On the predictability and robustness of Galileo disposal orbits

The end-of-life disposal of Galileo satellites is needed to avoid collisions with operational spacecraft and to prevent the generation of space debris. Either disposal into stable graveyard orbits or disposal into the atmosphere exploiting eccentricity growth caused by lunisolar resonances is possible. However, there is a concern about the predictability of medium Earth orbits because of possible chaotic behaviour caused by the overlap of resonances. In this work, we investigate if Galileo disposal orbits are predictable and robust, that is, if safe disposal is possible under uncertainties. For this, we employ finite-time Lyapunov exponents (FTLEs) to study the chaoticity of orbits. In addition, sensitivity analysis is used to quantify the effect of uncertainties on the orbital evolution and to determine if safe disposal is possible. Whether the disposal orbits are chaotic or not could not be concluded from the FTLE analysis, because the observed divergence between neighbouring orbits can also be caused by hyperbolicity of the dynamics. Nevertheless, because the resonance dynamics are perturbed and resonances may overlap, all disposal orbits are expected to be chaotic. Regarding robustness, we found that the majority of the investigated re-entry disposal trajectories (including low $\Delta V$ solutions) is robust. On the other hand, we find that the investigated graveyard orbits and a small portion of the assessed re-entry orbits are not robust under uncertainties in the disposal manoeuvre and in the dynamical model. Therefore, it is mandatory to assess the sensitivity of a disposal orbit to uncertainties to ensure safe disposal.