Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

IAF-00-S.6.04TETHER SYSTEMS FOR SATELLITE DEPLOYMENT AND DISPOSALRobert P. HoytTethers Unlimited, Inc., Seattle, Washingtonwww.tethers.comAbstractSpace tether systems have strong potential for providing significant reductions in the cost of propulsionfor a number of important applications, including spacecraft deployment, post-mission spacecraftdisposal, and satellite orbital maintenance. Tether systems can provide propulsion to space systems boththrough electrodynamic interactions with the Earth's magnetic field and through momentum-exchange interactionsbetween two objects in orbit. This paper summarizes recent work by Tethers Unlimited, Inc.to develop a product line of tether-based technologies to service markets for LEO microsatellite propulsion,LEO satellite disposal, and deployment of spacecraft to geostationary orbits.IntroductionTether systems can provide propellantless in-spacepropulsion for a wide range of space missions. A spacetether is a long thin cable constructed of either highstrengthfibers or conducting wires that is extendedbetween two or more objects in space. Tethers can accomplishpropulsion missions through two differentmechanisms. First, current flowing in conducting tetherscan generate forces through electrodynamic interactionswith the Earth's magnetic field. Second, tetherscan provide a mechanical link between two objects inspace, enabling orbital momentum and energy to beexchanged between the objects. Both mechanisms canaccomplish significant ÆV operations without consumptionof propellant. By minimizing the need forpropellant to be carried into orbit, tether systems cangreatly reduce the total propulsion costs for many missions.Tethers Unlimited, Inc. (TUI) is currently developinga line of tether products to provide cost-effective propulsioncapabilities for applications including end-ofmissionLEO satellite deorbit, microsatellite orbitraisingand stationkeeping, and deployment of largepayloads to geostationary and lunar transfer orbits.Small Electrodynamic Tether DevicesThe first two products in development at TUI willutilize electrodynamic tether techniques to provide lowmassand low-cost propulsion for satellites operating inLEO. The principle of electrodynamic tethers are illustratedin Figure 1. In an electrodynamic tether system,a long tether constructed of conducting wire is extendedfrom a spacecraft. Gravity-gradient forces will tend toorient this long flexible structure along the local verticaldirection. The orbital motion of the tethered systemacross the Earth's magnetic field generates a voltagealong the tether. If the tether system provides a mechanismfor electrically contacting the ionospheric spaceplasma at both ends of the tether, the induced voltagecan drive a current up the tether, as illustrated in Figure1 a. This current interacts with the magnetic field togenerate a JxB force on the tether which opposes themotion of the tether system. This force is thus a "drag"force which drains orbital energy from the system, loweringthe tethered system's orbit. If, however, the tethersystem applies a voltage down the tether sufficient toovercome the induced voltage, it can drive a currentdown the tether, resulting in a JxB force on the tetherthat raises the orbit of the tether system.The first product in development, the TerminatorTether ª , is a small electrodynamic tether drag deviceintended to enable the users of LEO space to mitigatethe growth of space debris by providing them a lowcostand reliable means of ensuring that their satellitesare removed from orbit after they has completed theirmissions. The second device, the Microsatellite PropellantlessElectrodynamic Tether (µPET ª ) PropulsionSystem, will provide an essentially infinite ÆV propulsioncapability for orbit raising, orbital modification,and long-term stationkeeping for small satellites operatingin LEO. The µPET ªPropulsion System buildsupon the Terminator Tether ª technologies by addingthe capability to process input power from the satelliteto drive the electrodynamic tether in a thrust mode,rather than a drag mode.dragorbitalvelocityvoltagecurrentthrustcurrentorbitalvelocityappliedvoltagemagneticfield lineFigure 1. a) Electrodynamic tether drag mode.magneticfield lineb) Electrodynamic tether thrust mode.Copyright©2000 by Tethers Unlimited, Inc. Published by the InternationalAstronautical Federation with permission.1