Tidal evolution of the Pluto–Charon system. Initially prograde Pluto and Charon with various initial ratios of rotation rates Ω_m0/Ω₀ and the 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 e; 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}}}﹩.