The radial motions of small moons in Saturn’s rings can provide excellent
tests of satellite  migration models.  In theory, radial drifts arise from
a torque exchange between these moons and ring particles.  We predict that
moons of radius 2–20 km in the A ring are massive enough to clear a gap in
the ring, yet are light enough to migrate through the ring in ~1000 yr.
Smaller moons, such as the propellers, are trapped by the inertia of ring
particles diffusing onto corotating orbits.  Larger moons, such as Pan and
Atlas, have too much of their own inertia for torque-induced migration to
operate efficiently.  The fast migration of the 2-20 km moons may explain
the observed large-radius cutoff around 1-2 km in the size distribution of
A ring moonlets.  However, Daphnis, a ~4 km moon in the A ring, is not
evidently experiencing radial drift, thus challenging this picture.
Possibly, Daphnis may not migrate because it is being stirred by distant,
massive moons (e.g., Mimas).  Or it may be a recent addition to the ring
that is orbitally settling into the ring plane prior to the onset of fast
migration.  We use numerical simulations to make measurable predictions for
Daphnis to distinguish between these scenarios.  Our simulations can be
applied to satellite migration in general, so we conclude by extending the
lessons from the A ring to the growth of planets in the early Solar System
and in protoplanetary disks around other stars.