Editing Magnetic sail (section)

=== In a planetary magnetosphere ===
Inside or near a planetary [[magnetosphere]], a magnetic sail can thrust against or be attracted to a planet's [[magnetic field]] created by a [[Dynamo theory|dynamo]], especially in an [[orbit]] that passes over the planet's magnetic poles.<ref name=":5">{{Cite journal |last=Zubrin |first=Robert |date=1991-06-24 |title=The use of magnetic sails to escape from low earth orbit |url=https://www.researchgate.net/publication/234453772 |journal=27th Joint Propulsion Conference |language=en |location=Sacramento, CA |publisher=American Institute of Aeronautics and Astronautics |doi=10.2514/6.1991-3352}}</ref> When the magnetic sail and planet's magnetic field are in opposite directions an attractive force occurs and when the fields are in the same direction a repulsive force occurs, which is not stable and means to prevent the sail from flipping over is necessary.

The thrust that a magnetic sail delivers within a magnetosphere decreases with the fourth power of its distance from the planet's internal magnetic field. When close to a planet with a strong [[magnetosphere]] such as [[Earth]] or a [[gas giant]], the magnetic sail could generate more thrust by interacting with the magnetosphere instead of the solar wind. When operating near a planetary or stellar magnetosphere the effect of that magnetic field must be considered if it is on the same order as the gravitational field.

By varying the magnetic sail's field strength and orientation a "[[perigee]] kick" can be achieved raising the altitude of the orbit's [[apogee]] higher and higher, until the magnetic sail is able to leave the planetary magnetosphere and catch the solar wind. The same process in reverse can be used to lower or circularize the apogee of a magsail's orbit when it arrives at a destination planet with a magnetic field.

In theory, it is possible for a magnetic sail to launch directly from the surface of a planet near one of its magnetic poles, repelling itself from the planet's magnetic field. However, this requires the magnetic sail to be maintained in an "unstable" orientation. Furthermore, the magnetic sail must have extraordinarily strong magnetic fields for a launch from Earth, requiring superconductors supporting 80 times the current density of the best known high-temperature superconductors as of 1991.<ref name=":5" />

In 2022 a spaceflight trial dubbed Jupiter Observing Velocity Experiment (JOVE) proposed using a [[plasma magnet]] to decelerate against the magnetosphere of Jupiter.<ref name=":11" />