Quality factor of tidal energy dissipation within Uranus in order for a satellite to migrate from a = 0.8 r_M to a = r_M. The ratio k₂/Q is computed from classical formulas and plotted here assuming a Love number k₂ = 0.1 for Uranus (Gavrilov & Zharkov 1977). The mass of the satellite is labelled above the curves. For comparison, the dotted blue line shows the case of a satellite migration from a = 0.96 r_M to a = r_M (i.e. a migration range reduced to Δa = 2 R_eq), as obtained in the numerical experiments of Sect. 3.3.