Physics and Technology of the Feasibility of Plasma Sails

Plasma sail technology, in which an artificial magnetosphere is generated around a spacecraft and is coupled to and dragged by the solar wind, has been proposed as a propulsion technique to rapidly travel to the outer regions of the solar system and heliosphere. An examination of the physics and planned implementation of plasma sail propulsion indicates that the proposed technology does not provide a functional, resource-competitive and viable propulsion mechanism using current technologies or technologies expected to be available in the foreseeable future. This conclusion is primarily based on application of the conservation of magnetic flux for the plasma sail system, which follows from the requirement that the divergence of the magnetic field vanish. A functional plasma sail must intercept and couple to a large area of the solar wind, and spacecraft thrust is derived from momentum transfer from the solar wind. To obtain the magnetic flux necessary for a large bubble requires a much larger magnet than those described in the literature. Even for a highly conservative implementation having a minimal 10 km radius of the artificial magnetosphere, along with an optimistic assumption of a 1/r fall-off in the magnetic field and a boundary condition of ~50 nT at the edge of the sail that is needed to balance the solar wind pressure (~2 nPa), one finds that the magnetic flux in the magnetic coil on the spacecraft must be at least ~30 Wb,