  We consider a scenario where the small satellites of Pluto and Charon grew
within a disk of debris from an impact between Charon and a trans-Neptunian
Object (TNO). After Charon's orbital motion boosts the debris into a disk-like
structure, rapid orbital damping of meter-size or smaller objects is essential
to prevent the subsequent re-accretion or dynamical ejection by the binary.
From analytical estimates and simulations of disk evolution, we estimate an
impactor radius of 30-100 km; smaller (larger) radii apply to an oblique
(direct) impact. Although collisions between large TNOs and Charon are unlikely
today, they were relatively common within the first 0.1-1 Gyr of the solar
system. Compared to models where the small satellites agglomerate in the debris
left over by the giant impact that produced the Pluto-Charon binary planet,
satellite formation from a later impact on Charon avoids the destabilizing
resonances that sweep past the satellites during the early orbital expansion of
the binary.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics