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 } } \sim 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. , 88 ) – 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 } } \sim 1 –2 km in the size distribution of the A ring ’ s propeller moonlets .
Although the presence of Daphnis ( r _ { { } _ { H } } \approx 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 \sim 10 ^ { 3 } yr prior to a phase of rapid migration .
We provide predictions of observational constraints required to discriminate among possible scenarios for Daphnis .