Electric sail, solar power satellites and energy production

Pekka Janhunen, Kumpula Space Centre/Finnish Meteorological Institute, Helsinki, Finland

English translation of originally Finnish article in June 2007 issue of "Arkhimedes" journal of the Finnish Physical Society. Translated by the author.

Abstract: We consider how the author's electric sail invention could be used for solar system research and utilisation. Our particular emphasis is in how to develop solar power satellites into a competitive form of electricity production.

The electric sail

The electric sail is a Finnish propulsion invention [1,2] which utilises the solar wind (speed 300-800 km/s) for producing thrust for a spacecraft. An electric sail spacecraft rotates slowly and deploys a large number (50-100) of long (~ 20 km), conducting tethers. An onboard solar-powered electron gun keeps the spacecraft and the tethers at a high positive potential (~ +20 kV) so that the tethers repel solar wind protons and deflect their orbits. Momentum transfer then takes place from the proton stream to the tethers and the spacecraft. The tethers are made of e.g. fourfold thin (20 um) metal wire. The redundancy guarantees that the whole tether does not break when a micrometeoroid cuts a single wire filament. For active guiding, there is a potentiometer between each tether and the spacecraft. By changing the potentiometer resistances one can control the potentials of the tethers individually. As the thrust exerted by the solar wind on the tether depends on the tether potential, changing the potentiometer settings enables one to control the thrust experienced by each tether, which gives a way of guiding the spacecraft. The magnitude of the thrust can be regulated by changing the current or voltage of the electron gun. The thrust direction can be controlled within some limits by turning the tether spin plane with respect to the solar wind flow. There is a continuous electron current from the solar wind plasma to the tethers, but because the plasma density of the solar wind is low, the plasma is very collisionless. In such a plasma, particles move in ballistic orbits so that they have a difficult time hitting the thin wires. Hence the required electron gun power is rather modest (~ 500 W). It is especially nice that the power requirement of the electron gun scales similarly (1/r^2) as the power produced by the solar panels as a function of solar distance r. This is because the solar wind plasma density decays as 1/r^2 and the electron current is proportional to it.

The basic electric sail produces 0.1-0.2 N thrust which gives 1-2 mm/s^2 acceleration to a 100 kg spacecraft. In one year this acceleration changes the velocity vector of the spacecraft by 30-60 km/s which is already an excellent achievement. The speed 50 km/s corresponds to 10 AU per year, at which rate a trip to Neptune would take three years and to Pluto a bit longer. The electric sail hardware (tethers, electron gun with their power supply systems and the solar panels) weighs about 50 kg or even less, so that in this example there is room for a 50 kg payload (which includes the spacecraft body). One can increase the thrust by increasing the number of tethers, their length and the power of the electron gun. In addition, it may be possible to use part of the electric power for radio frequency modulation of the electron beam, which may give a possibility to heat the electron population which is trapped in the potential well of the tethers. The heating expands the electron cloud, in other words the Debye length increases. Then the electric field of the tether penetrates a longer distance into the surrounding solar wind plasma so that the effective sail area of the tether increases, which increases the thrust. Modelling the electron heating is challenging, but testing it in space would be straightforward. For this reason, an electric sail test mission should be built as soon as possible. After becoming familiar with electric sail technology, one could increase its thrust perhaps even hundredfold, that is, to some tens of newtons, by using these techniques. Further increases seem to be prohibited by the yield strength and conductivity of presently available materials. Progress of material physics into industrial production of tethers made of carbon nanowires could possibly increase the upper limit of the thrust even further.

Sailing in the solar wind

With the electric sail it is possible to tack along a spiral orbit inward or outward in the solar system with a modest payload. The traveltimes are then of the same order as with traditional methods, but the cost is lower because no fuel is needed and the sail hardware is lightweight. On the other hand, if the payload is small one can fly radially outward with a high speed, although without a possibility to stop or return. Perhaps the most important limitation of the electric sail is that it hardly works inside Earth's magnetosphere where there is no solar wind.

