Benefits and risks of using electrodynamic tethers to de-orbit spacecraft

Abstract

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: