We present numerically-derived orbits and mass estimates for the inner
Saturnian satellites, Atlas, Prometheus, Pandora, Janus and Epimetheus from a
fit to 2580 new Cassini ISS astrometric observations spanning February 2004 to
August 2013. The observations are provided in a supplementary table. We
estimate GM_ Atlas=0.384+/-0.001 x 10^(-3)km^3s^(-2), a value 13% smaller than
the previously published estimate but with an order of magnitude reduction in
the uncertainty. We also find GM_ Prometheus=10.677+/-0.006x10(-3)km^3s^(-2),
GM_Pandora=9.133+/-0.009x10^(-3)km^3s^(-2),
GM_Janus=126.51+/-0.03x10^(-3)km^3s^(-2) and
GM_Epimetheus=35.110+/-0.009x10^(-3)km^3s^(-2), consistent with previously
published values, but also with significant reductions in uncertainties. We
show that Atlas is currently librating in both the 54:53
co-rotation-eccentricity resonance (CER) and the 54:53 inner Lindblad (ILR)
resonance with Prometheus, making it the latest example of a coupled CER-ILR
system, in common with the Saturnian satellites Anthe, Aegaeon and Methone, and
possibly Neptune's ring arcs. We further demonstrate that Atlas's orbit is
chaotic, with a Lyapunov time of ~10 years, and show that its chaotic behaviour
is a direct consequence of the coupled resonant interaction with Prometheus,
rather than being an indirect effect of the known chaotic interaction between
Prometheus and Pandora. We provide an updated analysis of the second-order
resonant perturbations involving Prometheus, Pandora and Epimetheus based on
the new observations, showing that these resonant arguments are librating only
when Epimetheus is the innermost of the co-orbital pair, Janus and Epimetheus.
We also find evidence that the known chaotic changes in the orbits of
Prometheus and Pandora are not confined to times of apse anti-alignement.Comment: 23 pages, 16 figures. Accepted for publication in The Astronomical
Journal 23 September 2014 (corrected Fig. 11