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.
Principles of operation and design
The solar wind is a tenuous 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 kilometres per second (250 to 370 mi/s). The magnetic sail introduces a magnetic field into this plasma flow, perpendicular to the motion of the charged particles, 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.
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.
For a sail in the solar wind at 1 AU away from the Sun, the field strength required to resist the dynamic pressure of the solar wind is 50 nT (Template:Convert/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 weight of such a coil is constrained by material strength limitations at roughly 40 tonnes, and it would generate 70 newtons (16 lbf) of thrust , giving a mass/thrust ratio of 600 kg/N. It is not clear how such a coil would be cooled.
The operation of magnetic sails, using plasma wind, is analogous to the operation of solar sails using the radiation pressure of photons emitted by the Sun. Although solar wind particles have rest mass, and photons do not, sunlight has thousands of times more momentum than the solar wind. Therefore, a magnetic sail must deflect a proportionally larger area of the solar wind, than a comparable solar sail, to generate the same amount of thrust. However, it need not be as massive as a solar sail, because the solar wind is deflected by a magnetic field instead of a large physical sail. Conventional materials for solar sails weigh around 7 grams per square metre (0.0014 lb/ft²), giving a thrust of 1e-5 N/m2 at 1 AU. This gives a mass/thrust ratio of at least 700 kg/N, similar to a magnetic sail, neglecting other structural components.
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