The novel propellantless electric solar wind sail concept promises efficient low thrust transportation in the Solar System outside Earth’s magnetosphere. Combined with asteroid mining to provide water and synthetic cryogenic rocket fuel in orbits of Earth and Mars, possibilities for affordable continuous manned presence on Mars open up. Orbital fuel and water enable reusable bidirectional Earth-Mars vehicles for continuous manned presence on Mars and allow smaller fuel fraction of spacecraft than what is achievable by traditional means. Water can also be used as radiation shielding of the manned compartment, thus reducing the launch mass further. In addition, the presence of fuel in the orbit of Mars provides the option for an all-propulsive landing, thus potentially eliminating issues of heavy heat shields and augmenting the capability of pinpoint landing. With this E-sail enabled scheme, the recurrent cost of continuous bidirectional traffic between Earth and Mars might ultimately approach the recurrent cost of running the International Space Station, ISS.
The electric solar wind sail (E-sail) provides thrust in the solar wind without consuming fuel. The E-sail uses charged tethers to extract momentum from the solar wind ions (mainly protons) by electrostatic Coulomb interaction with the plasma flow to produce thrust. According to current estimates, the E-sail is 2-3 orders of magnitude more efficient than traditional propulsion methods (chemical rockets and ion engines) in terms of produced lifetime-integrated impulse per propulsion system mass, if the mission duration is 10 years and the mission does not proceed too far into the outer solar system. This is based on numerical simulations. When these simulations are run for plasma representing LEO conditions, their predicted electron sheath width (which is a proxy for E-sail thrust per length) is in good agreement with laboratory measurements of the sheath width in LEOlike conditions.
To enable maneuvering and trajectory control, the E-sail thrust can be vectored in a cone of ∼ 30◦ around the solar wind velocity vector. It is possible to adjust the magnitude of the thrust between 0-100 % by modifying the current and voltage of the electron gun. Turning E-sail propulsion off is possible at any time by turning off the electron gun. A strategy of maximizing available thrust by matching the electron gun current with the current gathered by the tethers leads by certain natural negative feedback mechanisms to a situation where the thrust varies much less than the solar wind dynamic pressure. Even with simple trajectory control law, the maneuverability of the E-sail is sufficient to allow navigation to, for example, Mars.
A large E-sail, that can provide 1 N of thrust at 1 au from Sun, can travel from the Earth to the asteroid belt in a year, and bring back three tonnes of water in three years. One E-sail based spacecraft is capable of repeating the journey multiple times within its estimated lifetime of at least ten years.
Outline of EMMI, showing the locations of the orbital fuel stations where the water is split and condensed into LOX/LH2 fuel. On these fuel stations manned vehicles traveling between Earth and Mars can be fueled, dramatically reducing the overall mission fuel ratio at launch. Cargo transport and mining activities take place propellantlessly using E-sails.
The Esail Asteroid mining missions could thus be done on separate stages, filling up the fuel tank in-between each leg. First, a launcher is used to lift the passengers/payload to LEO where the first tanking occurs. Alternatively, the lift is continued with electric propulsion engines through the magnetosphere. The craft is lifted to L1, L2 or a high orbit, where again fuelling from the awaiting fuel reservoirs occurs. The last leg from Earth orbit to Mars and capture to Mars orbit would again consume the fuel.
The target asteroids would be sought from the vicinity of the Martian orbit, at around 1.5 au, as this is where majority of the mining products are headed. Yearly water extraction pace of roughly 50 000 kg would suffice for a manned bidirectional trip between Earth and Mars taking place every other year. To achieve this, 3.2 kW of electric power on the asteroid is required. At the distance of 1.5 au from the Sun, this translates into 230 kg of solar panels assuming a horizontal panel on the equatorial surface of a rotating asteroid and a characteristic mass for the solar panels of 100 W/kg at 1 au (Joel Ponzy, private communication). As power is also needed for other purposes, such as moving, communications and countering power system aging, we will assume a 50% higher power consumption, thus arriving at the whole extractor power system weighing 340 kg. We estimate that the whole extraction vessel would in this case weigh around 2000 kg, which is much lower than the expected mass of the orbital fuel factories.
Background on Esail tethers and systems
The electric sail (ESAIL), invented by Dr. Pekka Janhunen at the Finnish Kumpula Space Centre in 2006, produces propulsion power for a spacecraft by utilizing the solar wind. The sail features electrically charged long and thin metal tethers that interact with the solar wind. Using ultrasonic welding, the Electronics Research Laboratory at the University of Helsinki successfully produced a 1 km long ESAIL tether. Four years ago, global experts in ultrasonic welding considered it impossible to weld together such thin wires. The produced tether proves that manufacturing full size ESAIL tethers is possible.
