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This paper focuses on the nonlinear dynamics and control for the spin-up maneuver of an electrodynamic tethered satellite system in an inclined elliptical orbit. A nonsingular formulation is established by using a unit vector along the tether to define the tether orientation and avoid the Euler angle singularity, and is adopted to describe the attitude dynamics of the system and to facilitate the controller design. The spin-up problem of concern is to accelerate the electrodynamic tether system into the prescribed spinning motions and maintain the expected angular velocity. A nonlinear model predictive control law accounting for nonlinear dynamics and system constraints is proposed to achieve this control goal by actively regulating the changing rate of the bounded tether current. Finally, numerical simulations are performed to validate the effectiveness of the proposed control law for the spin-up maneuver and evaluate the control performance. Meanwhile, comparative studies are presented to demonstrate the advantages of the proposed control law in stabilizing the out-of-plane libration motions of the system in inclined orbits.

Space missions in the environments of the outer planets require a large amount of propellant or assistant flybys. Moreover, the quadratic decrease of the radiation intensity with the distance from the sun makes solar panels less convenient as power source. Electrodynamic tether (EDT) technology is receiving increasing attention since it allows for obtaining both power and propulsion profiting from the high magnetic field and plasma density in the proximity of the giant planets. In particular, it has been shown that a self-propelled vehicle with a single EDT could be captured by Jupiter, were it not for the relativistic effects in the electron collection and the overheating of the tether due to the interaction with the environment. Here, we show that a multi EDT (MEDT) propulsion system can be used to circumvent these limitations. The suitable number, shape, and length of the tethers, depending on the size of the vehicle, are selected by structural and thermal analysis. A possible MEDT-propelled mission to Jupiter is described, including the design of the interplanetary trajectory, the orbital insertion, and the subsequent orbits around the planet. The results obtained demonstrate that such a bare MEDT system is a feasible option to provide propulsion and power in outer planets environments.

State-of-the-art electrodynamic tethers reach a steady electric current by using a bare segment to capture electrons passively from the ambient plasma (anodic contact) and an active electron emitter or a tether segment coated with a low-work-function material (cathodic contact) to emit electrons back and close the electrical circuit. This work proposes to take advantage of recent developments on thin-film solar cells and insert a photovoltaic (pv) tether segment in between the anodic and the cathodic contacts. Since thin-film solar cells can be folded and manufactured with any desired length and the same cross-section dimensions as the bare segment, i.e. width and thickness around few centimeters and tens of microns, the resulting device is compact and preserves bare tether simplicity. Detailed analysis of the current and voltage profiles throughout the tether shows that the electrical power introduced by the pv-segment into the tether-plasma circuit improves the performance and makes them less dependent on ambient conditions. The pv-segment decreases considerably the tether-to-plasma bias at the cathodic contact, thus opening the possibility to emit substantial current while using consumable-less electron emitters like thermionic and electron field emitters. The pv-segment also favors the current collection by increasing the tether-to-plasma bias at the bare segment. Propulsion and power generation applications and alternative architectures of bare-pv tethers are briefly discussed. This work received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 828902 (E.T.PACK project). GSA work is supported by the Ministerio de Ciencia, Innovación y Universidades of Spain under the Grant RYC-2014-15357.

The space debris occupies the orbit resources greatly, which seriously threats the safety of spacecraft for its high risks of collisions. Many theories about space debris removal have been put forward in recent years. The Electro Dynamic Tether (EDT), which can be deployed under gravity gradient, is considered to be an effective method to remove debris in low orbit for its low power consumption. However, in order to generate sufficient Lorentz force, the EDT needs to be deployed to several kilometers, which increases the risks of tangling and the instability of the EDT system. In the deployment process, different initial in-plane/out-of-plane angles, caused by direction error at initial release or the initial selection of ejection, affect the motion of EDT system seriously. In order to solve these problems, firstly, this paper establishes the dynamic model of the EDT system. Then, based on the model, safety metrics of avoiding tangling and assessing system stability during EDT deployment stage are designed to quantitatively evaluate the EDT system security. Finally, several numerical simulations are established to determine the safety ranges of the initial in-plane/out-of-plane angles on the EDT deployment.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2020-03-10, 資料番号: PA2010045000

Electrodynamic tethers are proposed as propulsion and energy harvesters for space probes orbiting planets with a magnetic field and ionosphere, however there are no descriptions in the technical literature of the design of an electrical power system for such an application at subsystem and circuit detail. This paper presents a proposal for such a power system that extracts energy from the kinetic energy of a spacecraft using a bare electrodynamic tether, i.e. an unsheathed conductive wire or band, in low-Earth orbit. The application of the system is the powering of the spacecraft while in its final de-orbiting maneuvers at end-of-life with no reliance on the main spacecraft bus.

