Analysis of Propellantless Tethered System for the De-Orbiting of Satellites at End of Life

The increase of orbital debris and the consequent proliferation of smaller objects through fragmentation is driving the need for mitigation strategies that address this issue at its roots. The present guidelines for mitigation point out the need to deorbit new satellites injected into low Earth orbit (LEO) within a 25-year time. The issue is then how to deorbit the satellite with an efficient system that does not impair drastically the propellant budget of the satellite and, consequently, reduces its operating life. In this contest a passive system, which makes use of an electrodynamics tether to deorbit a satellite through Lorentz forces, has been investigated. The system collects electrons from the ionosphere at its anodic end (the conductive tether itself left bare) and emits electrons through a plasma contactor at the cathodic end. The current that circulates in the tether produces the Lorentz drag force through the interaction with the Earth’s magnetic field. Power can also be tapped from the tether for running the cathode and other ancillary on-board equipment. The deorbiting system will be carried by the satellite itself at launch and it will be deployed from the satellite at the end of its life. From that moment onward the system operates passively without requiring any intervention from the satellite itself. This thesis summarizes the results of the analysis carried out to show the deorbiting performance of the system starting from different orbital scenarios and for satellite configurations, and describing the tethered system by means of different mathematical models in order to include the lateral flexibility and increase the accuracy of the results, which can be easily scaled. Moreover high-fidelity and latest environmental routines has been used for magnetic field, ionospheric density, atmospheric density and a 4×4 gravity field model, since the environment is very important for describing appropriately each external interaction, in particular the electrodynamic one. The electric properties of the wire depends on its temperature, which is computed dynamically by a thermal model that considers all the major input fluxes and the heat emitted by the tether itself. At last the electric current along the rope is constantly evaluated during the reentry, since large variations happens passing from sunlight to shadow regions, and vice-versa. Without any control the system goes rapidly into instability, because the electrodynamic torque pumps continuously energy into the system enlarging the libration of the tether. So ad hoc strategies must be thought and included. In the past several techniques have been proposed, but with a lot of assumptions and limitations. In this work a new concept has been implemented, mounting in the satellite at the basis of the tether a damping mechanism for dissipating the energy associated with the lateral motion. At last the whole deployment of a tape tether has been analyzed. Several configurations have been studied, and the tradeoff analysis concluded that a non-motorized reeling deployer is well suited for a 1-3 cm wide tape like the tapes. Optimal reference profiles have been evaluated for two class of tether (3 and 5km), and are then used to regulate the brake mechanism mounted on the deployer itself to control the deployment. Different conditions have been analyzed to demonstrate the capabilities of the control law to provide a successful deployment in the presence of various error