A magnetic sail is a proposed method of spacecraft propulsion where an onboard magnetic field source interacts with a plasma wind (e.g., the solar wind) to form an artificial magnetosphere (similar to Earth's magnetosphere) that acts as a sail, transferring force from the wind to the spacecraft requiring little to no propellant as detailed for each proposed magnetic sail design in this article.

The animation and the following text summarize the magnetic sail physical principles involved. The spacecraft's magnetic field source, represented by the purple dot, generates a magnetic field, shown as expanding black circles. Under conditions summarized in the overview section, this field creates a magnetosphere whose leading edge is a magnetopause and a bow shock composed of charged particles captured from the wind by the magnetic field, as shown in blue, which deflects subsequent charged particles from the plasma wind coming from the left.

Specific attributes of the artificial magnetosphere around the spacecraft for a specific design significantly affect performance as summarized in the overview section. A magnetohydrodynamic model (verified by computer simulations and laboratory experiments) predicts that the interaction of the artificial magnetosphere with the oncoming plasma wind creates an effective sail blocking area that transfers force as shown by a sequence of labeled arrows from the plasma wind, to the spacecraft's magnetic field, to the spacecraft's field source, which accelerates the spacecraft in the same direction as the plasma wind.

These concepts apply to all proposed magnetic sail system designs, with the difference how the design generates the magnetic field and how efficiently the field source creates the artificial magnetosphere described above. The History of concept section summarizes key aspects of the proposed designs and relationships between them as background. The cited references are technical with many equations and in order to make the information more accessible, this article first describes in text (and illustrations where available) beginning in the overview section and prior to each design, section or groups of equations and plots intended for the technically oriented reader. The beginning of each proposed design section also contains a summary of the important aspects so that a reader can skip the equations for that design. The differences in the designs determine performance measures, such as the mass of the field source and necessary power, which in turn determine force, mass and hence acceleration and velocity that enable a performance comparison between magnetic sail designs at the end of this article. A comparison with other spacecraft propulsion methods includes some magnetic sail designs where the reader can click on the column headers to compare magnetic sail performance with other propulsion methods. The following observations result from this comparison: magnetic sail designs have insufficient thrust to launch from Earth, thrust (drag) for deceleration for the magsail in the interstellar medium is relatively large, and both the magsail and magnetoplasma sail have significant thrust for travel away from Earth using the force from the solar wind.

An overview of many of the magnetic sail proposed designs with illustrations from the references was published in 2018 by Djojodihardjo. The earliest method proposed by Andrews and Zubrin in 1988, dubbed the magsail, has the significant advantage of requiring no propellant and is thus a form of field propulsion that can operate indefinitely. A drawback of the magsail design was that it required a large (50–100 km radius) superconducting loop carrying large currents with a mass on the order of 100 tonnes (100,000 kg). The magsail design also described modes of operation for interplanetary transfers, thrusting against a planetary ionosphere or magnetosphere, escape from low Earth orbit as well as deceleration of an interstellar craft over decades after being initially accelerated by other means, for example. a fusion rocket, to a significant fraction of light speed, with a more detailed design published in 2000. In 2015, Freeland validated most of the initial magsail analysis, but determined that thrust predictions were optimistic by a factor of 3.1 due to a numerical integration error.

Subsequent designs proposed and analyzed means to significantly reduce mass. These designs require little to modest amounts of exhausted propellant and can thrust for years. All proposed designs describe thrust from solar wind outwards from the Sun. In 2000, Winglee and Slough proposed a Mini-Magnetospheric Plasma Propulsion (M2P2) design that injected low energy plasma into a much smaller coil with much lower mass that required low power. Simulations predicted impressive performance relative to mass and required power; however, a number of critiques raised issues: that the assumed magnetic field falloff rate was optimistic and that thrust was dramatically overestimated.

Starting in 2003, Funaki and others published a series of theoretical, simulation and experimental investigations at JAXA in collaboration with Japanese universities addressing some of the issues from criticisms of M2P2 and named their approach the MagnetoPlasma Sail (MPS). In 2011, Funaki and Yamakawa authored a chapter in a book that is a good reference for magnetic sail theory and concepts. MPS research resulted in many published papers that advanced the understanding of physical principles for magnetic sails. Best performance occurred when the injected plasma had a lower density and velocity than considered in M2P2. Thrust gain was computed as compared with performance with a magnetic field only in 2013 and 2014. Investigations and experiments continued reporting increased thrust experimentally and numerically considering use of a Magnetoplasmadynamic thruster (aka MPD Arc jet in Japan) in 2015, multiple antenna coils in 2019, and a multi-pole MPD thruster in 2020.

Slough published in 2004 and 2006 a method to generate the static magnetic dipole for a magnetic sail in a design called the Plasma magnet (PM) that was described as an AC induction motor turned inside out. A pair of small perpendicularly oriented coils acted as the stator powered by an alternating current to generate a rotating magnetic field (RMF) that analysis predicted and laboratory experiments demonstrated that a current disc formed as the rotor outside the stator. The current disk formed from electrons captured from the plasma wind, therefore requiring little to no plasma injection. Predictions of substantial improvements in terms of reduced coil size (and hence mass) and markedly lower power requirements for significant thrust hypothesized the same optimistic magnetic field falloff rate as assumed for M2P2. In 2022, a spaceflight trial dubbed Jupiter Observing Velocity Experiment (JOVE) proposed using a plasma magnet based sail for a spacecraft named Wind Rider using the solar wind to accelerate away from a point near Earth and decelerate against the magnetosphere of Jupiter.