Proceeding of: Sixth International Conference on Tethers in Space, Madrid, June 12-14, 2019 The Electrodynamic Tether Technology for Passive Consumable-less Deorbit Kit (E.T.PACK) is a project aimed at the development of a deorbit kit based on low-work-function Tether (LWT) technology, i.e., a fully passive and electrically floating system made of a long conductive tape coated with a low-work-function material. The LWT interacts passively with the environment (ambient plasma, magnetic field, and solar radiation) to exchange momentum with the planet's magnetosphere, thus enabling the spacecraft to de-orbit and/or re-boost without the need for consumables. The main goal is to develop a deorbit kit and related software with Technology Readiness Level 4 and promote a follow-up project to carry out an in-orbit experiment. The planned kit in the experiment has three modes of operation: fully passive LWT and conventional electrodynamic tether equipped with an active electron emitter in passive and active modes. Several activities of the project pivot around the C12A7 : e- electride, which will be used in four hardware elements: (i) LWT (ii) hollow cathode, (iii) photo-enhanced thermionic emission device to convert solar photon energy into electrical energy, and (iv) a hollow cathode thruster. These elements, some of which do not belong to the deorbit kit, are synergetic with the main stream of the project and common to some tether applications like in-orbit propulsion and energy generation. This work explains the activities of E.T.PACK and the approach for solving its technological challenges. After reviewing past progresses on electrodynamic tethers and thermionic materials, we present a preliminary concept of the kit for the in-orbit experiment, some simulation results, and the key hardware elements. This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 828902 (E.T.PACK project). GSA work is supported by the Ministerio de Ciencia, Innovaci on y Universidades of Spain under the Grant RYC-2014-15357. SN work is supported by Comunidad de Madrid (Spain) under the Grant 2018/T2IND/11352

Spinning electrodynamic tether systems are considered one of the most promising potential applications of tethered satellite systems, which require little fuel and avoid the equilibrium limit of conventional vertical electrodynamic tether systems. Therefore, spinning electrodynamic tether systems have good prospects in debris removal, orbit reboost, payload transportation, and so on. It has been found that, if not carefully controlled, tethers easily become slack or sagging during the transition process from equilibrium state into spin. In this regard, this paper mainly focuses on the transition process of spinning electrodynamic tether systems from the initial equilibrium state into the final spinning state with expected angular velocities. Conditions of dynamic equilibrium position and minimum current for acceleration (critical current) are firstly derived in this paper, which provide references for future space tether experiments. The motion of spinning electrodynamic tether systems is described by the Lagrangian model, which takes orbital motion into consideration. By considering power limits of electrodynamic tether systems, this paper proposes two open-loop control methods for the safe transition into spinning state as nominal control algorithms with different mission objectives. The first method (direct transition) provides a near time-minimum solution, and the second method (swinging transition) provides a minimum current-energy solution for acceleration into spin. Considering perturbations in space, including inhomogeneous distribution of magnetic induction, varying mass distribution of the system and so on, an adaptive sliding mode controller is proposed to regulate the system and to track nominal trajectories of acceleration. The effectiveness of the proposed control methods is validated by numerical results.

This paper studies the deployment control of the electrodynamic tether system by means of tether tension and electric current regulation. Design of the control strategy has been implemented based on the simplified dumbbell model. In order to improve the robustness of the control system to the possible external disturbances, an adaptive sliding mode control is proposed to deploy the tether to the local vertical with the consideration of input limitations, which are introduced by a pair of saturation functions to ensure that the tether tension is always non-negative and the current is within limits. In addition, the proposed adaptive law is intended to estimate the mass parameter of the model, which is with uncertainty caused by the difficulty in accurately determining the masses of the end-bodies. The stability characteristic of the system under the proposed hybrid controller is studied based on the Lyapunov theory. Numerical case studies in the different orbital inclinations are conducted to illustrate the effectiveness of the proposed control strategy. Moreover, the performance of the controller is presented in the presence of the initial perturbations, the external disturbances and the uncertainty of mass parameter of the system.

