Magnetic sail

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, the solar wind from the Sun contains several million protons and electrons per cubic meter, and flows at 400 to 600 kilometers per second. The magnetic sail introduces a magnetic field into this plasma flow, perpendicular to the motion of a 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 1AU away from the sun, the field strength required to resist the dynamic pressure of the solar wind is 50nT. Zubrin's proposed magnetic sail design would create a bubble of space of 100km in diameter where solar wind ions are substantially deflected, using a hoop 50km in radius. The minimum weight of such a coil is constrained by material strength limitations at roughly 40 metric tonnes, and it would generate 70N of thrust[1], giving a mass/thrust ratio of 600kg/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 meter, giving a thrust of 1e-5 N/m at 1AU. This gives a mass/thrust ratio of at least 700kg/N, similar to a magnetic sail, neglecting other structural components.

The solar and magnetic sails have a thrust that falls off as the square of the distance from the sun.

When close to a planet with a strong magnetosphere, e.g. Earth or a Gas giant, the magsail could generate more thrust by interacting with the magnetosphere instead of the solar wind, and may therefore be more efficient.

Mini-magnetospheric plasma propulsion

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 running through the plasma 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. The original NASA design proposes a spacecraft containing a can-shaped electromagnet, into which a plasma is injected. The plasma pressure stretches the magnetic field and inflates a bubble of plasma around the spacecraft. The current in the plasma in this case augments and partially replaces currents in the coils. The plasma then generates a kind of minaturised magnetosphere around the spacecraft, analogous to the magnetosphere that surrounds the earth. The protons and electrons which make up the solar wind are deflected by this magnetosphere, and the reaction accelerates the spacecraft. The thrust of the M2P2 device would be steerable to some extent, potentially allowing the spacecraft to tack into the solar wind, and allowing efficient changes of orbit.

In the case of the (M2P2) system the spacecraft releases gas to create the plasma needed to maintain the somewhat leaky plasma bubble. The M2P2 system therefore has an effective specific impulse which is the amount of gas consumed per newton of thrust. This is a figure of merit usually used for rockets, where the fuel is actually reaction mass. Robert Winglee, who originally proposed the M2P2 technique, calculates a specific impulse of 200 kN·s/kg (roughly 50 times better than the space shuttle main engine). These calculations suggest that the system requires on the order of a kilowatt of power per newton of thrust, considerably lower than electric thrusters, and that the system generates the same thrust anywhere within the heliopause because the sail spreads automatically as the solar wind becomes less dense. However, this technique is less well understood than the simpler magnetic sail, and issues of how large and heavy the magnetic coil would have to be([2],), or whether the momentum from the solar wind can be efficiently transferred to the spacecraft([3]), are under dispute.

The expansion of the magnetic field using plasma injected has been successfully tested in a large vacuum chamber on Earth, but the development of thrust was not part of the experiment. A beam-powered variant, MagBeam, is also under development.

Modes of operation

A magnetic sail in a wind of charged particles. The sail generates a magnetic field, represented by red arrows, which deflects the particles into the page. The force on the sail is out of the page.

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.

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 could maneuver to most orbits.

In this mode, the amount of thrust generated by a magnetic sail falls off with the square of its distance from the Sun as the flux density of charged particles reduces. 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. 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

A magnetic sail in a spatially-varying magnetic field. Because the vertical external field B_ext is stronger on one side than the other, the leftward force on the left side of the ring is smaller than the rightward force on the right side of the ring, and the net force on the sail is to the right.

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.

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 to 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. 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

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.

Thus, in interstellar spaceflight outside the heliopause of a star, a magnetic sail could act as a parachute, to decelerate a spacecraft. This removes any fuel requirements for the deceleration half of an interstellar journey, which would benefit interstellar travel enormously. The magsail was first proposed for this purpose in 1985 by Robert Zubrin and Dana Andrews, predating other uses, and evolved from a concept of the Bussard ramjet which used a magnetic scoop to collect interstellar material.

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 due to the electrostatic repulsion of its component particles. This dispersion problem could potentially be resolved by accelerating a stream of sails which then in turn transfer their momentum to a magsail vehicle, as proposed by Jordin Kare.

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.