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Magnetic sail

A magnetic sail or magail is a proposed method of spacecraft propulsion. A spacecraft would deploy a large loop of superconducting wire to generate a magnetic field, and possibly auxiliary loops for steering or to reduce radiation hazards from the charged particles. Magnetic sails are an attractive propulsion technology because calculations show that superconducting magnetic sails could have a better thrust-to-mass ratio than solar sails.

Table of contents

Principles of operation and design

When a charged particle such as a proton or electron moves through a magnetic field perpendicular to the field lines, it is deflected from its path.

The solar wind from the Sun has several million protons and electrons per cubic meter near Earth. The protons move away from the Sun at 400 to 600 kilometers per second. A magnetic sail can accelerate these particles, and gain thrust.

This is similar to how solar sails use the pressure of photons emitted by the Sun, but protons mass more than photons. Therefore more momentum is available in the same volume. The sail is also a magnetic field, which has no mass. Therefore the mass of a magnetic sail could be much less than a light sail.

A magnetic sail can also thrust directly against planetary and solar magnetospheres.

A magnetic sail would be stored coiled up on board a spacecraft when not in use. It would just be a spool of superconducting wire. To deploy it, an electric current would be started in the loop. Once the current is started no more energy is required. The magnetic field created by the current circulating in the loop attempts to expand the loop, helping to "inflate" it and force it into a circular shape.

Since the magnetic sail would operate at low field strengths, typically around 0.00001 Tesla (approximately 1/3 Earth's magnetic field strength at its equator), the cable can be weak.

In planetary magnetospheres and in a plasma wind a magnetic sail is more efficient with lower currents and a larger, weaker magnetic field. The result is a thinner, lighter cable and a larger loop radius.

One way to exploit this property might be to use an electrically-excited gas-plasma field loop instead of a wire loop. This approach is called mini-magnetospheric plasma propulsion.

Modes of operation

In a plasma wind

When operating away from planetary magnetospheres, a magnetic sail forces the positively charged protons of the solar wind to curve as they pass through. The change of momentum of the protons thrusts against the magnetic field, and thus against the field coil.

Just as with solar sails, magnetic sails can "tack." If a magnetic sail orients at an angle relative to the solar wind, charged particles are deflected preferentially to one side and the magnetic sail is pushed laterally. This mens that magnetic sails can maneuver to most orbits.

In this mode, the efficiency of the magnetic sail falls off with the square of its distance from the Sun. Solar weather also has major effects on the sail. It is possible that the plasma eruption from a severe solar flare could damage an efficient, fragile sail.

Some people believe that a magnetic sail cannot exceed the speed of the plasma pushing it, but this is false. As the speed of a magnetic sail increases, its acceleration becomes more dependent on its ability to tack efficiently. At high speeds, the plasma wind's direction will seem to come increasingly from the front of the spacecraft. Advanced sailing spacecraft might deploy field coils as "keels," so the spacecraft could use the difference in vector between the solar magnetic field and the solar wind, much as sailing yachts do.

Inside a planetry 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.

The range of maneuvers available to a magnetic sail inside a planetary magnetosphere are more limited than in a plasma wind. Just as with the more familiar small-scale magnets used on Earth, a magnetic sail can only be attracted towards the magnetosphere's poles or repelled from them, depending on its orientation.

When the magnetic sail's field is oriented in the opposite direction as the magnetosphere it experiences a force inward and toward the nearest pole, and when it is oriented in the same direction as the magnetosphere it experiences the opposite effect. It is important to note that a magnetic sail oriented in the same direction as the magnetosphere is not stable, and will have to prevent itself from being flipped over to the opposite orientation by some other means.

The thrust that a magnetic sail delivers within a magnetosphere decreases with the fourth power of its distance from the planet's internal magnetic dynamo.

This limited maneuvering capability is still quite useful. By varying the magnetic sail's field strength over the course of its orbit, a magnetic sail can give itself a "perigee kick" raising the altitude of its orbit's apogee.

Repeating this process with each orbit can drive the magnetic sail'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.

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 its "unstable" orientation. A launch from Earth requires superconductors with 80 times the current density of the best known high-temperature superconductors.

Interstellar travel

In interstellar spaceflight outside the heliopause of a star, a magenetic sail could act as a parachute, to decelerate a spacecraft. This would save the deceleration half of an interstellar spacecraft's fuel, an immense benefit.

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. The cyclotron radiation from the acceleration of particles would be an easily detected howl in radio frequencies.

Magnetic sails could also be used with beam-powered propulsion, by using a high-power particle accelerator to fire a beam of charged particles at the spacecraft. The magsail would deflect this beam, transferring momentum to the vehicle. This would provide much higher acceleration than a solar sail driven by a laser, but a charged particle beam would disperse in a shorter distance than a laser.

Related concepts

A related concept is mini-magnetospheric plasma propulsion, which uses a cloud of plasma to form its magnetic sail instead of a loop of cable.

See also: spacecraft propulsion

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