This paper investigates spacecraft orbital boost maneuver by a fully insulated electrodynamic tether using a coupled multiphysics dynamic model that considers electric-magnetic-dynamic energy coupling. The current-potential characteristics of the electrodynamic tether is described by a simplified analytical model, where revised Parker-Murphy model is employed to evaluate electric current generation at plasma contactors together with libration dynamics of tether. The 13th-order Earth's magnetic field model is used to account influence of inhomogeneity of Earth's magnetic field on orbital parameters of electrodynamic tether system, especially in inclined orbits. Parametric analysis has been conducted for orbital boost maneuver from 400 km to 1,200 km at four different orbital inclinations. The results show that (i) the orbit of electrodynamic tether system will change from circular to elliptical orbits, especially in inclined orbits, (ii) the voltage of onboard power supply at the anode affect electric current generation, (iii) the coupled multiphysics model is necessary to characterize the interaction of electrodynamic tether with the surrounding space environment and the controllability of electric current in electrodynamic tether, (iv) the motion of orbital boost maneuver by electrodynamic tether is stable, which is different from the deorbit by electrodynamic tether.

It is well known that the libration motion of the electrodynamic tether is unstable. Libration control of the electrodynamic tether system using the Lorentz force produced by current in the tether is studied. In the libration dynamic equation, the Lorentz force is virtually divided into two parts. One part takes the role of keeping the essential function of the electrodynamic tether system. The other part takes the role of controlling the libration motion. The control scheme of the Lorentz force is based on the extended time-delay autosynchronization method. In this scheme, the control current which is the control parameter is determined by real-time states of libration and angular velocities for delay time of previous periods. The validity of the control scheme proposed in this study is confirmed by numerical simulations in the case of circular orbit, elliptical orbit, orbit in the equatorial plane and noises existing in measuring the states of libration. The results show that the perturbed libration motions converge to the periodic solution with time growing in examples of circular orbit, elliptical orbit, and noises existing. The perturbed libration motion converges to an approximate periodic orbit in the example of equatorial orbit. The control current is in a reasonable range during the process of control, and converges to zero in the end. The curve of the current in the tether is continuous and smooth which can be easily achieved in an actual system.

The libration dynamics of the electrodynamic tether system is studied for 13° International Geomagnetic Reference Field. Using the International Geomagnetic Reference Field including up to 13 t h order and 13° terms to describe the geomagnetic field, the attitude dynamic equations of the system in the elliptical orbits are built. The generalized forces produced by this magnetic model are derived. The generalized forces related to the in-plane and out-of-plane angles are sum of generalized forces for nontilted dipole model and generalized forces for higher order geomagnetic model terms. In the analysis of the libration dynamic characteristics, the generalized forces for higher order geomagnetic model terms are regarded as perturbations to the dynamic equations for the nontilted dipole model. The simulation results show that differences of components of these two geomagnetic model and differences of generalized forces related to them are all small. Failure time of the libration motion is defined to measure the influence of the perturbation to the system. Examples for different electrodynamic parameters and orbital parameters are simulated. The results show that the perturbations have obvious effects on the attitude dynamics. The influences of perturbations caused by higher order terms of 13° International Geomagnetic Reference Field for different parameters are all obtained.

This paper investigates the applicability of five active methods for removal of large low Earth orbit debris. The removal methods, namely net, laser, electrodynamic tether, ion beam shepherd, and r ...

This paper studies the control of libration stability of an insulated electrodynamic tether system in orbital boost. The electrodynamic tether is assumed rigid and the tethered spacecraft are modeled as lumped masses, while a complete electric circuit model is established by the Parker-Murphy model and Kirchhoff's first law. The electric current regulation is realized by adjusting the resistance of a resistor in the circuit. The impact of periodic variations of electrodynamic force on the libration of electrodynamic tether is minimized by a simple fuzzy-based continuous electric current control. The proposed controller maintains the merits of continuous electric current control and simplicity of on-off current control. The effectiveness of the fuzzy-based controller has been demonstrated by analyzing the libration dynamics of electrodynamic tether with continuous and discrete on-off current profiles in orbital boost. Numerical results show that the proposed approach is much effective than the on-off current control in stabilizing both in-plane and out-of-plane libration of the electrodynamic tether system subjected to periodic and non-periodic perturbations.

