Robert P. Hoyt

Terminator Tether™: A Spacecraft Deorbit Device

This paper investigates the use of passive electrodynamic tether drag as a method for quickly removing spent or dysfunctional spacecraft from low Earth orbits (LEO). The fundamental physical principles underlying the operation of an electrodynamic drag Terminator Tether TM are developed, some practical considerations are discussed, and calculationsof the area-time productare made forspacecraft orbits representative of those that will beused in the LEO satellite constellations of the next few decades. These calculations indicate that electrodynamic drag can remove a spacecraft from a typical 700 ‐2000-km LEO constellation orbit within a few months using a Terminator Tether system massing less than 3% of the spacecraft dry mass. Although the tether increases the cross-sectional area of the satellite system during the deorbit phase, the electrodynamic drag is so many times greater than atmospheric drag at these altitudes that the total area-time product can be reduced by several orders of magnitude, reducing the risks of collisions with other satellites. Concerns regarding tether survivability can be solved by using a multiline, fail-safe Hoytether TM construction. The Terminator Tether may thus provide a cost-effective method of mitigating the growth of debris in valuable constellation orbits.

Cislunar Tether Transport System

We describe a space systems architecture for repeatedly transporting payloads between low Earth orbit and the surface of the moon without significant use of propellant. This architecture consists of one rotating tether in elliptical, equatorial Earth orbit and a second rotating tether in a circular low lunar orbit. The Earth-orbit tether picks up a payload from a circular low Earth orbit and tosses it into a minimal-energy lunar transfer orbit. When the payload arrives at the Moon, the lunar tether catches it and deposits it on the surface of the Moon. Simultaneously, the lunar tether picks up a lunar payload to be sent down to the Earth orbit tether. By transporting equal masses to and from the Moon, the orbital energy and momentum of the system can be conserved, eliminating the need for transfer propellant. Using currently available high-strength tether materials, this system could be built with a total mass of less than 28 times the mass of the payloads it can transport. Using numerical simulations that incorporate the full threedimensional orbital mechanics and tether dynamics, we have verified the feasibility of this system architecture and developed scenarios for transferring a payload from a low Earth orbit to the surface of the Moon that require less than 25 m/s of thrust for trajectory targeting corrections.

FAILSAFE MULTILINE HOYTETHER LIFETIMES

We are currently developing a failsafe multiline tether system for long-duration, highvalue, and crew-rated missions. The lifetimes of current single-line tethers are limited by damage due to meteorite and orbital debris impactors to periods on the order of weeks. Although single-lme tether lifetimes can be improved by increasing the diameter of the tether, this incurs a prohibitive mass penalty. We have developed a tether structure composed of multiple lines with redundant interlinking that is able to withstand many impacts. Analytical modeling and numerical simulation of this design indicate that this tether structure can achieve lifetimes of tens of years without incurring a mass penalty. Moreover, while single-line tether survival probability drops exponentially with time, redundant linkage in failsafe multiline tethers keeps the tether survival probability very high until the tether lifetime is reached. d

The Terminator Tether - Autonomous deorbit of LEO spacecraft for space debris mitigation