A practical implementation of the electric sail contains a few technically somewhat challenging pieces, such as reliable reeling of a multiline tether and an industrial fabrication of long tethers. On the other hand, these technical questions might get resolved even easily; we do not know before the engineering work has been properly initiated. In any case, many share the opinion that an electric sail which is based on one-dimensional tethers is easier to implement than a traditional solar sail (solar radiation pressure sail) which requires a two-dimensional surface. Besides, the solar sail should be fabricated of a membrane which is only a few nanometres thick in order to be competitive with the electric sail.

If one can build an electric sail, it would seem to make the outer solar system, and the interstellar space beyond the heliopause, accessible for small research probes with similar traveltimes as what are nowadays common in flights in the inner solar system and to the innermost giant planets. Flights staying in the inner solar system would become cheaper as well, although the traveltimes would not change. In addition one could fly to study the Sun at close distance or to build probes which float between Earth and Sun, predicting space weather with a longer warning period than what is possible using traditional space weather probes at the Lagrange L1 point.

Fuel factory at high orbit

The electric sail can also transport payloads which are much heavier than the spacecraft itself, if given enough time and if the mission delta-v requirement (time integral of the absolute value of the propulsive acceleration) is modest. A hundred kilogramme electric sailcraft could freight two tons of payload from an asteroid to a high Earth orbit in about four years. It makes sense to take a load of asteroid water ice as return payload and to have a reception point at high Earth orbit. At the reception point there is a solar-powered, water-decomposing electrolytic factory which produces liquid hydrogen and liquid oxygen rocket fuel. We still do not know which asteroids contain ice or how far one has to go to find the first one that does, but if needed, the electric sail can fetch a payload even from beyond Mars orbit where one can almost certainly find icy objects. The number of asteroids is large and for our application it is sufficient to find one ice-containing body, even a small and untypical one will do. The number of kilometre-size and larger bodies is about half a million. One kilometre-size object whose water content is 1% contains 10 million tons of water. (Icy objects are probably old cometary nuclei, but here we do not care about the origin of the bodies, calling them all simply asteroids.)

Mining water from an icy asteroid needs only warming, and then one has to collect the outcoming water vapour in a tank or bag where one lets it freeze or condense. The fuel factory at high Earth orbit is also not complicated. For example, with 100 kW solar panels (same order of magnitude as on the International Space Station) one can produce more than 150 tons of fuel per year. Storing the cold liquid fuels behind solar shields is not a big problem because the factory is located far from the Earth and its harmful infrared radiation.

The most problematic issue with the fuel factory is an economical transfer of water from an asteroid to high Earth orbit. The transfer can be done with a hydrogen-burning rocket, but then most of the generated fuel is consumed during the transfer (the fraction depends exponentially on the delta-v velocity space distance behind which the icy asteroid resides). If the transfer is done with an ion engine, one needs less propellant, but the noble gas like propellants which are the most suitable for ion engines (e.g. because they do not form harmful deposits to solid surface) are unfortunately lacking from the atmosphereless asteroids. A thermal fission rocket using hydrogen as propellant would be still another possibility. Because of the light molecular weight of hydrogen, the so-called specific impulse of a fission rocket is about two times higher than the specific impulse of a hydrogen-oxygen chemical rocket, so that less propellant is consumed. But the hydrogen must be made from asteroidal water using electrolysis, and 89% of the mass of water is oxygen which one has to abandon in space in the fission version, whereas in the chemical version it is stored and utilised. For this reason, at least a solid core nuclear rocket would not be better than a chemical rocket in this application.

Thus, the electric sail would seem to solve these logistic problems. The electric sail needs no propellant and its payload can be of the order 20 times larger than the Earth-launched spacecraft, if the bidirectional traveltime is five years and if the target resides at Martian orbital distance.