A comprehensive tour of the complex outer planet systems is a central goal in space science. However, orbiting multiple moons of the same planet would be extremely prohibitive using traditional propulsion and power technologies. In this paper, a new mission concept, named Magnetour, is presented to facilitate the exploration of outer planet systems and address both power and propulsion challenges. This approach would enable a single spacecraft to orbit and travel between multiple moons of an outer planet, without significant propellant or onboard power source. To achieve this free-lunch ‘Grand Tour’, Magnetour exploits the unexplored combination of magnetic and multi-body gravitational fields of planetary systems, with a unique focus on using a bare electrodynamic tether for power and propulsion. Preliminary results indicate that the Magnetour concept is sound and is potentially highly promising at Jupiter.

This paper developed a new multiphysics finite element method for the elastodynamic analysis of space debris deorbit by a bare flexible electrodynamic tether. Orbital motion limited theory and dynamics of flexible electrodynamic tethers are discretized by the finite element method, where the motional electric field is variant along the tether and coupled with tether deflection and motion. Accordingly, the electrical current and potential bias profiles of tether are solved together with the tether dynamics by the nodal position finite element method. The newly proposed multiphysics finite element method is applied to analyze the deorbit dynamics of space debris by electrodynamic tethers with a two-stage energy control strategy to ensure an efficient and stable deorbit process. Numerical simulations are conducted to study the coupled effect between the motional electric field and the tether dynamics. The results reveal that the coupling effect has a significant influence on the tether stability and the deorbit performance. It cannot be ignored when the libration and deflection of the tether are significant.

This paper developed a high-fidelity dynamic model of flexible space tether systems by considering the elastic, thermal, and electrical coupling effects on the dynamics and stability of bare electrodynamic tether systems. A simple and effective control strategy based on the libration energy of the flexible tether is derived and applied in the deorbit process. Numerical results show that the newly proposed energy control strategy is effective in controlling the libration and transverse elastic motion of the electrodynamic tether. In addition, the effect of the mass of sub-satellite on the system dynamics and stability is investigated. It is found that the libration and transverse motion of the electrodynamic tether can be stabilized by the proposed control strategy even without the sub-satellite when the electrodynamic force is dominant. This new finding shows the satellite deorbit with the electrodynamic tethers is advantageous in term of mass saving.

This paper focuses on the feasibility of constellation maintenance for large-scale satellite constellations (i.e., mega-constellations) by the electrodynamic tethered multi-agent system with consideration of electric-magnetic-dynamic energy coupling in the orbital maneuver process. The electrodynamic tethered multi-agent system consists of electrodynamic tethers and agents (satellites, robots, or spacecrafts). The nonlinear dynamic characteristics of the electrodynamic tethered multi-agent system are investigated here, to analyze the performance of orbital maneuver. The electrodynamic tether is divided into two parts, such that, the fully insulated tether and the fully bare tether, which used to boost and deboost the electrodynamic tethered multi-agent system respectively. In the orbital boost phase, the real-time electric current generated from the segment of the fully insulated tether. Similarly, in the orbital deboost phase, the electric current generated from the fully bare tether segment. The numerical results show that the system could accelerate or decelerate the motion of the electrodynamic tethered multi-agent system by regulating the magnitude and polarity of electric current carrying tether to generate a Lorentz force, leading to orbital boost or deboost. The electrodynamic tether propulsion is useful for the constellation maintenance, such that, the Starlink, LeoSat and Project Kuiper satellite constellations.

The amount of space debris will increase in the future because of collisions with other debris, even if no future space launches are carried out. Therefore, debris removal is essential for sustainable space development. The electrodynamic tether system is a promising method to remove debris because it does not require any propellants; however, it can be severed by a collision with a small piece of debris. As a tape tether is wider and thinner than a conventional solid cylindrical tether, it may overcome this drawback. The tape tether, however, can be twisted and transformed to a non-ideal shape. This paper considers a scenario in which the tape tether is twisted to create a loop shape. Every impact generates a debris cloud, which may subsequently impact another part of the tether that happens to be located in the direction the debris cloud is emitted to. This could cause a cascade effect where a single impact results in multiple collisions that lead to significant damage of the tether. Through hypervelocity impact experiments, collisions on the loop part are evaluated to gain new insights into the loop phenomenon of the tape tether system. Based on the experimental results, the rate of severance of the tape tether was compared with that of the conventional solid cylindrical tether. When only a small part of the tape tether is in loops, the rate of severance of the tape tethers is much lower than that of the conventional solid cylindrical tethers. Hence, tape tethers have a considerable advantage over conventional tethers when the looped range is narrow.

