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December 09, 2010

EU funds Electric solar sail project

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

In December 8-9, 2010, the kickoff meeting of the electric sail EU project was held at the Finnish Meteorological Institute. The ESAIL project will last for three years, its EU funding contribution is 1.7 million euros and its goal is to build the laboratory prototypes of the key components of the electric sail.

Esail website

When accelerating, the theoretical limit is the solar wind speed 400-800 km/s (895,000 mph - 1,790,000 mph) which is about 0.1% of speed of light. It might be possible to use an electric sail as a brake for an interstellar probe which has been accelerated to high speed by some other method such as the laser sail.



The electric solar wind sail may enable faster and cheaper access to the solar system. In the longer run it may enable an economic utilisation of asteroid resources. A related but simpler device (the so-called 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 the Finnish Aalto-1 nanosatellites.

According to estimates, a full scale electric sail will produce one newton continuous thrust and weigh only 100 kg. In certain missions the performance level of the electric sail is 100-1000 times larger than that of present chemical rockets and ion engines. The electric sail consists of long and thin metallic tethers which are kept in a high positive potential by an onboard solar-powered electron gun. The charged tethers repel solar wind protons so that the solar wind flow exerts a force on them and pushes the spacecraft in the desired direction.

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 (67,000 mph). 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 (11,000-22,000 mph), 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.

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