Tidal evolution of the Pluto–Charon system. An initially prograde Pluto and initially retrograde Charon are considered, with various initial eccentricities and fixed initial spin-orbit ratios of Ω₀/n₀ = −50 and Ω_m0/n₀ = 50. 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}}}﹩.