Rotational Modeling of Hyperion

Details Rotational Modeling of Hyperion Rotational Modeling of Hyperion

May 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dda....41.0802h&link_type=abstract

American Astronomical Society, DDA meeting #41, #8.02; Bulletin of the American Astronomical Society, Vol. 41, p.932

Astronomy and Astrophysics

Astronomy

Scientific paper

Saturn's moon, Hyperion, is subject to strongly-varying solid body torques from its primary and lacks a stable spin state resonant with its orbital frequency. In fact, its rotation is chaotic, with a Lyapunov timescale on the order of 100 days. In 2005, Cassini made three close passes of Hyperion at intervals of 40 and 70 days, when the moon was imaged extensively and the spin state could be measured. Shifts in the spin state of the moon were observed, and Hyperion was shown to spin at a varying rotation rate between 4.2 and 4.4 times synchronous. Curiously, the spin axis was observed at the same location within the body, within errors, during all three fly-bys -- approximately 30° from the long axis of the moon.
Dynamical modeling, presented at DDA in 2009 by the authors, predicted that the rotation axis should be precessing within the body, with a period of approximately 16 days. If the spin axis indeed retains its orientation during all three fly-bys, then this puts a strong constraint on the in-body precessional period, and thus the moments of inertia, of Hyperion. However, the location of the primary rotation axis in our model is derived from the shape model of Hyperion, assuming a uniform composition. This may not be a valid assumption, as Hyperion likely has significant void space, as shown by its low density (Thomas et al., 2007). This talk will examine both a rotation model with primary axes fixed by the shape model, and one with offsets from the shape model.

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