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Magnetic sail

From Academic Kids

A magnetic sail or magsail 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.

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.

Momentum

This is similar to how solar sails use the 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. It might not be more massive than a solar sail, however, because the solar wind is deflected by a magnetic field instead of a material sail. It still has a mass from the superconducting loop and power source used to generate the field. A magnetic sail can also thrust directly against planetary and solar magnetospheres.

Storage

A magnetic sail could be stored coiled up on board a spacecraft when not in use. It could 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.

Low field strengths

Since the magnetic sail would operate at low field strengths, typically around 0.00001 tesla (10 microteslas, 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

Missing image
Magsail-particle-wind.png
A magnetic sail in a wind of charged particles. The sail generates a magnetic field, represented by red arrows, which deflects the particles out of the page. The force on the sail is into the page.
A magnetic sail in magnetic field (represented by magenta arrows).  The sail generates its own magnetic field, represented a small bar magnet.  The ambient magnetic field pulls on the magnetic sail like any other magnet. The force on the sail is to the left.
A magnetic sail in magnetic field (represented by magenta arrows). The sail generates its own magnetic field, represented a small bar magnet. The ambient magnetic field pulls on the magnetic sail like any other magnet. The force on the sail is to the left.


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 means 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.

A common misconception is 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 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.

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 magnetic sail could act as a parachute, to decelerate a spacecraft. This removes any need to accelerate fuel for the deceleration half of an interstellar journey. This is 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.

Fictional uses

The magnetic sail features prominently in the science-fiction novels of Michael Flynn, particularly in The Wreck of the River of Stars; this book is the tale of the last flight of a magnetic sail ship when fusion rockets based on the Farnsworth-Hirsch Fusor have become the preferred technology.

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