An Electric sail (also called electric solar wind sail or E-Sail) is a proposed form of spacecraft propulsion using the dynamic pressure of the solar wind as a source of thrust. It uses an electric field for deflecting solar wind protons and extracting momentum from them. It was invented by Pekka Janhunen from Finland in 2006 at the FMI and creates a "virtual" sail by forming an electric field on small wires.[1]. To test the technology a new European Union-backed electric sail study project is underway.[2] The 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. The research project called E-Sail involves five European countries and will last for three years. In the EU evaluation, the ESAIL project got the highest marks in its category.[3] The electric solar wind sail could enable faster and cheaper access to the solar system and in the longer run may enable an economic utilisation of asteroid resources.[4] The working principles of the electric sail will be tested in the coming years on the Estonian ESTCube-1 and the Finnish Aalto-1 nanosatellites.[5]
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The electric sail consists of a number of thin, long and conducting tethers which are kept in a high positive potential by an onboard electron gun. The positively charged tethers repel solar wind protons, thus deflecting their paths and extracting momentum from them. Simultaneously they also attract electron from the solar wind plasma. The arriving electron current is compensated by the electron gun. A way to deploy the tethers is to rotate the spacecraft and have the centrifugal force keep them stretched. By fine-tuning the potentials of individual tethers and thus the solar wind force individually, the attitude of the spacecraft can be controlled.
The electric sail probably cannot be used inside planetary magnetospheres because there is no solar wind there, only slower plasma flows and magnetic fields. While modest variation of the thrust direction can be achieved by inclining the sail, the thrust vector always points more or less radially outward from the Sun. It has been estimated that maximum operational inclination would be 60°, resulting in a thrusting angle of 30° from the outward radial direction [6].
The electric solar wind sail has little in common with the traditional solar sail. First of all the E-Sail gets its momentum from the solar wind ions, whilst photonic sail is propelled by photons. In E-Sail, the part of the sails is played by straigthened conducting tethers (wires) which are placed radially around the host ship. The wires are electrically charged and thus an electric field is created around the wires. The electric field of the wires extends a few dozen metres into the surrounding solar wind plasma.
Because the solar wind electrons react on the electric field (similarly to the photons on a traditional solar sail), the functional radius of the wires is based on the electric field that is generated around the wire rather than the actual wire itself. This fact also makes it possible to maneuver a ship with an electric solar wind sail by regulating the electric charge of the wires. A full-sized operational electric solar wind sail would have 50-100 straightened wires with a length of about 20 km each.
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