Magnetic sail

A magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft. A magnetic sail could also thrust directly against planetary and solar magnetospheres.

The magnetic sail was proposed by Dana Andrews and Robert Zubrin working in collaboration in 1988. At that time, Andrews was working on a concept to use a magnetic scoop to gather ions to provide propellant for a nuclear electric ion drive spacecraft, allowing the craft to operate in the same manner of a Bussard ramjet, but without the need for a proton-proton fusion propulsion drive. He asked Zubrin to help him compute the drag that the magnetic scoop would create against the interplanetary medium. Zubrin agreed, but found that the drag created by the scoop would be much greater than the thrust created by the ion drive. He therefore proposed that the ion drive component of the system be dropped, and the device simply used as a sail. Andrews agreed, and the magsail was born. The two then proceeded to elaborate their analysis of the magsail for interplanetary, interstellar, and planetary orbital propulsion in a series of papers published from 1988 through the 1990s.

The magsail operates by creating drag against the local medium (planet's magnetic field, solar wind, or interstellar winds), thereby allowing a spacecraft accelerated to very high velocities by other means, such as a fusion rocket or laser pushed lightsail, to slow down – even from relativistic velocities – without requiring the use of onboard propellant. It can thus reduce the delta-V propulsion required for an interstellar mission by a factor of two. This capability is the most unusual feature of the magsail, and perhaps the most significant in the long term.

In typical magnetic sail designs, the magnetic field is generated by a loop of superconducting wire. Because loops of current-carrying conductors tend to be forced outwards towards a circular shape by their own magnetic field, the sail could be deployed simply by unspooling the conductor and applying a current through it.

The solar wind is a continuous stream of plasma that flows outwards from the Sun: near the Earth's orbit, it contains several million protons and electrons per cubic meter and flows at 400 to 600 km/s (250 to 370 mi/s). The magnetic sail introduces a magnetic field into this plasma flow which can deflect the particles from their original trajectory: the momentum of the particles is then transferred to the sail, leading to a thrust on the sail. One advantage of magnetic or solar sails over (chemical or ion) reaction thrusters is that no reaction mass is depleted or carried in the craft.

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