  The motions of small moons through Saturn's rings provide excellent tests of
radial migration models. In theory, torque exchange between these moons and
ring particles leads to radial drift. We predict that moons with Hill radii r_H
~ 2-24 km should migrate through the A ring in 1000 yr. In this size range,
moons orbiting in an empty gap or in a full ring eventually migrate at the same
rate. Smaller moons or moonlets -- such as the propellers (e.g., Tiscareno et
al. 2006) -- are trapped by diffusion of disk material into corotating orbits,
creating inertial drag. Larger moons -- such as Pan or Atlas -- do not migrate
because of their own inertia. Fast migration of 2-24 km moons should eliminate
intermediate-size bodies from the A ring and may be responsible for the
observed large-radius cutoff of r_H ~ 1-2 km in the size distribution of the A
ring's propeller moonlets. Although the presence of Daphnis (r_H ~ 5 km) inside
the Keeler gap challenges this scenario, numerical simulations demonstrate that
orbital resonances and stirring by distant, larger moons (e.g., Mimas) may be
important factors. For Daphnis, stirring by distant moons seems the most
promising mechanism to halt fast migration. Alternatively, Daphnis may be a
recent addition to the ring that is settling into a low inclination orbit in
~10^3 yr prior to a phase of rapid migration. We provide predictions of
observational constraints required to discriminate among possible scenarios for
Daphnis.
