The E-Sail mass is expected to only weigh in the range of hundreds of kilograms, hence the E-Sail is 100 – 1000 times more efficient than traditional techniques. To produce the same total impulse one would need 100 tons of chemical fuel (specific impulse 300 s) or 10 tons of ion engine propellant (specific impulse 3000 s). Instead of a 13 ton launch of one solar electric propulsion system, one could launch fifty or one hundred of the E-Sails which could combine towing to provide 50 Newtons of towing capacity. The E-Sails would be able to capture 20 to 40 times the mass of asteroids for equivalent launches. Also, the E-Sails can be used repeatedly if there is a long term power source for the electron gun, they would not have other consumables and could keep capturing the solar wind.
The electric solar wind sail (E-Sail) is a new propulsion method for interplanetary travel which was invented in 2006 and is currently under development. 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 number of positively charged tethers are radially deployed from a rotating spacecraft and stretched by the centrifugal force. Because the tethers are charged, they deflect charged particles of the streaming solar wind (from here also referred to as SW), thus producing a Coulomb drag interaction which transfers momentum from the particles to the tethers. Most of the momentum comes from the protons, where the majority of the solar wind momentum flux is. Solar wind electrons will continuously impact the positively charged tethers, making it necessary to maintain the tether charging by actively pumping out electrons from the system. The onboard electron gun, typically of few hundred watts of power, is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential.
Upcoming solar electric sail projects
ESTCube-1 student satellite project – There will be a 10 meter long test tether onboard the ESTCube-1 satellite, to be launched in 2012.
Aalto-1 student satellite project – There will be a 100 meter long test tether onboard the Aalto-1 satellite, to be launched in 2013 or 2014.
SWEST (Solar Wind Electric Sail Test) is a proposal to the EU whose purpose is to build a flight-ready 60 kg satellite which is able to measure the E-sail effect in the solar wind with four 1 km long tethers. The satellite is mainly built by the Alta space company in Italy. It would have 25 milliNewtons of thrust.
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.
The effectiveness of E-Sail will be tested in 2012 onboard the Estonian satellite ESTCube-1.
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.
Also, different kinds of net deployment possibilities might be considered. For example assisting tethers could be inserted between individual radial tethers connecting them with each other. This approach might allow better coverage of the now empty space between the furthest ends of the tethers. It could also provide more stability to the system, even helping the sail in keeping its overall form for example in that undesired scheme where one tether was to loose its maneuvering ability or be cut by a micrometeorite impact. The lines connecting the tethers with each other would need to have some part of them insulated so that individual tethers could still be steered by simply varying their voltage. Packaging and deployment problems might cause some trouble, but if they can be solved, this approach could considerably increase the effectivity of a single E-Sail without increasing its dimensions. Moreover, an E-Sail with more compact dimensions should be easier to manoeuvre (for example when adjusting the plane of operation).