Tethers for Deorbit of Objects in High Eccentricity Orbits at the End-of-Life

The present PhD research investigates the removal from orbit (i.e. deorbit) of inactive objects in high eccentricity orbits, considering as specific target Sylda, a large dual-payload adapter used onboard Ariane 5 heavy launches to Geostationary Earth Orbit (GEO), and released in Geostationary Transfer Orbit (GTO). The population of space debris always increased since the start of space activities, due to the fact that deorbit procedures were never implemented so far. The adoption of a deorbit system, onboard payloads or other objects delivered into space in the future, is an effective solution to significantly attenuate the otherwise exponentially increasing debris density and consequent collision risk in space. The deorbit is performed with a dedicated system that dissipates orbital energy, causing a gradual decrease of orbital altitude until the object degrades while reentering the Earth’s atmosphere. Deorbit from Low Earth Orbits (LEO) has been widely studied in literature, whereas very little is present regarding deorbit from GTO or other high eccentricity orbits, even if such objects are very important targets for removal since the cross continuously both highly valuable zones of LEO and GEO. International guidelines for space debris attenuation set 25 years as maximum time for removal from orbit. The present research employs a Bare Electrodynamic Tether (BET) as deorbit device: such tape tether creates a Lorentz drag from the interaction between the current circulating along the tether, from the collection of electrons from space plasma, and the geomagnetic field. Sylda is a major threat from the collision risk standpoint, given its very large exposed area and high mass, and therefore it is an optimal candidate for deorbit implementation, also due to the high rate of dual-payload launches to GEO. The research covers both the numerical simulation and analysis of deorbit and of pre-deorbit procedures. Regarding deorbit, two main simulation campaigns are performed: the first simulates the deorbit on the orbital plane, neglecting the out-of-plane oscillations of the system. One of the main outcomes from this first batch of analyses is that the optimal configuration for the tethered system is having one electrodynamic tether subdivided in two segments deployed along opposite directions from Sylda: this is called butterfly configuration. A second phase of deorbit analysis follows, with a different computational model, adopting the butterfly configuration, and including also the out-of-plane dynamics of the system. Before starting the deorbit from GTO, two procedures must be performed: at first the detumbling of Sylda, i.e. the minimization of the residual angular velocity of rotation of Sylda around its center of mass. This procedure is analyzed proposing a set of magnetic torquers to accomplish this task. After detumbling is complete, the deployment of the tethers can start. The deployment is then simulated, accounting for three different options: deployment at apogee, at perigee, or at mid-way along the initial orbit, the last resulting as the most favorable location. Another part of research is dedicated to the study of the attitude detection of the two tether segments of the butterfly configuration, during deployment, using a vision system that tracks the tipmass at the end of each tether. An analysis is performed to estimate the uncertainty affecting the measure, considering two main configurations for the vision system, i.e. a monocular system and a stereo system.