The research activities of the Space Tether Group (STG) are focussed on tether modelling, design and analysis of new concepts and missions, dynamics, and new materials for tether applications. The members of the group have more than 20 years of experience on the subject and have participated in several National and European projects. They include the FP7/Space project BETs and the Coordination of three projects funded by the European Innovation Council (E.T.PACK, E.T.PACK-F and E.T.COMPACT) for a total of 9.5 M€. The most important result of these project is an autonomous deorbit device with a volume of 12U and a mass of 24 kg that will be on-orbit demonstrated in late 2025/early 2026 thank to a launch opportunity in the framework of the ESA/EC Flight Ticket Initiative. Members of the research team are cofounders of the UC3M spinoff PERSEI Space, which is focussed at the development and commercialization of space tether products. In 2019, STG hosted the Sixth International Conference on Tethers in Space (TiS2019).

The STG is specialized in electrodynamics tethers and the modeling of its interaction with the ambient plasma. Previous and on-going works include a variety of topics like nonlinear wave excitation by using electrodynamic tethers, modelling of current collection and emission using kinetic eulerian solvers, mitigation of dynamic instabilities, optimal mission design in deorbiting scenarios, and two disruptive subclasses of electrodynamic tether named the low work-function tether (LWT) and the Bare-Photovoltaic tether (BPT).

Research capabilities

Development of Electrodynamic Tether Systems

The STG group, through its participation and coordination of the E.T.PACK, E.T.PACK-F and E.T.COMPACT projects, acquired an unique expertise on the development of electrodynamic tether system from the point of view of the system engineering as well as for the specific elements that are involved like cathodes, deployment mechanism, and avionics. The figure below shows the Engineering Qualification Model of the deorbit device of E.T.PACK-F, which has been developed by UC3M, TU Dresden,  the University of Padova and SENER Aeroespacial

Mission Analysis

The STG has developed BETsMA, a friendly software for the preliminary design and analysis of deorbiting missions by using electrodynamic tethers. The code has two modules. The Optimization Module implements the π-algorithm and can be used to find the optimal tether length, width and thickness for a given mission (spacecraft mass and elements of the initial orbit). The Deorbiting Module computes, for a given tether geometry, the deorbiting manoeuvre and provides key quantities like the deorbit time, and the temporal evolution of the orbital elements, average and maximum electric current, tether temperature and forces, etc. BETsMA v1.0 was developed under the European Commission FP7 Space Project Propellantless deorbiting of Space Debris by bare electrodynamic tethers (BETs, 262972). Its physical models, performance, and result analysis have been upgraded in BETsMA v2.0 in the framework of the E.T.PACK project. BETsMA v2.0 can simulate three different types of electrodynamic tethers (standard bare tethers, LWTs and BPTs), with different type of cross-sections, tether attitude dynamics, and mission goals (deorbiting, reboost and station keeping). The software provide dynamic, thermal, electrical and risk analysis, among many others capabilities. BETsMA v2.0 has been cross-verified by comparing its modules with other tether codes developed by worldwide institutions.

Tether Dynamics

The STG has developed simulators of different complexity to study the dynamics of space tethers and has used them to study several tether applications like deorbiting at the end of life, drag compensation scenarios, among others. It has also expertise on modelling advanced and non-conventional tether concepts. The animation below shows the dynamics of a Low Work-Function Tether with a loop shape orbiting in equatorial orbit and the corresponding Poincare section.

Tether-plasma interaction simulators

The STG has background on key topics of plasma physics related to electrodynamic tethers like Langmuir and emissive probe theory and nonlinear wave propagation in magnetized plasmas. The correct modelling of the charge exchange with the plasma is critical for bare tethers because current and voltage profiles along the tether impact on tether performance and dynamics. The group has developed a suit of Vlasov-Poisson solvers that allow a kinetic analysis of the plasma sheath around tethers and the determination of the current collected and emitted by bare and coated tethers. The suit has currently three tools:

• Stationary Vlasov-Poisson solver for cylindrical tethers. Works are currently in progress to construct a database with characterics curves (current versus bias) for cylindrical tethers with and without electron emission to be used in both tether and plasma diagnostics applications.

• Stationary Vlasov-Poisson solver for tape-like tethers. It allows to study the current collection and emission of tethers and probes with an arbitrary cross-section.

• Non-stationary Vlasov-Poisson solver for cylindrical tethers immersed in flowing plasmas. It allows to study advanced effects like the particle trapping during transient phases. An example is given in the animation in the right hand side, which shows the evolution of the electron distribution at two points of the sheath around a positively polarized tether.