Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

IAF-00-S.6.04Terminator Tether ª Deorbit DeviceTUI's first tether product is the Terminator Tether ª , asmall electrodynamic tether drag device designed toprovide a cost-effective method for autonomously deorbitinglow Earth orbit (LEO) spacecraft to mitigate thegrowth of orbital debris. 1The Terminator Tether ª is asmall, lightweight, low-cost device that will be attachedto satellites and upper stages before launch. Thedevice contains a conducting tether, a tether deployer,an electron emitter, and electronics to monitor the hostspacecraft and control the deployment and operation ofthe tether. During the operational period of the hostspacecraft, the tether will be stored in the deployer andthe Terminator Tether ªelectronics will be dormant,waking up periodically to check the status of the hostspacecraft. When the device receives an activationcommand, or when it determines that the host spacecraftis defunct, the Terminator Tether ªwill activatesprings in the deployer to kick the device down andaway from the spacecraft, deploying the tether.A schematic of the device is shown in Figure 2. Oncedeployed, the motion of the conducting tether throughthe EarthÕs magnetic field will generate a voltage alongthe length of the tether; in a direct orbit, the top of thetether will be charged positively relative to the ambientionospheric plasma. Most of the tether length will beleft uninsulated, so that the bare wires can efficientlycollect electrons from the ionosphere. 2These electronswill flow down the tether to the Terminator Tether ªendmass, where the electron emitter will expel themback in to the ionosphere. Thus a current will flow upthe tether, and the current ÒloopÓ will be closed byplasma waves in the ionosphere. 3,4This current willthen interact with the EarthÕs magnetic field to generatea Lorentz JxB force on the tether. This force will opposethe orbital motion of the tether. Through its mechanicalconnection to the host spacecraft, the tetherwill thus drain the orbital energy of the spacecraft, loweringits orbit until it disintegrates in the upper atmosphere.The Terminator Tether ªSatellite Deorbit Systemwill be composed of several subsystems: a conducting,survivable tether, a tether deployment system, a devicefor emitting electron current, and an electronic controlsystem called the Tether Control Unit (TCU).TetherIn order to electrically insulate the host spacecraftfrom the tether, a short section of the tether nearest thespacecraft will be constructed of high-strength, nonconductingyarns. The rest of the tether will be a survivableHoytether ª structure constructed of thin aluminumor copper wires, shown in Figure 3. The tether designwill vary depending upon the mass and orbit of thehost spacecraft, but for a typical LEO constellation satellitemassing 1500 kg, the tether will be 5 km longand mass approximately 15 kg (1% of the host spacecraftmass). In the Hoytether ª design, the wires areknitted together in an open-net structure that providesredundant paths to carry the mechanical load and current.This design will enable the tether to provide avery high probability of surviving the orbital debrisenvironment for the period of several weeks or monthsrequired to deorbit the spacecraft. 5Tether Control UnitThe TCU is part of the endmass that is deployedbelow the host spacecraft at the end of the tether. TheTCU carries the responsibilities of monitoring the hostspacecraft during the spacecraftÕs operational phase (thedormant phase for the Terminator Tether ª ), activatingthe deployment system when it is time to deorbit thehost, monitoring and controlling the tether dynamics tooptimize the descent rate, and responding to groundcontrol signals to perform avoidance maneuvers.Electron EmitterThe electron emitter will be a Field Emission ArrayCathode (FEAC) device, also known as a Spindt Cathode.6This device will be designed to emit up to 1ÊAof electron current. Electron emission is achieved byapplying a gate voltage of approximately 75-100 Vbetween an array of millions of microscopic needlepoints and a gate electrode. The emitted current can beFigure 2. The Terminator Tether ª .2Figure 3. Photo of a 20-cm length of the conducting Tri-Line Hoytapemade of 30 gauge aluminum wires knitted together with 22-TexP.T.F.E. thread.