A paper analyses the potential of the electric solar wind sail for solar system space missions. Applications studied include fly-by missions to terrestrial planets (Venus, Mars and Phobos, Mercury) and asteroids, missions based on non-Keplerian orbits (orbits that can be maintained only by applying continuous propulsive force), one-way boosting to outer solar system, off-Lagrange point space weather forecasting and low-cost impactor probes for added science value to other missions. We also discuss the generic idea of data clippers (returning large volumes of high resolution scientific data from distant targets packed in memory chips) and possible exploitation of asteroid resources. Possible orbits were estimated by orbit calculations assuming circular and coplanar orbits for planets. Some particular challenge areas requiring further research work and related to some more ambitious mission scenarios are also identified and discussed.
The electric solar wind sail (E-sail) is an advanced concept for spacecraft propulsion, based on momentum transfer from the solar wind plasma stream, intercepted by long and charged tethers. The electrostatic field created by the tethers deflects trajectories of solar wind protons so that their flow-aligned momentum component decreases. The flow-aligned momentum lost by the protons is transferred to the charged tetherby a Coulomb force (the charged tether is pulled by the plasma charge separation electric field) and then transmitted to the spacecraft as thrust. The conceptis attractive for applications because no propellant is needed for travelling over long distances. The E-sail’s operating principle is different from other propellantless propulsion technologies such as the solar photon sail and the solar wind magnetic sail. The former is based on momentum transfer from sunlight (solar photons), while the latter is based on a large loop-shaped superconductive wire whose magnetic field deflects solar wind protons from their originally straight trajectories.
The main purpose of this article is to analyse the potential of E-sail technology in some of the envisaged possible applications for solar system space activities. To a limited extent we also adopt a comparative approach,estimating the added value and other advantages stemming from E-sail technology in comparison with present chemical and electric propulsion systems and(in some cases) with other propellantless propulsion concepts. When making such comparisons a key quantity that we use for representing the mission cost is the total required velocity change, Av, also called delta-v.The Sail Propulsion Working Group, a joint working group between the Navigation Guidance and Control Section and the Electric Propulsion Section of the European Space Agency, has envisaged the study of three reference missions which could be successfully carried out using propellantless propulsion concepts.
Currently, the demonstration mission ESTCube-1 is being developed by the University of Tartu to provide a practical proof of the E-sail concept in a low Earthorbit (LEO). In LEO the E-sail would sense a plasma stream moving at a relative velocity of ~ 7 km/s which is much slower than in the solar wind (300–800 km/s), but at the same time with much higher plasma density (about10^10–10^11 m−3 versus about 5 × 10^6 m−3 in the solar wind).
Mass budgets of E-sails of various thrust levels and for different scientific payload masses were estimated and tabulated. For example, to yield characteristic acceleration (E-sail thrust divided by total spacecraft mass at 1 au distance from the Sun in average solar wind) of 1 mm/s2, the spacecraft total mass was found to be 391 kg (including the 20% margin),of which 143 kg is formed by the E-sail propulsion system consisting of 44 tethers of 15.3 km length each. Characteristic acceleration of 1 mm/s2 corresponds to 31.5 km/s of Av capability per year.
Outside the Earth’s magnetosphere, the E-sail can provide propulsive thrust almost everywhere in the solar system. The only restrictions are that the thrust direction cannot be changed by more than about ±30 degrees and that inside giant planet magnetospheres special considerations are needed.
The E-sail thrust magnitude decays as ~ 1/r, where r is the solar distance. Noticethat the E-sail thrust decays slower than the photonic sail and solar electric propulsion thrust because the latter ones decay as 1/r2. The reason is that while the solar wind dynamic pressure decays as 1/r2, the effective are a of the sail is proportional to the electron sheath width surrounding the tethers, which scales similarly to the plasma Debye length that in the solar wind scales as ~ r.
Since the E-sail thrust is proportional to the product of the dynamic pressure and the effective sail area, it scales as 1/r.The solar wind is highly variable and at first sight one might think that this would set restrictions to applying the solar wind as a thrust source for space missions.However, if the electron gun voltage is controlled inflight so as to produce maximal thrust with available electric power, the resulting E-sail thrust varies much less than the solar wind dynamic pressure and accurate navigation is possible.As mentioned above, by inclining the sail the thrust direction can be modified by up to ~ 30 degrees. This makes it possible to spiral inwards or outwards in the solar system by tilting the sail in the appropriate direction to decrease or increase the heliocentric orbital speed, respectively. Thus, even though the radial component of the E-sail thrust vector is always positive, one can still use the system also to tack towards the Sun. A similar tacking procedure is possible also with photonic sails. Unlike most photon sails, the E-sail thrust can be throttled at will between zero and some maximum by controlling the power of the electron gun.
The number of potential E-sail applications is large.Here we use a categorization into five main groups:
(1) asteroid and terrestrial planets,
(2) non-Keplerian orbits (e.g., off-Lagrange point solar wind monitoring to achieve longer warning time for space weather forecasts),
(3) near-Sun missions,
(4) one-way boosting to the outer solar system, and
(5) general ideas for impactors or penetrators, “data clippers” carrying data as payload and in situ resource utilization.
The E-sail needs to be raised beyond the Earth’s magnetosphere before it can generate propulsive thrust.
The E-sail needs the solar wind or other fast plasma stream to work. Therefore the E-sail cannot in practice be used inside the Earth’s magnetosphere where the plasma generally does not stream rapidly. An exception to this limitation is that one can use an E-sail like apparatus for plasma braking in LEO, utilizing the ~ 7 km/s speed difference between the orbiting satellite and nearly stationary ionosphere and the fact that the plasma density in LEO is high so that the process is relatively efficient even though the speed difference is much less than the solar wind speed of 400–800 km/s. In giant planetmagnetospheres the plasma corotates rapidly with the planet, which might enable some form of E-sailing also inside giant planet magnetospheres; this question should be addressed in future studies.
The obtainable performance (characteristic acceleration) depends on how large fraction the E-sail propulsion system forms of the spacecraft total mass. There is also a practical upper limit of E-sail size beyond which complexity would increase and performance would drop. At the present level of technology this soft limit is likely to be ~ 1 N thrust at 1 au solar distance. For small and moderate payloads up to a few hundred kilograms of mass, high performance is typically available, of order 1 mm/s2 characteristic acceleration corresponding to ~ 30 km/s of Av capability per year. The numbers must be scaled by 1/r when going beyond 1 au.
The E-sail can be used to make most planetary missions cheaper or faster or both. It really excels in asteroid missions and makes two-way missions feasible (sample return and data clippers). It can also be used as a booster for outer solar system missions. The E-sail is enabling technology for multi-asteroid touring and non-Keplerian orbit missions.All E-sail missions must start from a near or full escape orbit. Also, any escape orbit is good for any E-sail probe. Hence, E-sails can be piggybacked on other escape orbit launches and E-sails going to different targets can be launched together in any combination.In the longer run, E-sails might enable economic asteroid resource utilization and asteroid-derived propellant manufacture for satellite orbit raising, for lifting E-sail missions themselves to escape condition and for manned Mars and asteroid exploration