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May 31, 2013

Electric Solar Sail

ESTCube-1, launched earlier early May 2013, is proving out the electric solar sail. Even though it uses but a single 10-meter wire, its rotation rate should change once the tether is fully extended and powered up. Bear in mind that ESTCube-1 is deep within the Earth’s magnetosphere, so the charged particles it will be interacting with are not from the solar wind, but a proof of principle is sought here that could make electric sailing a candidate for outer system-bound spacecraft.

Numerical results show that the E-sail propulsion system, once qualified for flight, could be an interesting option for a wide class of deep space missions that include scientific payloads in the range 30 to 1000 kg, and require a characteristic acceleration up to about 3 mm=s^2.

Moreover, some rather straight-forward near-term component level improvements have the potential of reducing the effective E-sail mass further (28% in the specific case) with a consequent improvement in mission performance. Future work will concentrate on prototyping and testing the E-sail subsystem as well as measuring the E-sail performance in small scale in the real environment, that is, within the solar wind.

The electric sail takes advantage of the solar wind, the stream of charged particles that streams constantly from the Sun at speeds ranging from 300 to 800 kilometers per second. The sail’s tethers would be thinner than a human hair but would extend tens of meters into the solar wind flow, with each tether yielding the effective area of a sail roughly a square kilometer in size. Using multiple tethers like these, Janhunen’s team believes speeds of up to 100 kilometers per second (20 Astronomical units per year) are possible, fast enough to reach Pluto in just four years and to push deeply into the nearby interstellar medium in fifteen. The solar wind cannot be used in interstellar space, but a mission to another star propelled by other means could use an electric sail like this do decelerate, braking against the destination star’s own solar wind as it arrives.



A 2009 status report on the electric solar sail.

Without trapped electrons, a 20kV charged tether at 1 AU distance in average solar wind achieves 500nN=m thrust per length. For example, an electric sail composed of 100 tethers 20km long each would then achieve 1N thrust. Such tethers weigh 11kg if made of 25mm aluminium using the four-fold Hoytether construction and the electron gun requires 400W power, so assuming that the whole propulsion system mass is less than 100kg in this case appears to be justified. Such a device would give 1mm/s acceleration to a 1000kg spacecraft of which 90% is payload. Alternatively, for a small probe of 50kg total mass, the same acceleration would be provided by a small electric sail composed of only 10 tethers 10km long each, with 50mN total thrust at 1 AU.

The tether specifications used to calculate the system performance seem to be fairly conservative. New materials (perhaps carbon nanotubes) seem likely to enable stronger, lighter and higher performing solar electric sails. It seems that acceleration of 30 mm/s^2 and top speeds of 800 kilometers per second (160 AU per year) should become feasible.

Ways to calculate pure electric sail trajectories to planetary and asteroid targets have been analysed. The standard 1N electric sail could be useful in four different tasks: (1) providing a fast one-way ride for a small payload (200kg) at 50km/s out of the solar system, (2) providing a relatively fast trip to a giant planet orbit for 500kg payload, using a chemical orbit insertion burn near the planet and possibly also E-sailing and/or ED tethering in the giant planets magnetosphere, (3) providing a back and forth sample return trip for a 1000kg payload in the inner solar system (at most the main asteroid belt distance), (4) providing a non-Keplerian orbit for special purposes such a off-Lagrange point space weather monitoring or off-ecliptic solar orbit for helio seismological measurements.

Commercially, utilisation of asteroid resources such as water could become economical by using electric sails as a logistic chain for returning material from asteroids to Earth orbit. This is so because the impulse per mass unit produced by the electric sail over its lifetime may be
1000 times higher than for a chemical rocket and 100 times higher than for a contemporary ion engine (e.g., 1N thrust over 10 years lifetime per 100kg propulsion system mass gives 3000km/s impulse over mass figure of merit, compared to 3km/s for space-storable chemical propellant). In the role of an asteroid material tug hauling heavy payloads, the sail remains all the time near 1AU so that the lifetime impulse is not limited by the sail travelling too far from the sun.

Conclusions

Unless some currently unforeseen technical or physical difficulties will emerge and prohibit the realisation of its currently held potential, the electric sail technology is on its way to markedly improving our access to the solar system. For the electric sail, space is not empty, but a radially flowing plasma stream in which one can fly and manoeuvre almost at will and for arbitrarily long periods without propellant or other consumables

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