Tidal evolution of the Pluto–Charon system. An initially prograde Pluto and initially retrograde Charon are considered, with various initial rotation rates and a fixed initial eccentricity e₀ = 0.4. Panel (a) semimajor axis ratio a/a_p (where a_p is the present semimajor axis value); panel (b): eccentricity; panel (c): spin rates Ω/n (shades of blue) and Ω_m/n (shades of red); panel (d): tidally dissipated power, ﹩\dot{E}﹩ and ﹩{\dot{E}}_{{\rm{m}}}﹩. In accordance with the adopted sign convention, panel (c) shows the spin rate Ω of an actually prograde Pluto as negative, while the depicted spin Ω_m of a retrograde Charon is positive.