An electric solar wind sail, a.k.a electric sail, consists of long, thin (25 to 50 microns) electrically conductive tethers manufactured from aluminium wires. A full-scale sail can include up to 100 tethers, each 20 kilometres long. In addition, the craft will contain a high-voltage source and an electron gun that creates a positive charge in the tethers. The electric field of the charged tethers will extend approximately 100 metres into the surrounding solar wind plasma. Charged particles from the solar wind crash into this field, creating an interaction that transfers momentum from the
solar wind to the spacecraft. Compared with other methods, such as ion engines, the electric sail produces a large amount of propulsion considering its mass and power requirement. Since the sail consumes no propellant, it has in principle an unlimited operating time.
The electric sail is raising a lot of interest in space circles, but until now it has been unclear whether its most important parts, i.e. the long, thin metal tethers, can be produced.
ESAIL EU FP7 project
The ESAIL EU FP7 project (2011-2013) develops laboratory prototypes (TRL 4-5) of the key components of the E-sail. The project involves five countries, nine institutes and has a budget of about 1.7 million Euros.
* Deploy and confirm the deployment of a 10 m conductive Hoytether from a 1U CubeSat
* Test of a 100 m tether deployment on Aalto-1 3U CubeSat (2014)
Solar Wind Electric Sail Test (SWEST)
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.
A single metal wire is not suitable as an ESAIL tether, as micrometeoroids present everywhere in space would soon cut it. Therefore the tether must be manufactured from several wires joined together every centimetre [Image 1]. In this way, micrometeoroids can cut individual wires without breaking the entire tether.
The tether factory has so far produced ultrasonic welds for one kilometre of aluminium tether
The Electronics Research Laboratory team started studying the production problem four years ago. At the time, the view of international experts in ultrasonic welding was that joining thin wires together was not possible. However, the one-kilometre-long tether produced now, featuring 90,000 ultrasonic welds, shows that the method works and that producing long electric sail tethers is possible.
The wire is produced with a fully automated tether factory, a fine mechanical device under computer control, developed and constructed by the team itself. [Image 2]. The tether factory at the Kumpula Science Campus in Helsinki, Finland, was integrated into a modified commercial ultrasonic welding device. Ultrasonic welding is widely in the electronics industry, but normally it is used for joining a wire to a base.
-We have a challenging task, as keeping thin wires repeatedly in the precisely correct position is hard, says Timo Rauhala who works in the laboratory.
Approximately three metres of tether is currently produced per hour. Its quality is verified optically with a real-time measurement that inspects the connection of every individual joint. In the future, the production speed is to be raised and the weld quality will be assured during the production process.
The products of the tether factory will soon see action in space. The first opportunity will be the ESTCube-1 satellite, an Estonian small satellite to be launched in March 2013. ESTCube-1 will deploy a 15-metre long tether in space and measure the ESAIL force it is subjected to. This is ground-breaking as, so far, the theoretically predicted electric sail force has not yet been experimentally measured.
Next in turn will be the Aalto-1 small satellite from the Aalto University, to be launched in 2014, which will deploy a 100-metre long tether.
The deployed tethers are kept straight in space by the centrifugal force, the magnitude of which is five grams in a full-scale electric sail. The wire-to-wire welds of the ESAIL tether produced at the University of Helsinki will tolerate a pull of 10 grams.
Potential of the ESail
We had previously looked at an asteroid capture analysis using solar electric power. The analysis involved a 40 KW solar electric system using an ion thruster (2.4 Newton of power for up to 10,000 hours). They described capturing a 10 ton asteroid to low earth orbit or a 508 ton asteroid to high earth orbit. A near term increase in the ion system can get to 400 KW and 24 Newtons which would enable capturing asteroids ten times as large.
Here is an eight page analysis of Moving an asteroid with electric solar wind sail. The analysis is mainly for deflecting a 3 million ton asteroid over 5 to 10 years.
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.
Background on the ESail
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.
Scaling up Esails by hundreds of times
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.
There is a 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.
SOURCES – Arxiv, electric-sailing.fi, NASA, European Space Agency, estcube.eu, wiki.aalto alphagalileo
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
Known for identifying cutting edge technologies, he is currently a Co-Founder of a startup and fundraiser for high potential early-stage companies. He is the Head of Research for Allocations for deep technology investments and an Angel Investor at Space Angels.
A frequent speaker at corporations, he has been a TEDx speaker, a Singularity University speaker and guest at numerous interviews for radio and podcasts. He is open to public speaking and advising engagements.