Journal of The British Interplanetary Society, Vol 43, pp. 265-272,1990 MAGNETIC SAILS AND INTERSTELLAR TRAVEL* DANA G. ANDREWS * and ROBERT M. ZUBRIN** *Boeing Aerospace, Seattle, Washington 98124, USA ** Martin Marietta Astronautics, Denver, Colorado 80201, USA. A new concept, the magnetic sail, or "Magsail", is proposed which propels spacecraft by using the magnetic field generated by a loop of superconducting cable to deflect interplanetary or interstellar plasmas winds. A description is given of the computer code used to model the performance of such a device and results of a series of investigations are presented. It is found that a Magsail sailing on the solar wind at a radius of one astronautical unit (A.U.) can attain accelerations on the order of 0.01 m/s 2 , much greater than that available from a conventional solar lightsail. When used as a brake for an interstellar spacecraft, the Magsail can reduce spacecraft velocity by a factor of e every five years. A systems performance code was used to analyze the utility of the Magsail when used in conjunction with either fusion rocket or laser lightsail accelerated interstellar spacecraft. It is found that the Magsail can reduce flight times by forty to fifty years and propellant requirements by thirty percent for fusion rocket propelled ten lightyear missions. The Magsail also provides an efficient method for decelerating laser lightsail propelled missions that are otherwise simply impossible. 1. INTRODUCTION 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 hundreds of kilometres in diameter is stored on a drum attached to a payload spacecraft. When the time comes for operation the cable is played out into space 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 10 -5 Tesla, so little structural strengthening is required. Two different configurations were examined as shown in fig. 1. In the axial configuration (fig. la), the axis of the dipole is aligned with the direction of flight. In the normal configuration (fig. 1 b) the axis of the dipole is normal (or perpendicular) to the direction of flight. In operation charged particles entering the field are deflected according to the В-field they experience, thus imparting momen- tum 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 earth is a flux of several million protons and electrons per cubic meter at a velocity of 300 to 600 km/sec. This can be used to accelerate a spacecraft radially away from the sun and the maximum speed available would approximate that of the solar wind itself. While inadequate for interstellar missions these velocities are certainly more than adequate for interplanetary missions. The dipole field of the normal configuration also generates a force perpendicular to the wind (i.e. lift). While not crucial for interstellar applications, lift greatly enhances the usefulness of the Magsail for interplanetary operations. Additional interplane- tary maneuvering capability could be attained by using gravita- tional swingbys of the major planes. The second application, and the one which will receive the majority of our attention in this paper, is as a brake for an interstellar spacecraft travelling at fractions of the speed of light. The rapidly moving magnetic field of the Magsail ionises the interstellar medium and then deflects *This paper was presented at the 39th IAF Congress in Banga- lore, October 1988.