The Terminator Tether is a lightweight, low-cost device that will use electrodynamic drag generated by a conducting tether to remove satellites and upper stages from low Earth orbit when they have completed their missions. In order to investigate and optimize the performance of the device, we developed a detailed numerical simulation that includes models for tether dynamics, electrodynamic interactions with the EarthÕs ionosphere, field emission aray cathode operation, and other relevant physics. Using this simulation, we examined the electrical behavior of the tether-plasma circuit, and found that a device with a tether length of 5-10 km can utilize some of the power generated by the tether to drive its own circuitry without severely affecting the deorbit rate. Thus the device can be independent of the host spacecraftÕs power systems during deorbit. Because an uncontrolled electrodynamic tether is dynamically unstable, we developed a feedback-control scheme and verified its operation using simulations. Using the same models and control scheme, we investigated the performance of the device for disposing of spacecraft from various orbital inclinations and altitudes. We found that a tether device massing 2% of the host spacecraft mass can deorbit an upper stage from a 50¡, 400 km orbit in under two weeks, a mid-LEO satellite from a 50¡, 850 km orbit in under three months, or a high-LEO satellite from a 50¡, 1400 km orbit in less than a year. Introduction Electrodynamic tether drag can provide a costeffective method for autonomously deorbiting low Earth orbit (LEO) spacecraft to mitigate the growth of orbital debris. The Terminator Tether is a small, lightweight, low-cost device that will be attached to satellites and upper stages before launch. The device contains a conducting tether, a tether deployer, an electron emitter, and electronics to control the deployment and operation of the tether. During the operational period of the host spacecraft, the tether will be stored in the deployer and the Terminator Tether electronics will be dormant, waking up periodically to check the status of the host spacecraft. When the device receives an activation command, or when it determines that the host spacecraft is defunct, the Terminator Tether will activate springs in the deployer to kick the device down and away from the spacecraft, deploying the tether. A schematic of the device is shown in Figure 1. The principle of electrodynamic tether drag is illustrated in Figure 2. The motion of the conducting tether through the EarthÕs magnetic field will generate a voltage along the length of the tether; in a direct orbit, the top of the tether will be charged positively relative to the ambient ionospheric plasma. Most of the tether length will be left uninsulated, so that the bare wires can efficiently collect electrons from the ionosphere. These electrons will flow down the tether to the Terminator Tether endmass, where the electron emitter will expel them back in to the ionosphere. Thus a current will flow up the tether, and the current ÒloopÓ will be closed by plasma waves in the ionosphere. This current will then interact with the EarthÕs magnetic field to generate a Lorentz JxB force on the tether. This force will oppose the orbital motion of the tether. Through its mechanical connection to the host spacecraft, the tether will thus drain the orbital energy of the spacecraft, lowering its orbit until it disintegrates in the upper atmosphere.

The Multi-Application Survivable Tether (MAST) Experiment

Tethers Unlimited, Inc (TUI) and Stanford Universitys Space Systems Development Laboratory (SSDL) are collaboratively developing the Multi-Application Survivable Tether (MAST) experiment, which will obtain data on tether performance, survivability, and dynamics. This data is crucial to the de- velopment of operational tether systems for propellantless propulsion and deorbit, formation-flying, and momentum-exchange transportation applications. The first objective of the MAST experiment is to obtain detailed on-orbit data on the survivability of space tethers and other gossamer spacecraft structures in the micrometeorite/orbital (M/OD) debris environment. The MAST experiment will deploy three 1-kg Cube- Sats along a 1-km Hoytether that incorporates both conducting and nonconducting materials. The middle CubeSat will then slowly translate along the tether, inspecting the tether as it moves and returning data on the rate of damage to the tether by M/OD impacts. The second objective of the experiment will be to study the dynamics of tethered formations of spacecraft and rotating tether systems. This data is required to en- able the validation of space tether simulation tools such as TetherSim and GTOSS. The third objective of the experiment will be to demonstrate momentum-exchange tether concepts. In this paper we will present results of initial design studies and analyses of MAST system dynamics and performance.

APPLICATION OF THE TERMINATOR TETHER™ ELECTRODYNAMIC DRAG TECHNOLOGY TO THE DEORBIT OF CONSTELLATION SPACECRAFT

The Terminator TetherTM is a small, lightweight system that will use passive electrodynamic tether drag to rapidly deorbit spacecraft from low Earth orbit. Studies of the application of electrodynamic drag to the deorbit of constellation satellites indicates that the Terminator TetherTM offers significant mass savings compared to conventional rocket-based deorbit systems. Moreover, because it uses passive electrodynamic drag to achieve deorbit, i t can deorbit the spacecraft even if the host has lost power and control functions. Numerical analyses of the performance of the Terminator TetherTM indicate that a 5 to 10 km long conducting tether weighing only 2% of the host spacecraft mass can deorbit a typical constellation satellite within a few months. Although the tether increases the total collision cross-sectional area of the satellite system during the deorbit phase, the electrodynamic drag is so many times greater than atmospheric drag at constellation altitudes that the tether can reduce the collisional Area-Time product for the satellite by several orders of magnitude. The Terminator TetherTM thus can provide a low-cost and reliable method of mitigating the growth of debris in valuable constellation orbits.

