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Rotational modeling of Hyperion

机译：Hyperion的旋转建模

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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 67 days, when the moon was imaged extensively and the spin state could be measured. Curiously, the spin axis was observed at the same location within the body, within errors, during all three fly-bys—~ 30° from the long axis of the moon and rotating between 4.2 and 4.5 times faster than the synchronous rate. Our dynamical modeling predicts that the rotation axis should be precessing within the body, with a period of ~ 16 days. If the spin axis 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. However, the location of the principal axes in our model are derived from the shape model of Hyperion, assuming a uniform composition. This may not be a valid assumption, as Hyperion has significant void space, as shown by its density of 544± 50 kg m−3 (Thomas et al. in Nature 448:50, 2007). This paper will examine both a rotation model with principal axes fixed by the shape model, and one with offsets from the shape model. We favor the latter interpretation, which produces a best-fit with principal axes offset of ~ 30° from the shape model, placing the A axis at the spin axis in 2005, but returns a lower reduced χ 2 than the best-fit fixed-axes model.
机译：土星的卫星Hyperion受到来自其主要部分的固体扭矩的强烈变化，并且缺乏与其轨道频率共振的稳定自旋状态。实际上，它的旋转是混乱的，李雅普诺夫（Lyapunov）时标大约为100天。 2005年，卡西尼号以40天和67天的间隔进行了3次海波龙近距离探测，当时对月球进行了广泛成像并可以测量其自旋状态。奇怪的是，在所有的三个飞越过程中，都在人体内相同的位置上观察到了自旋轴，误差不大-距月球长轴约30°，旋转速度比同步速度快4.2至4.5倍。我们的动力学模型预测旋转轴应在体内进动，周期约为16天。如果自旋轴在所有三个飞越期间均保持其方向，则这将对体内进动周期以及惯性矩施加强大的约束。但是，在我们的模型中，主轴的位置是从Hyperion的形状模型中得出的，假设它们的成分相同。这可能不是一个有效的假设，因为Hyperion具有显着的空隙空间，如其密度544±50 kg m -3 所示（Thomas等人，Nature 448：50，2007）。本文将研究主轴由形状模型固定的旋转模型和偏移形状模型的旋转模型。我们支持后一种解释，该解释产生的最佳拟合的主轴偏离形状模型约30°，在2005年将A轴放置在旋转轴上，但返回的归因较低χ 2 比最适合的固定轴型号。

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