The Magnetic Sail
The magnetic sail or magsail, is a device which can be used to accelerate or decelerate a spacecraft by using a magnetic field to accelerate/deflect the plasma naturally found in the solar wind and interstellar medium. Its principle of operation is as follows:
A loop of superconducting cable perhaps tens of kilometers in diameter is stored on a drum attached to a payload spacecraft. When the time comes for operation, the cable is played out and a current is initiated in the loop. This current once initiated, will be maintained indefinitely in the superconductor without further power. The magnetic field created by the current will impart a hoop stress to the loop aiding the deployment and eventually forcing it to a rigid circular shape. The loop operates at low field strengths, typically 0.0001 Tesla, so little structural strengthening is required. The loop can be positioned with its dipole axis at any angle with respect to the plasma wind, with the two extreme cases examined for analytical purposes being the axial configuration, in which the dipole axis is parallel to the wind, and the normal configuration, in which the dipole axis is perpendicular to the wind. A magsail with payload is depicted in Fig. 1.
In operation, charged particles entering the field are deflected according to the B-field they experience, thus imparting momentum to the loop. If a net plasma wind, such as the solar wind, exists relative to the spacecraft, the magsail loop will always create drag, and thus accelerate the spacecraft in the direction of the relative wind. The solar wind in the vicinity of the Earth is a flux of several million protons and electrons per cubic meter at a velocity of 400 to 600 km/s. This can be used to accelerate a spacecraft radially away from the sun and the maximum speed available would be that of the solar wind itself. While inadequate for interstellar missions, these velocities are certainly more than adequate for interplanetary missions.
However if the magsail spacecraft has somehow been accelerated to a relevant interstellar velocity, for example by a fusion rocket or a laser-pushed lightsail, the magsail can be used to create drag against the static interstellar medium, and thus act as an effective braking device. The ability to slow spacecraft from relativistic to interplanetary velocities without the use of rocket propellant results in a dramatic lowering of both rocket mass ratio and the mission time.
If the magsail is utilized in a non-axial configuration, symmetry is destroyed and it becomes possible for the magsail to generate a force perpendicular to the wind, i.e. lift. Lift can be used to alter the magsail spacecraft's angular momentum about the sun, thus greatly increasing the repertoire of possible maneuvers. In addition, lift can be used to provide steering ability to a decelerating relativistic interstellar spacecraft.
The magsail as currently conceived depends on operating the superconducting loop at high current densities at ambient temperatures. In interstellar space, ambient is 2.7 degrees K, where current low temperature superconductors NbTi and Nb3Sn have critical currents (depending upon temperature and local magnetic field) of approximately 1.0x1010 and 2.0x1010 Amps/m2 respectively. In interplanetary space, where ambient temperatures are above the critical temperatures of low temperature superconductors, these materials would require expensive and heavy refrigeration systems However the new high temperature superconductors such as YBa2Cu3O7 have demonstrated comparable critical currents in microscopic samples at temperatures of 77 K or more, which would make them maintainable in interplanetary space using simple multi-layer insulation and highly reflective coatings. Assuming that this performance will someday be realizable in bulk cable, we can parameterize the problem of estimating potential magsail performance by assuming the availability of a high temperature superconducting cable with a critical current of 1010 Amps/m2, i.e. equal to that of NbTi. Because the magnets are operating in an ambient environment below their critical temperature, no substrate material beyond that required for mechanical support is needed. Assuming a fixed magnet density of 5000 kg/m3 (copper oxide), such a magnet would have a current to mass density (j/pm) of 2.0x106 Amp-m/kg.
By interacting with the Earth's magnetic poles, the magsail can generate sufficient force to allow it to drive both itself and a substantial payload up to escape velocity via a series of perigee kicks. Once escape has been reached, the magsail will find itself in interplanetary space where the solar wind is available to enable further propulsion. Magsail operations can enable both maneuvering in heliocentric space and deceleration of ultra-high velocity interstellar spacecraft without the use of propellant. It also provides an option for lowering the orbit of a spacecraft by creating drag against a planetary ionosphere.
Pioneer’s work on Magnetic Sails was supported by funding from the NASA Institute for Advanced Concepts (NIAC.)