The Electric Solar Wind Sail (E-sail) was invented by Pekka Janhunen, Finnish Meteorological Institute. He first published “Electric sail for spacecraft propulsion” in the AIAA Journal of Propulsion and Power. (2004)
* Uses solar wind momentum for producing thrust
* Consists of a number of long thin conducting tethers
* An electron gun is used to keep the wires at high positive potential
* The electric field of the wires extends tens of meters into the surrounding solar wind plasma
There is now more details of the first ESTCube-1 space mission to deploy a ten meter solar electric sail wire. It will be followed by a 100 meter test in 2014 and then a satellite with 4 – one kilometer long solar electric sail wires.
* Test of a 100 m tether deployment on Aalto-1 3U CubeSat (2014)
* CubeSat mission to the solar wind
* To deploy and confirm the deployment of a 10 m conductive Hoytether from a 1U CubeSat
* To measure the electric sail force, interacting with the tether. The success criteria for this objective is the measured effect on the satellite attitude resulting from electric sail force
Launch planned for 2013
The E-Sail uses charged tethers to extract momentum from the solar wind particles to obtain propulsive thrust. According to current estimates, the E-Sail is 2-3 orders of magnitude better than traditional propulsion methods (chemical rockets and ion engines) in terms of produced lifetime-integrated impulse per propulsion system mass.
A typical E-Sail powered spacecraft might weight 200 kg and have 100 charged tethers, each of 20 km in length. The sail tethers are themselves knitted out of four 25−50μm diameter metal wires in a crossed “Hoytether” pattern in order to minimise the possible destructive effects of micrometeoroids cutting a vulnerable single wire (Hoyt and Forward, 2001). These tethers, if made out of aluminium (2.7 g/cm3) wires, would weigh less than 30 kg for the whole E-Sail. Here the central 25μm wires are assumed to have a 30 angle with respect to the bordering 50μm wires. With 70 kg reserved for the mass of the spacecraft bus, electron gun, solar panels and other E-Sail system parts, one would be left with a payload of 100 kg. With other tether materials of lower density or thickness, the mass taken by the wires can be significantly reduced or the length of the wires risen to produce more force for the same mass. Newest results show that the force produced by the solar sail is five times larger than what was estimated at first, 500 nN/m (Janhunen, 2009). For our default E-Sail this would amount to a force of about 1 N.
Enlarging the size of one E-Sail would directly transfer into higher towing force. The maximum length achieved with normal metals used as E-Sail tether wires is around 100 km, beyond which both the resistivity of the wire and its tensile strength might become an issue. Greater lengths might be achieved with novel materials having much improved strength and lower density when compared to the copper considered here. 100 km long tethers would produce five times the tow of our default sail with 20 km long tethers. Tethers could also be spaced in higher angular density, for example 200 tethers around the sail instead of the default 100 proposed, again roughly doubling the tow. The steering of high number of such a long wires could be problematic though. It might even be possible to upgrade the E-Sail force up to hundreds of Newton’s and even beyond, which would make the E-Sail technology very attractive for various other uses as well as for towing bigger asteroids.