Tiscareno, M. (2012). "Propeller Peregrinations: Ongoing Observations of Disk-Embedded Migration In Saturn's Rings" American Astronomical Society, DPS meeting #44 #513.05.
The "propeller" moons within Saturn's rings are the first objects ever to have their orbits tracked while embedded in a disk, rather than moving through empty space (Tiscareno etal. 2010, ApJL; hereafter T10). The ~km-sized "giant propellers" whose orbits have been tracked in the outer-A ring, as well as their smaller ~0.1-km-sized brethren swarming in the mid-A ring, are not seen directly; rather, their locations are inferred by means of the propeller-shaped disturbances they create in the surrounding ring material (Tiscareno etal. 2006, Nature; Sremcevic etal. 2007, Nature; Tiscareno etal. 2008, AJ). The orbits of giant propellers are primarily Keplerian, but with clear excursions of the semimajor axis on the order of +-0.15 degrees longitude for the largest and best-studied, and +-several degrees longitude for others (T10).|
Most theories that have been proposed to explain the non-Keplerian motion of propeller moons rely on gravitational and/or collisional interactions between the moon and the surrounding disk, and thus hold out the prospect for directly observing processes that are important in protoplanetary scenarios and other disk systems. One model suggests that the moons migrate due to azimuthal variations in the disk that they themselves create (Pan and Chiang 2010, ApJL; 2012, AJ). Although the classical form of "Type I migration," relying on the asymmetries in the dynamics themselves, is not powerful enough to explain the observed motions (Crida etal. 2010, AJ), modifications of that model relying on temporal (Rein and Papaloizou 2010, A&A, Pan etal. 2012, MNRAS) or radial (Tiscareno 2012, P&SS) variations in the disk have been suggested. The different models make different predictions, and future data will likely distinguish among them.
In 2012, after a nearly two-year hiatus, Cassini left Saturn's ringplane and resumed tracking the propellers. We will report early results of the new observing campaign.