On the longitudinal librations of Hyperion

We study a nonlinear, second order ordinary dierential equation that models the longitudinal librations of the longest axis L of a satellite with respect to the planet-satellite center line C. Combining theoretical arguments and numerical evidence we prove that, in the case of Hyperion, a satellite of Saturn, the angle A between L and C can change in a chaotic manner at the moments when the distance from Hyperion to Saturn reaches its minimum value. More precisely, given an arbitrary sequence of zeros and ones, we show that there is at least one initial velocity of A such that its successive positions reproduce the given sequence.