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 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 t he craft.
For a sail in the solar wind one AU away from the Sun, the field strength required to resist the dynamic pressure of the solar wind is 50 nT. Zubrin’s proposed magnetic sail design would create a bubble of space of 100 km in diameter (62 mi) where solar-wind ions are substantially deflected using a hoop 50 km (31 mi) in radius.
The minimum mass of such a coil is constrained by material strength limitations at roughly 40 tonnes (44 tons) and it would generate 70 N (16 lbf) of thrust, giving a mass/thrust ratio of 600 kg/N. If operated within the solar system, high temperature superconducting wire would be required to make the magsail practical. If operated in interstellar space conventional superconductors would be adequate.
In order to reduce the size and weight of the magnet of the magnetic sail, it may be possible to inflate the magnetic field using a plasma in the same way that the plasma around the Earth stretches out the Earth’s magnetic field in the magnetosphere.
In this approach, called mini-magnetospheric plasma propulsion (M2P2), currents that run through the plasma will augment and partially replace the currents in the coil. This is expected to be especially useful far from the Sun, where the increased effective size of a M2P2 sail compensates for the reduced dynamic pressure of the solar wind.
Modes of operation:
In a plasma wind- When operating away from planetary magnetospheres, a magnetic sail would force the positively charged protons of the solar wind to curve as they passed through the magnetic field. The change of momentum of the protons would thrust against the magnetic field, and thus against the field coil.
Inside a planetary magnetosphere- Inside a planetary magnetosphere, a magnetic sail can thrust against a planet’s magnetic field, especially in an orbit that passes over the planet’s magnetic poles, in a similar manner to an electrodynamic tether.
Interstellar travel- Interstellar space contains very small amounts of hydrogen. A fast-moving sail would ionize this hydrogen by accelerating the electrons in one direction and the oppositely charged protons in the other direction. The energy for the ionization and cyclotron radiation would come from the spacecraft’s kinetic energy, slowing the spacecraft.