It is well known that libration motion of electrodynamic tethers operating in inclined orbits is affected by dynamic instability due to the electromagnetic interaction between the tether and the geomagnetic field. We study the application of two feedback control methods in order to stabilize the periodic attitude motions of electrodynamic tethers in elliptic inclined orbits. Both control schemes are based on the time-delayed autosynchronization of the system. Numerical simulations of the controlled libration motion show that both control techniques are able to transform the uncontrolled unstable periodic motions into asymptotically stable ones. Such stabilized periodic attitude motions can be taken as starting points for the operation of the tether. The control domains of both methods have been computed for different values of the system parameters, as functions of the two control parameters shared by both control schemes. The relative effectiveness of the two techniques in the stabilization of the periodic attitude motion has also been studied.

The amount of space debris is ever increasing, and pollution of the space environment has become a serious problem that can no longer be ignored. Consequently, the active removal of large space debris from crowded economically useful orbits should begin as soon as possible. The Japan Aerospace Exploration Agency has been investigating an active debris removal system that employs highly efficient electrodynamic tether (EDT) technology for orbital transfer. This study investigates the tether deployment from a spool-type reel using thrusters by means of numerical simulations of an EDT system. The thrusters are used in order to ensure the deployment of a tether with the length of several kilometers. In the simulations using a multiple mass tether model, the key parameters are estimated from various on-ground experiments. By means of the numerical simulations, the dynamics of tether deployment is studied and requirements of thruster needed for the deployment, such as the thrust forces and the periods of thruster activation, are clarified.

The chaotic librational motion of a tether system in elliptic orbit has been studied and controlled by delayed feedback control up to date. Contrary to the past literatures that addressed the periodic stability of tether systems, this paper studies the local stability of in-plane/out-of-plane motion of electrodynamic tether (EDT) systems in elliptic orbits with respect to the true anomaly. From the viewpoint of the local stability of tether systems, in this paper, provided that the electric current is used only for controlling the librational motion, a new simple delayed feedback control, named “switching delayed-feedback control (SDFC)” is proposed, which combines a delayed-feedback control (DFC) and variable structure system (VSS) concept. The effectiveness of the proposed method to stabilize the librational in-plane/out-of-plane periodic motion of an electrodynamic tether system in an inclined elliptic orbit with high eccentricity was demonstrated, and compared with that of the traditional delayed-feedback controllers by numerical simulations.

This paper analyzes the stability of in-plane/out-of-plane motion of electrodynamic tether (EDT) systems in elliptic and inclined orbits. The chaotic librational motion of an EDT system in inclined elliptic orbits has been studied extensively and it is believed that delayed feedback control cannot stabilize the librational motion of an EDT in an inclined and elliptic orbit with high eccentricity. Contrary to previously published results, the analytical results in this paper reveal that delayed feedback control can stabilize the in-plane/out-of-plane coupling librational motion of EDT systems in inclined elliptic orbits with high eccentricity to a periodic motion because the in-plane and out-of-plane coupling motions do not have the odd number condition if the electric current can flow on tether in both direction, and appropriate control gains are chosen. The validity of the stability analyses in this paper is confirmed through numerical simulations in which a delayed feedback control with appropriate control gains is applied to the in-plane/out-of-plane coupling periodic motion of an EDT system in inclined elliptic orbit with high eccentricity.

