  We consider the formation of satellites around the Pluto-Charon binary. An
early collision between the two partners likely produced the binary and a
narrow ring of debris, out of which arose the moons Styx, Nix, Kerberos and
Hydra. How the satellites emerged from the compact ring is uncertain. Here we
show that a particle ring spreads from physical collisions and collective
gravitational scattering, similar to migration. Around a binary, these
processes take place in the reference frames of "most circular" orbits, akin to
circular ones in a Keplerian potential. Ring particles damp to these orbits and
avoid destructive collisions. Damping and diffusion also help particles survive
dynamical instabilities driven by resonances with the binary. In some
situations, particles become trapped near resonances that sweep outward with
the tidal evolution of the Pluto-Charon binary. With simple models and
numerical experiments, we show how the Pluto-Charon impact ring may have
expanded into a broad disk, out of which grew the circumbinary moons. In some
scenarios, the ring can spread well beyond the orbit of Hydra, the most distant
moon, to form a handful of smaller satellites. If these small moons exist, New
Horizons will find them.
