Magnetic sail (also in short magsail) - a device which can be theoretically used to propel (accelerate or decelerate) a spacecraft by using magnetic field to accelerate or deflect plasma naturally occurring in the solar wind and interstellar space.1)
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A current is initiated in the loop and the associated magnetic field (at the order of 0.1 mT) and mechanical forces will help in holding the circular shape of the loop, without other structural support. The loop can be positioned and used at any angle with respect to the plasma wind.
Charged particles entering the field are deflected according to magnetic field generated by the loop, hence transferring momentum to the loop, and the payload.
The magnetic sail loop will always create drag in a net plasma wind (such as the solar wind) relative to the spacecraft. This can be used to accelerate the spacecraft in the direction of such wind.
The Sun's solar wind in the vicinity of the Earth is at the order of several million protons and electrons per cubic meter at a velocity of 400-600 km/s, which would be a maximum speed available for such acceleration. The velocity would be too low for interstellar travel, but it might be sufficient for interplanetary missions.
If the magnetic sail is oriented sideways to the solar wind it becomes possible to provide steering ability to the interstellar spacecraft.
As of 2013, the magnetic sail has not been used yet in practical application. Theoretical calculations show that for interplanetary missions the transfer would be slower than by using ordinary ballistic methods. But the magnetic sail offers trust to payload ratio one to two orders of magnitude greater. However, at the time of investigation (year 2000) the existing superconducting technology did not offer required current density of superconductors, but there was enough progress suggested by the Malozemoff's law to recommend an “aggressive pursue” of magnetic sail technology in the future.