By using electrodynamic drag to greatly increase the orbital decay rate, an electrodynamic space tether can remove spent or dysfunctional spacecraft from low Earth orbit rapidly and safely. Moreover, the low mass requirements of such tether devices make them highly advantageous compared to conventional rocket-based de-orbit systems. However, a tether system is much more vulnerable to space debris impacts than a typical spacecraft and its design must prove to be safe to a certain confidence level before being adopted for potential applications. To assess the space debris related concerns, a new task (Action Item 19.1) on the “Potential Benefits and Risks of Using Electrodynamic Tethers for End-of-life De-orbit of LEO Spacecraft” was defined by the Inter-Agency Space Debris Coordination Committee (IADC), in March 2001. Two tests were proposed to compute the fatal impact rate of meteoroids and orbital debris on space tethers in circular orbits, at different altitudes and inclinations, as a function of the tether diameter, and to assess the survival probability of an electrodynamic tether system during typical deorbiting missions. IADC members of three agencies, the Italian Space Agency (ASI), the Japan Aerospace Exploration Agency (JAXA) and the US National Aeronautics and Space Administration (NASA), participated in the study and different computational approaches were specifically developed in the framework of this IADC task. This paper summarizes the content of the IADC AI 19.1 Final Report. In particular, it introduces the potential benefits and risks of using tethers in space, it describes the assumptions made in the study plan, it compares and discusses the results obtained by ASI, JAXA and NASA for the two tests proposed. Some general conclusions and recommendations are eventually highlighted as a result of a massive and intensive study. DE-ORBITING SPACECRAFT WITH ELECTRODYNAMIC TETHERS Over nine thousand satellites and other trackable objects are currently in orbit around the Earth, along with many smaller particles. As the low Earth orbit (LEO) is not a limitless resource, some sort of debris mitigation measures are needed to solve the problem of unusable satellites and spent upper stages. Despite a small number of full-scale experiments made so far using space tethers, the possibility of de-orbiting spacecraft by means of electrodynamic tethers has been on the drawing board of theorists for almost a decade. Various conducting tether configurations have been studied and their deorbiting performances have been extensively assessed by several authors. The electrodynamic drag concept is based on the exploitation of the Lorentz force due to the interaction between the electric current flowing in a conductive tether and the geomagnetic field. The decelerating Lorentz force F r (electrodynamic drag) depends in a complex way on the design parameters of the system, the orbit and the characteristics of the local ionosphere

An overview of a sounding rocket, S-520-25th, project on space tether technology experiment is presented. The project is prepared by an international research group consisting of Japanese, European, American, and Australian researchers. The sounding rocket will be assembled by the ISAS/JAXA and will be launched in the summer of 2009. The sounding rocket mission includes two engineering experiments and two scientific experiments. These experiments consist of the deployment of bare electrodynamic tape tether in space, a quick ignition test of hollow cathode system in space, the demonstration of bare electrodynamic tether system in space, and the test of the OML (orbital-motion-limit) current collection theory.

56th International Astronautical Congress(October 17th-21th, 2005), Fukuoka, Japan, 資料番号: ARDS05259000, レポート番号: IAC-05-C4.4.07

56th International Astronautical Congress(October 17th-21th, 2005), Fukuoka, Japan, 資料番号: ARDS05294000, レポート番号: IAC-05-C1.2.07

This study presents the analytical results of the dynamic behavior of an electrodynamic tether applied to a deorbit system in elliptical orbits. Parametric analyses of tether dynamics with variations in the ballast mass, the mass of the mother ship, and the tether length were performed in order to determine the range of eccentricity of the elliptical orbit in which the deorbit mission is possible. In addition, a new method for stabilizing the tether dynamics by regulating the tether current was proposed in order to expand the limit of eccentricity of the elliptical orbit in which the deorbit mission can be performed. The duration of the deorbit mission and the possibility of the tether being cut by debris during the mission were also analyzed in order to demonstrate the deorbit performance of the system.

As a countermeasure for suppressing space debris growth (P. Eighler, A. Bade, Chain Reaction of Debris Generation by Collisions in Space—A Final Threat to Spaceflight? in: 40th Congress of the International Astronautical Federation, IAA-89-628, October 1989), the National Aerospace Laboratory of Japan is investigating a satellite capture, repair and removal system for non-cooperative satellites, part of which involves assessing the viability of electrodynamic tether (EDT) technology as an orbital transfer system. In this paper, some results concerning the time required to remove existing satellites, the behavior of flexible tethers during the debris separation phase, and orbital transfer strategies of EDT systems during space debris removal operations are described. From numerical simulations, it is found that EDT systems can transfer satellites from LEO to orbits with a short lifetime within a realistic timeframe. It is also found that the stability of EDT systems is compromised when debris separation occurs both while a tether current is running and when the ratio of the end mass to that of the service satellite is high. To ensure stability, the end mass should be selected from the target debris group with due regard for the maximum possible mass that can be maneuvered safely. Moreover, it is also found that orbital elements (a, e, i) can be changed independently with an adequate current control strategy.

In the future, a plasma contactor using a hollow cathode will be applied to charging mitigation of large structures and electron emitter/collector of electrodynamic tether systems in space. In this study, ground-based experiments on the plasma contactor were carried out to examine basic operational characteristics. In electron emission, the same operational performance as that only with a simple assembly of the hollow cathode was obtained in the simulating low-earth-orbit (LEO) plasma. In electron collection, the excellent effect of collection current enhanced in the simulating LEO plasma existed. Furthermore, the mechanism of electron collection by the plasma contactor was examined from plasma parameter distributions near the plasma contactor. As a result, an intensive interaction between the contactor plasma and the simulating LEO plasma was observed.

Bare-tether systems are one of the greatest-efficiency electrodynamic tethered systems. The system with an uninsulated portion of the metallic tether itself to collect electrons from the space plasma is operated as a thruster or a power generator on a satellite. Ground experiments were carried out to understand the phenomenon of electron collection by a bare tether in space. Metallic tether samples were exposed to a simulating low-Earth-Orbit plasma flow as varying tether sample diameter and length, and plasma velocity. A magnetic field was also applied. The normalized collection current increased with plasma velocity. The existence of magnetic field increased the normalized collection current because of the edge effect of a tether sample or the three-dimensional effect. A high collection current above the orbital-motion-limited current could be achieved with a magnetic field. The current characteristics of a bare tether strongly depended on the plasma velocity and surrounding magnetic field.

Hypergeometric series solutions are presented for the electrodynamic response of bare-wire tether systems, and are used to generate closed-form expressions for the sensitivity coefficients in the generator configuration. For the thruster configuration, the governing equations are in the form of three simultaneous differential equations, which do not allow closed-form solutions for the sensitivity coefficients to be generated, and therefore numerical solutions for the sensitivity coefficients are presented.

The bi-national, US-Italian, Tethered Satellite System (TSS) program was designed to provide a unique opportunity to explore certain space plasma- electrodynamic processes and the orbital mechanics of a gravity-gradient stabilized system of two satellites linked by a long conducting tether. The second flight, TSS-LR, was launched February 22, 1996 on STS-75 and satellite deployment began at MET 3/00:27. A unique data set was obtained over the next five hours, as the tether was deployed to a length of 19695 meters, which has allowed significant science to be accomplished. This presentation will focus on electrodynamic processes generated by the tether--in particular, the collection of electrical current from the ionospheric plasma. Of particular significance is an apparent transition of the physics of current collection when the potential of the collecting body becomes greater than the ram energy of the ionospheric atomic oxygen ions. Previous theoretical models of current collection were electrostatic--assuming that the orbital motion of the system, which is highly subsonic with respect to electron thermal motion, was un- important. This may still be acceptable for the case of relatively slow-moving sounding rockets. However, the TSS-LR results show that motion relative to the plasma must be accounted for in orbiting systems.

It is proposed that magnetobraking may be used to dissipate hyperbolic excess velocity from a spacecraft returning from Mars to Earth orbit. In magnetobraking, an electrodynamic tether is deployed from the spacecraft. The Earth's magnetic field produces a force on electrical current in the tether, which can be used to either brake or accelerate the spacecraft without expenditure of reaction mass. The peak acceleration on the Mars return is 0.007 m/s 2 , and the amount of braking possible is dependent on the density and current-carrying capacity of the tether, but is independent of length. Since energy is produced as the spacecraft velocity decreases, no on-board power source is required. As the spacecraft approaches the Earth, the magnetic field increases and the power produced by the tether increases, reaching a maximum of about 800 W per kg of spacecraft mass at closest approach.

The tether systems are very attractive for space activity. They may be used for maintaining the large fixed distances between basic satellite and subsatellites with a variety of scientific instruments. The problem of such systems application to lower ionospheric layers measurements and electrodynamic objectives is discussed. It is shown that the application efficiency of long or extra-long tethered systems for space experiments both for instrumentation delivery to the lower ionosphere and for different electrodynamic purposes is very low. Moreover in spite of small radius the great length of the tethering rope has big interacting surface with tiny particles floating in the space. So the collision probability of the tether with such particles is rather high that can cause soon the rope break.

