Solar electric sails could have their first prototype launched in 2012 The electric sail is a new space propulsion concept which uses the solar wind momentum for producing thrust. A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Therefore the solar wind ions “see” the wires as rather thick, about 100 meter wide obstacles.
This article discusses the roadmap for solar electric sails to achieve hundreds and even thousands of newtons of propulsion. For comparison a VASIMR plasma rocket needs 40 kW/Newton, so 40 Megawatt VASIMR engines would be producing ~1,000 Newton. Five to ten 40 Megawatt VASIMR engines would be used for a 39 day manned mission to Mars scenario tied to three nuclear reactors.
The 39 day VASIMR mass-breakdown is…
476 tons propellant
100 tons power-supply
24 tons payload/structure
600 tons total
A 100 ton advanced material E-sail with 1000 km tethers could achieve 1000 newtons of propulsion and achieve similar performance.
The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun. This is about 5000 times weaker than the solar radiation pressure. Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft’s electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind. For example, by equipping a 1000 kg spacecraft with 100 such wires, one may produce acceleration of about 1 mm/s^2. After acting for one year, this acceleration would produce a significant final speed of 30 km/s.
In order to attach more E-Sails and thus to scale up the force, it could be possible to launch several smaller E-Sail stations that would then transfer their pulling power to the asteroid by separate or shared towing cords. This would look like a cosmic equivalent of towing boats, but would introduce technical challenges into steering and controls of the E-Sails to prevent them from clashing with each other. The stabilisation of the rotation of an asteroid might also be an issue. If the rotation is out of control, there is a risk of multiple wires getting distorted and knotted. A gravity tractor discussed in the previous chapter would not have this problem, although some issues might arise with controlling the heavy tractor mass itself. The best option though might be to tie several E-sails onto the same towing cord.
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.
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 effectiveness of a single E-Sail without increasing its dimensions. Moreover, an E-Sail with more compact dimensions should be easier to maneuver (for example when adjusting the plane of operation).
According to our estimates, the baseline E-sail, which is the target of our investigations, provides 1 N thrust, which scales in distance to the Sun as ~1/ r, weighs ~100 kg, provides ~30° of thrust vectoring capability, and needs no propellant or other consumables. Such a system is two to three orders of magnitude more efficient than currently used systems (chemical rockets and ion engines), and thus it has the potential to be quite a revolutionary step in future space technology. The E-sail would enable at moderate cost level a number of exciting solar system science missions such as a mission out of the heliosphere, rapid flyby missions of Kuiper belt or other objects, sample return from many targets and non-Keplerian orbit missions. It could also make asteroid resource utilization economically advantageous by providing a very efficient way of transporting the mined materials. With more advanced tether materials than aluminum, the performance of the E-sail could be larger than the baseline system since for fixed E-sail mass, the reached thrust level is roughly proportional to the usable tensile strength per mass density of the tethers.
If one wants a specific acceleration of 10 mm/ s2, with aluminum tethers (10 MPa usable tensile strength) the thrust is 1 N, while with a tensile strength of 100 MPa, the thrust is over 10 N. 100 MPa usable tether strength should not be unrealistic since, e.g., carbon fibers and silicon carbide fibers routinely reach several gigapascal strength. With hypothetical carbon nanotube tethers strength more than 10 GPa, a 100 ton E-sail with 1000 km tethers might haul 10^6 kg payload at 1 mm/ s2 acceleration, assuming that control and other issues would also be solved.
When will the E-sail be ready? No major unsolved problems are currently known, which would hinder its realization, but some key items are yet to be demonstrated. Most fundamentally, the E-sail effect must be experimentally measured to confirm its existence and magnitude. Experimental investigation of the E-sail effect is the goal of ESTCube-1 Estonian nanosatellite whose planned launch is 2012.
Note: carbon nanotube tethers (1000 kilometers long, plus higher density of wires and other configuration advancements mentioned above) would lead to e-sails with tens of thousands of newtons of propulsion.
Likewise, an experimental demonstration of reliable reeling i.e., one-time reeling in at the factory and one-time reeling out in space of the E-sail tether is needed. This topic will be attacked as soon as long enough tether samples exist. Thus far, a few meter tether samples have been produced by an ultrasonic wire bonding technique whose scaling to at least kilometer length tethers is expected to be straightforward. By the end of 2010, at least 10 m of tether is planned to be ready, which is enough for ESTCube-1 and for starting the experimental reeling tests.