  Motivated by the New Horizons mission, we consider how Pluto's small
satellites -- currently Styx, Nix, Kerberos, and Hydra -- grow in debris from
the giant impact that forms the Pluto-Charon binary. After the impact, Pluto
and Charon accrete some of the debris and eject the rest from the binary orbit.
During the ejection, high velocity collisions among debris particles produce a
collisional cascade, leading to the ejection of some debris from the system and
enabling the remaining debris particles to find stable orbits around the
binary. Our numerical simulations of coagulation and migration show that
collisional evolution within a ring or a disk of debris leads to a few small
satellites orbiting Pluto-Charon. These simulations are the first to
demonstrate migration-induced mergers within a particle disk. The final
satellite masses correlate with the initial disk mass. More massive disks tend
to produce fewer satellites. For the current properties of the satellites, our
results strongly favor initial debris masses of 3-10 x 10^{19} g and current
satellite albedos of roughly 0.4-1. We also predict an ensemble of smaller
satellites with radii less than roughly 1-3 km, and very small particles, with
radii of roughly 1-100 cm and optical depth smaller than 10^-10. These objects
should have semimajor axes outside the current orbit of Hydra.
