Astronomy and Astrophysics – Astronomy
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dda....40.0803h&link_type=abstract
American Astronomical Society, DDA meeting #40, #8.03; Bulletin of the American Astronomical Society, Vol. 41, p.902
Astronomy and Astrophysics
Astronomy
Scientific paper
Saturn's moon, Hyperion, is possibly unique among all known natural satellites. Its significantly non-spherical shape, combined with its large orbital eccentricity ( 0.1,due to a 3:4 mean-motion resonance with Titan) subject it to strongly-varying solid-body torques from Saturn. As a result, the possibility of a stable spin state resonant with Hyperion's orbital frequency (including 3:2 and 2:1 spin-orbit resonances as well as the 1:1 "synchronous" state) is eliminated and the system is chaotic, with a Lyapunov timescale on the order of a few orbits. During its flyby of Hyperion in 1981, the Voyager 2 spacecraft observed Hyperion to be in a spin state 4.2 times faster than the synchronous frequency and with a spin axis located close to the satellite's long axis. Black et al. (1995) took observations from this single close pass and used them to model low-resolution photometric observations of Hyperion taken over an 18 day period before the pass. They showed that Hyperion was in a quasi-periodic state at the time of the flyby, with a roughly constant spin rate and a precessing spin axis.
In 2005, Cassini made three close passes of Hyperion at intervals of 40 and 70 days. Shifts in the rotation axis and spin rate of the moon are observable between fly-bys, but Hyperion appears to be in a similar quasi-periodic state to the one observed over two decades before. Attempts to use a dynamical model to interpolate between the three measurements will both assist in making an improved shape model and map of Hyperion and should enable us to refine our current estimates of Hyperion's moments of inertia. We will discuss the progress made in such modeling, and the limitations the chaotic nature of the problem imposes on the model.
Carcinch B.
Harbison Rebecca A.
Nicholson Philip D.
Thomas Peter C.
Tiscareno Matthew S.
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