As a customer of the orbital fuel factory one needs to design a reusable version of a liquid hydrogen burning upper stage booster rocket. This requires a certain amount of new engineering work, since long lifetime is now an important design criterion. After this we have an infrastructure with which one can transfer payloads between orbits without launching fuel from the Earth. For example, if launching a one-ton telecommunication satellite to geostationary orbit (GEO), nowadays one first has to lift a three-ton payload to low Earth orbit (LEO). About two thirds of the lifted mass is fuel which is used when propelling the payload itself to GEO. In the fuel factory concept, the booster rocket lifts only one ton to LEO, wherefrom a reusable orbital transfer vehicle (tanked with asteroid fuel) picks it up to GEO and returns itself (or is fetched) back to the high orbit fuel factory for refuelling and waiting for the next mission.

With the fuel factory, one can benefit from the mass efficiency of the electric sail also in those applications which require impulsive thrust (e.g., landing to large bodies), large payloads (e.g. manned Mars flight) or intramagnetospheric orbits (nearly all commercial space activities). Thus, although the electric sail produces only weak thrust and does not work in Earth's magnetosphere, its benefits can be felt in essentially all space activities.

Solar power satellites

A solar power satellite is a way to produce commercial electric power. A solar power satellite at GEO or other orbit sends the electric power gathered by its large solar panels to Earth using few GHz microwaves. The groundbased receiver is an antenna field of about ten square kilometre area [3]. The overall efficiency of microwave transfer from solar panel DC power to earthly grid AC power is about 50% and the power produced by a single satellite is of order one gigawatt. The power density of the microwave beam has been selected to be safely low so that for example a bird can conveniently fly through it without feeling a need to land and cool off. On the other hand, the microwave beam spreads during the long transfer distance, which is why the receiving antenna field has to be fairly large. From the constraints follows a characteristic unit power of GW order. This power level suits well for electricity production, although it complicates the building of small demonstration plants.

Taking the solar panels to space has two important benefits compared to installing them on ground. First, there is no night, clouds or winter in space, thus the plant can produce continuous electric power and consequently there is no energy storage problem. Second, in space one can use concentrator type solar collectors. The majority of the surface area of these collectors consists of lightweight parabolic reflector or Fresnel lens so that one needs much less of the expensive semiconductor. In ground-based panels, it is usually not economical to use concentrator type collectors because they produce no electricity in cloudy weather, require a sun-follower mechanism and support structures to withstand the wind load. If the production costs of solar panels come down markedly from their present level (at least by an order of magnitude), the latter benefit gets eliminated, but the first benefit, i.e. the absence of the energy storage problem, remains in any case.

One can estimate that by using present space technology (LEO launch cost 5000 euro/kg with expendable rocket and transfer to GEO at threefold price), electric power produced by solar power satellites would be roughly 100 times more expensive than nuclear power. The majority of the cost comes from launching, so that one can reduce the total cost either by bringing down the cost of LEO-launched kilogramme or by reducing the launched mass, or both.

One often considers that by employing reusable boosters (for example Kistler K-1 project), the LEO launch cost could be brought down by factor 10-50, i.e. to 100-500 euro/kg level. The technology of reusable booster systems is well known and their practical implementation is actually only waiting for a somewhat increased demand, i.e. a commercial need to launch payloads more frequently than nowadays. If one can reduce launch costs so much by reusable boosters, then why the Space Shuttle of NASA is in practise even more expensive than expendable rockets? The Shuttle is always manned because when it was designed in the 1970's, the level of computer technology did not quite enable the automatic landing of an aeroplane-like vehicle. The presence of a crew at every flight makes testing and going to the limits difficult and makes the administration of the project more rigid. Another reason for the large cost of the Shuttle is that of its two stages only the upper, smaller one is actually reused. One had decided to make the Shuttle a cheap spacecraft, no matter at what cost.

A reusable booster system does not necessarily by itself suffice to bring the cost of solar power satellites to competitive level. In addition it would be beneficial to reduce the launched mass. With the above-described fuel factory one could reduce GEO launch cost by factor 2-3 at maximum. Another way to reduce the launched mass (which acts multiplicatively with respect to the previous one) is to build the solar power satellites partly from raw materials that come from asteroids. Bringing the asteroid materials is possible with the same electric sail technology than what is used for fuel production. In addition, suitable metal-rich asteroids exist near the Earth in terms of delta-v. Building a suitable orbital factory which turns asteroid regolith into beams and trusses, for example, is of course a large investment, but it may well be commercially viable if the goal is a global energy production based on solar power satellites.