Remediation of radiation belts using electrostatic tether structures

Scattering of energetic charged particles by high-voltage electrostatic tether structures may present a technically and economically viable method of rapidly remediating radiation belts caused by both natural processes and manmade events. In this paper, we describe a concept for a system of electrostatic tether structures designed to rapidly remediate an artificial radiation belt caused by a high altitude nuclear detonation. We then investigate the scaling of the system size and power requirements with the tether voltage and other design parameters. These scaling analyses indicate that a conventional single-line tether design cannot provide sufficient performance to achieve a system design that is viable. We then propose innovative multiwire tether geometry and show that this tether design can significantly improve the overall performance of the electrostatic system, enabling the requirements for total power and number of satellite systems to be reduced to levels that are both technically and economically viable

Performance of the Terminator Tether for autonomous deorbit of LEO spacecraft

The Terminator Tether is a lightweight, low-cost device that will use electrodynamic drag generated by a conducting tether to remove satellites and upper stages from low Earth orbit when they have completed their missions. In order to investigate and optimize the performance of the device, we developed a detailed numerical simulation that includes models for tether dynamics, electrodynamic interactions with the EarthÕs ionosphere, Spindt cathode electron emission, and other relevant physics. Using this simulation, we examined the electrical behavior of the tether-plasma circuit, and found that a device with a tether length of 5-10 km can utilize some of the power generated by the tether to drive its own circuitry without severely affecting the deorbit rate. Thus the device can be independent of the host spacecraftÕs power systems during deorbit. Because an uncontrolled electrodynamic tether is dynamically unstable, we developed a feedback-control scheme and verified its operation using simulations. Using the same models and control scheme, we investigated the performance of the device for disposing of spacecraft from various orbital inclinations and altitudes. We found that a tether device massing 2% of the host spacecraft mass can deorbit an upper stage from a 50¡, 400 km orbit in under two weeks, a mid-LEO satellite from a 50¡, 850 km orbit in under three months, or a high-LEO satellite from a 50¡, 1400 km orbit in less than a year.

The Terminator Tape ™ : A Cost-Effective De-Orbit Module for End-of-Life Disposal of LEO Satellites

The rapid growth of the orbital debris population poses an increasing threat to military, commercial, and civilian science spacecraft in Earth orbit. NASA, the DoD, ESA, and other organizations have begun to respond to this problem by imposing requirements for debris mitigation upon new space systems. These requirements specify that spacecraft at end-oflife be disposed of by either atmospheric re-entry within 25 years, maneuver to a higher storage orbit, or direct retrieval. For most satellites operating in low Earth orbit (LEO), atmospheric re-entry is the most viable option. To provide a cost-effective means for satellite operators to comply with the 25-year post-mission orbital lifetime restriction, Tethers Unlimited is developing a lightweight de-orbit module called the “Terminator Tape ™ ”. The Terminator Tape is a small module that bolts onto any side of a spacecraft during satellite integration. At the completion of the satellite’s mission, the satellite will activate the Terminator Tape module. The module will then deploy a several-hundred-meter length of thin conducting tape. This tape will not only significantly enhance the aerodynamic drag experienced by the system, but will also generate electrodynamic drag forces through passive interactions with the Earth’s magnetic field and conducting ionospheric plasma, de-orbiting the satellite within 25 years. Two modules are currently in development, one sized for microsatellites operating at altitudes of less than 900 km, and the other sized for CubeSats. In this paper, we will present design overviews and concept of operations for both modules, as well as analyses of deorbit of satellites using these modules.

Stabilization of electrodynamic tethers

Electrodynamic tethers are susceptible to instabilities in a number of different modes, including pendulum librations, transverse wave oscillations, and “skip rope” oscillations. We have developed two simple feedback algorithms for controlling these oscillations. The first algorithm requires knowledge of the motion of several points along the length of the tether, and control is achieved by varying the tether current. Detailed simulations of electrodynamic tethers indicate that this feedback control algorithm is successful in stabilizing the dynamics of electrodynamic tethers during extended periods of operation. The second algorithm requires only knowledge of the motion of the tether end-mass. Simulations indicate that this simpler algorithm is also successful in stabilizing the dynamics of the tether, although it stabilizes the oscillations at a higher quasi-steady state level.