  A giant collision is believed to be at the origin of the Pluto-Charon system.
As a result, the initial orbit and spins after impact may have substantially
differed from those observed today. More precisely, the distance at periapse
may have been shorter, subsequently expanding to its current separation by
tides raised simultaneously on the two bodies. Here we provide a general 3D
model to study the tidal evolution of a binary composed of two triaxial bodies
orbiting a central star. We apply this model to the Pluto-Charon binary, and
notice some interesting constraints on the initial system. We observe that when
the eccentricity evolves to high values, the presence of the Sun prevents
Charon from escaping because of Lidov-Kozai cycles. However, for a high initial
obliquity for Pluto or a spin-orbit capture of Charon's rotation, the binary
eccentricity is damped very efficiently. As a result, the system can maintain a
moderate eccentricity throughout its evolution, even for strong tidal
dissipation on Pluto.
