A magnetic sail (MagSail) is a unique interplanetary propulsion system. To propel a spacecraft in the direction leaving the Sun, the MagSail produces a large-scale magnetic field to block the hypersonic solar wind plasma flow. Since thrust of the MagSail is proportional to the blocking area, a very large-scale interaction between the artificial magnetic field and the solar wind is required as in the case of the geomagnetic field to capture the energy of very low density solar wind flow. For example, the original MagSail by Zubrin requires an unrealistic spacecraft design with a large hoop coil of 100 km in radius to form 1000-km-radius blocking area (which corresponds to 10-N-class thrust). Due to this large structural requirement the idea of the MagSail did not draw attention so far. In 2001, however, the idea of the MagSail received a renewed interest when Winglee proposed Mini-Magnetospheric Plasma Propulsion (M2P2) concept, which inflates a weak original magnetic field made by a small coil of about 0.1 m in diameter with an assistance of a high-density plasma jet. Although the feasibility of this compact M2P2 design is denied by several researchers, we revised the M2P2 design by changing the coil to moderate sizes of 10 to 100 m in diameter to efficiently enlarge the blocking area. Such revised systems, which we call Magnetoplasma Sail (MPS), still has some both technical and physical issues to be clarified; most of them are reviewed in this article with the comprehensive list of the solar wind sail propulsion systems

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The interaction between the solar wind and the magnetic field created around the spacecraft is investigated by utilizing two-dimensional ideal magnetohydrodynamic (MHD) equations to estimate the thrust of Magneto Plasma Sail (MPS). In this propulsion system, the dynamic pressure of the solar wind is converted into the aerodynamic drag force to the magnetosphere created around the spacecraft through the MHD interaction. When the dynamic pressure of the solar wind is 0.753 [nPa] and the magnetic dipole moment is 5000 [Tm³], the estimated thrust of MPS is 1.45 [N], which is close to the theoretically estimated values.

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Numerical simulations of electromagnetic interaction between the solar wind and the moderately-sized magnetic sails are made to clarify the characteristics of reaction forces exerted on the magnetic sail when its characteristic scale is reduced below the continuum limit at which the magneto-hydrodynamic approximations of the plasma flow fail. The hybrid particle-in-cell (PIC) method is used to take into account the finite Larmor-radius effect of the electromagnetic interaction between the plasma flow and the magnetic field. The magnetic dipole intensity is changed so that the characteristic size of the magnetic sail changes from several kilometers to few thousand kilometers. The results show that the drag coefficient of the magnetic sail based on the representative radius of the magneto-hydrodynamic interaction decreases as the ratio of the ion Larmor radius to this representative radius becomes greater than unity. An approximate formula to estimate the drag coefficient in a wide range of magnetic sail dimension is presented.

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An air-breathing pulse laser powered launcher has been proposed as an alternative to conventional chemical launch systems. The trajectory from the ground to Geosynchronous Earth Orbit (GEO) by pulse laser propulsion is calculated by modeling the thrust during pulsejet, ramjet and rocket flight modes, and the launch cost is estimated. The results show that the pulse laser powered launcher can transfer 0.096 kg payload per 1 MW beam power to GEO, and 2,800 divisional launches of a payload are necessary to redeem the cost of its laser transmitter compared with conventional chemical launchers.

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The magnetic sail utilizes the momentum of the solar wind (high-speed plasma flow) to produce the thrust by an applied magnetic field. Since the momentum flux from the sun is transformed into the thrust like a solar sail, no propellant is required and infinite Isp performance is available. Hence, it has a potential to reduce drastically the duration for a deep space mission because it can accelerate the spacecraft up to its theoretical limit, that is, the speed of solar wind. In the present study, its acceleration performance and scales of the phenomena in relation to the solar wind flow around the magnetic sail are roughly estimated. Based on the estimation, the interaction of the solar wind with the applied magnetic field is numerically simulated by the full particle (PIC) method. Fundamental features of the flow field and the induced electromagnetic field around the magnetic sail are clarified. Force acting upon the magnetic sail is also estimated by the considering the momentum change of the solar wind.

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Trajectory design using low thrust propulsion system has to take into account of thrust direction angle. thrust magnitude, and thrust/power ratio as constraints. For example, the thrust direction angle of Magneto-Plasma Sail (MPS) is restricted to near the opposite direction of the Sun, while that of Ion Engine System (IES) is basically free if power and thermal conditions permit. The thrust to power ratio of MPS and IES are assumed 100-250mN/kW and 20-30mN/kW, respectively. This paper analyzes the characteristics of trajectories of low thrust propulsion systems in order to compare MPS with other low thrust propulsion systems. Low thrust propulsion mission performance is represented by three parameters; initial thrust/spacecraft mass ratio (acceleration), thrust angle and specific impulse, Isp. It is assumed that electrical power provided to the propulsion system is inversely proportional to the square of heliocentric distance, the thrust angle from transversal direction ranges from 0 to 360 deg. and the spacecraft mass decreases as the fuel is expended. The possible application of MPS to outer and inner solar system missions is finally discussed.

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