With the development of a manned space station permanently circling the earth, an efficient system is needed to provide both electrical power and propulsion. The electrodynamic tether system being developed by NASA will do this by using a long (e.g. 10 km) conducting tether to interact with the earth's magnetic field. In a low earth orbit, the induced voltage would be on the order of a few tenths of a kilovolt per kilometer of tether. Assuming sufficient electrical contact can be maintained with the ionosphere, a 10 km tether would supply several kilowatts of electrical power. The extraction of power via an associated current results in an I × B force that reduces the orbital speed of the spacecraft. Conversely, if a sufficiently large reverse voltage is applied to reverse the current, then the I × B force will increase the orbital speed and electrical energy will be converted into energy. Consequently, the electrodynamic tether system is expected to perform as a motor/generator system providing a simple, reliable and economical means of efficiently converting orbital energy into electrical energy in a fully reversible process. In addition to a general discussion of the electrodynamic tether system, this article will concentrate on the dynamics of the tether, the proposed electrical contacts or current brushes and the Alfven waves that will carry the charge through space.

Many novel electrodynamic and space plasma physics experiments can be done from the space shuttle using a gravity gradient stabilized subsatellite tethered 10-30 km above the Orbiter via a long, conducting wire which is insulated from the ionospheric plasma. This system, called the Shuttle Electrodynamic Tether System (SETS), is described in the present paper with emphasis upon the various electrical processes which determine its coupling to the ambient plasma. The three most important physical effects include sheath formation and electron collection by the subsatellite, the generation of a large emf through the orbital motion of the tether across geomagnetic field lines, and the active ejection of electrons from the Orbiter into the surrounding ionosphere. An electrical circuit analogy is presented for SETS together with a brief outline of possible areas related to the artificial generation of magnetohydrodynamic waves.

The plasma contacting process is described and experiments are discussed that suggest the key role that cold ions play in establishing a low impedance plasma bridge that can conduct current in either direction between a contactor electrode and a dilute plasma. A ring cusp contactor is shown to provide from 1000-mA of electron emission to 500-mA of electron collection as its bias relative to a simulated space plasma is varied through an 80-v range.

The solar terrestrial observatory (STO) concept is discussed as well as Space Station accommodations for STO. Instruments for the initial STO which can be used to excite and detect waves include the electrodynamic tether, waves in space plasmas, space experiments with particle accelerators, vehicle charge and potential experiments, recoverable plasma diagnostics package, magnetospheric multiprobes, and theoretical and experimental beam plasma physics. An example is given of a typical campaign mode of operation which is appropriate for active experiments.

Plasma contactors have been proposed as a means of making good electrical contact between biased surfaces, such as found at the ends of an electrodynamic tether, and the space environment. A plasma contactor is a plasma source which emits a plasma cloud which facilitates the electrical connection. The physics of this plasma cloud is investigated for contactors used as electron collectors. The central question that is addressed is whether the electrons that are collected by a plasma contactor come from the far field or by ionization of local neutral gas. This question is important to answer because the system implications are quite different for the two mechanisms. It is shown that contactor clouds in space will consist of a spherical core possibly containing a shock wave. Outside of the core the cloud will expand anisotropically across the magnetic field leading to a turbulent cigar-shape structure along the field. This outer region is itself divided into two regions by the ion response to the electric field. A two-dimensional theory for the outer regions of the cloud is developed. The current voltage characteristic of an argon plasma contactor cloud is estimated for several ion currents in the range of 1–100 A. It is suggested that the major source of collected electrons comes by ionization of neutral gas while collection of electrons from the far field is relatively small.

The motion of the long conducting wire of a tethered satellite across the geomagnetic field creates an emf of about 0.1–0.2 V/m along the length of the tether. This emf can be utilized to excite wave modes in the ionosphere ranging in frequency from the magnetohydrodynamic regime to the upper hybrid frequency. The emitted spectrum depends principally upon the size (in the direction perpendicular to the magnetic field) of the moving source. The amplitude of radiated waves having a frequency less than the ion cyclotron frequency has been calculated. The shear Alfven wave is always excited and forms an “Alfven-wing” structure whose amplitude diminishes as the length of the tether increases. In the near-field zone, the emitted waves propagate along magnetic field lines as packets but disperse into individual Fourier components as the far field is reached. However, as the distance to the far-field zone is at least 106 kilometers for typical tether configurations, the shear Alfven wave will be reflected by the earth's ionosphere with accompanying coupling to the compressional Alfven mode before the far field is reached.