiation pressure. Due to the very large effective area and very low weight per unit length of thin metal wire, the electric sail remains efficient.

A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but opened in space and connected to the spacecraft's electron gun, can produce several km2 effective solar wind sail area, capable of extracting about 10 millinewton force from the solar wind.

Left: The L'Garde Inc sail proposed for NASA's ST9 mission. Picture  by Tim Van Sant, ST9 Solar Sail Team Lead, NASA Goddard Space Flight Center. http: //www. andybrain. com/extras/solar-sail.htm

By equipping a 1,000kg 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.

Alternatively, to move medium size payloads at ordinary 5-10 km/s speed, but with lowered propulsion costs because the mass that has to be 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  direction of the thrust is basically away from the Sun, it can be varied within some limits by inclining the sail and tacking towards the Sun is therefore also possible.

A related but simpler device the plasma brake  can be used for deorbiting satellites to address the space debris issue. The working principles of the electric sail and the plasma brake will be tested in the coming years by the Estonian ESTCube-1 and Finnish Aalto-1 nanosatellites. 

The electric sail won the 2010 Finnish Quality Innovation Prize among Potential innovations.