Naturally, one can also reduce the launched mass by improving the power versus mass ratio of the solar power satellite.

Thus, overcoming the present hundredfold price gap of solar power satellites is a challenge, but it may well also succeed or even be surpassed. Numerically the largest cost reduction will probably be realised by switching to reusable booster systems, but also an orbital fuel factory based on asteroid resources and a material factory may contribute. As explained above, the electric sail is a technique that would seem to enable economical fetching of asteroid materials for orbital fuel factory and construction. Can a fuel production be based on small electric sailcraft each carrying few tons of payload, if the goal is to serve large-scale construction of solar power satellites, must still be regarded as an open question, however. This is because if the electric sail fleet becomes large, space traffic control and space debris problems might arise which have not yet been properly thought about. On the other hand, the size of the fleet would grow more modestly if the thrust of the electric sail could be grown hundredfold, as we have speculated above.

At the moment we still do not know whether the permanently shadowed craters of lunar polar regions contain a sufficient amount of water ice. If there is enough ice, an economical fuel factory could perhaps be based on lunar resources instead of asteroids, as has often been suggested. One could then implement a fuel factory without relying on electric sail technologies which are still at the development stage. The most important benefit of the Moon is a temporally short transfer distance, a drawback is the gravity field which forces one to use a chemical rocket when descending and landing (about half of the produced fuel would be consumed to the trafficking). The gravity field also increases the mass and logistic cost of surface infrastructure such as solar panels and the miner vehicle. Also the coldness of the lunar regolith in polar regions may be troublesome.

The above-mentioned usage pattern of the orbital fuel factory is not the only possible one. If the LEO launch costs get reduced sufficiently, it may be economical to handle transfers to GEO with earthly fuel also in the future. But even then, if one turns up using asteroid raw materials in the construction of the solar power satellites (which is, if successful, in principle a way to reduce the total costs indefinitely), one needs the fuel factory when bringing materials to GEO from above: If chemical rockets are used in the transfer, their fuel must be generated from extraterrestrial sources and if electric sails are used, one again needs some chemical fuel to stop the payload to GEO, because the GEO orbit resides inside the magnetosphere where the electric sail gives in principle no thrust.


One should study the electric sail, the orbital fuel factory concept and solar power satellites more carefully. Also one should clarify the situation concerning lunar and asteroid ice. If the electric sail works, it will in any case play a large role in basic research of the solar system. For example, the mapping of ice in asteroids could be implemented with it. The electric sail would also seem to enable a fuel factory which may cheapen all space activities, although the scaling of the method to very large fuel production rates is thus far somewhat unclear. On the other hand, if icy objects are found relatively close to Earth orbit in terms of delta-v, or if enough mineable ice resources are found on the Moon, an economical fuel factory can also be implemented by reusable chemical rockets. The scaling of the latter method is certain at least.

If LEO launch costs come down sufficiently, one can construct economical solar power satellites also directly, but an orbital fuel factory would possibly anyway lower the costs. Technically the most challenging path is to use asteroid or lunar materials in their construction; this approach has the potential of producing the cheapest end result. It seems that when bringing the materials to wanted Earth orbit, one almost certainly needs an orbital fuel factory. If and when the solar power satellite costs can be reduced sufficiently, a prospect for environmentally friendly, cheap, global and scalable electricity production without energy storage problems would open up.

[1] Janhunen, P., Electric sail for spacecraft propulsion, J. Prop. Power, 20, 763-764, 2004.

[2] Janhunen, P. and A. Sandroos, Simulation study of solar wind push on a charged wire: basis of solar wind electric sail propulsion, Ann. Geophys., 25, 755-767, 2007.

[3] http://www.ursi.org/WP/wp-SPS-1812061.doc, see also http://www.ursi.org/WP/SupportingDocument1.pdf