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 2013 on the Estonian ESTCube-1 and in 2014 on the Finnish Aalto-1 nanosatellites.^[5][6]

Principles of operation and design [edit]

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 electrons 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.

Intrinsic limitations [edit]

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 [2].

Electric solar wind sail [edit]

ESTCube-1 was the first satellite to test electric sail. Artist's rendering of the satellite.

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 straightened 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.

In order to minimise damage to the thin wires from micrometeoroids, the wires would be formed from a number of individual strands, 25–50 micrometer in diameter, welded together at regular intervals. Thus, even if one individual wire was severed, a conducting path along the full length of the braided wire would remain in place. The feasibility of carrying out these welds by ultrasonic welding was demonstration at the University of Helsinki in January 2013.^[7]

Applications [edit]

• Fast missions (>50 km/s or 10 AU/year) out of the Solar system and heliosphere with small or modest payload
• As a brake for a small interstellar probe which has been accelerated to high speed by some other means such as laser lightsail
• Inward-spiralling missions to study the Sun at a closer distance
• Two-way missions to inner Solar System objects such as asteroids
• Off-Lagrange point solar wind monitoring spacecraft for predicting space weather with a longer warning time than 1 hour

References [edit]

• Janhunen, P., Electric sail for spacecraft propulsion, Journal of Propulsion and Power, 20, 4, 763-764, 2004.
• Janhunen, P. and A. Sandroos, Simulation study of solar wind push on a charged wire: basis of solar wind electric sail propulsion, Annales Geophysicae, 25, 755-767, 2007
• "The electric solar wind sail by Pekka Janhunen". Retrieved 2008-04-18. 

External links [edit]

• Electric Sailing
• Kumpula Space Centre
• Finnish Meteorological Institute/Space Research
• "Electric Solar Wind Sail Could Power Future Space Travel In Solar System". ScienceDaily. 2008-04-17. Retrieved 2008-10-15.