Electric sail

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Not to be confused with magnetic sail or photon sail.

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]

Development history[edit]

To test the technology, a new European Union-backed electric sail study project was announced by the FMI in December 2010.[2] The EU funding contribution is 1.7 million euros. Its goal is to build laboratory prototypes of the key components. The E-Sail research project involved five European countries and ended in November 2013.[3] In the EU evaluation, the project got the highest marks in its category.[4] The technology could enable faster and cheaper access to the Solar System, and in the longer run may enable an economic utilisation of asteroid mining.[5] The working principles of the electric sail underwent testing in 2013 on the Estonian ESTCube-1, to be followed in 2014 on the Finnish Aalto-1 nanosatellites.[6][7]

An international research team that includes Pekka Janhunen, applied for funding through a 2015 NAIC Phase II solicitation for further development at NASA's Marshall Space Flight Center.[8]

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 and extracting momentum from them. Simultaneously they attract electrons from the solar wind plasma. The electron gun compensates for the arriving electric current.

One way to deploy the tethers is to rotate the spacecraft, using centrifugal force to keep them stretched. By fine-tuning the potentials of individual tethers and thus the solar wind force individually, the spacecraft's attitude can be controlled.

E-Sail missions can be launched at almost any time with only minor variations in travel time. By contrast, conventional slingshot missions must wait for the planets to reach a particular alignment.[9]

Intrinsic limitations[edit]

The electric sail probably cannot be used inside planetary magnetospheres because the solar wind does not penetrate them, allowing 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. Unlike conventional spacecraft that can orbit around their destinations, E-Sail vehicles have no way to reduce their velocity to match that required for orbit, limiting the amount of time for research.[9]

Electric solar wind sail[edit]

Artist's rendering of ESTCube-1, launched in May 2013 was the first satellite to test an electric sail.

The electric solar wind sail has little in common with the traditional solar sail. The E-Sail gets its momentum from the solar wind ions, whilst a photonic sail is propelled by photons. In the E-Sail, the part of the sail 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 affect 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 manoeuvre by regulating the wires' electric charge. A full-sized sail would have 50–100 straightened wires with a length of about 20 km each.[citation needed]

In order to minimise damage to the thin wires from micrometeoroids, the wires would be formed from multiple strands, 25–50 micrometers in diameter, welded together at regular intervals. Thus, even if one wire was severed, a conducting path along the full length of the braided wire would remain in place. The feasibility of using ultrasonic welding was demonstrated at the University of Helsinki in January 2013.[10]

Applications[edit]

Uranus entry probe[edit]

Janhunen has proposed a mission to Uranus powered by an electric sail. The mission could reach its destination in about the same time that the earlier Galileo space probe required to arrive at Jupiter, just over one fourth as far away. Galileo took 6 years to reach Jupiter at a cost of $1.6 billion, while Cassini-Huygens took 7 years to get to Saturn and cost almost as much. The sail is expected to consume 540 watts, producing about 0.5 newtons accelerating the craft by about 1 mm/s2. The craft would reach a velocity of about 20 km/s by the time it reaches Uranus, 6 years after launch.[9]

The proposed craft has three parts: the E-Sail module with solar panels and reels to hold the wires; the main body, including chemical thrusters for adjusting trajectory en route and at destination and communications equipment; and a research module to enter Uranus' atmosphere and make measurements for relay to Earth via the main body.[9]

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