Title: Analytical description of physical librations of Saturnian coorbital satellites Janus and Epimetheus Show Chinese title

Title: 土星共轨卫星Janus和Epimetheus的物理摆动的解析描述

Abstract: Janus and Epimetheus are famously known for their distinctive horseshoe-shaped orbits resulting from a 1:1 orbital resonance. Every four years these two satellites swap their orbits by a few tens of kilometers as a result of their close encounter. Recently Tiscareno et al. (2009) have proposed a model of rotation based on images from the Cassini orbiter. These authors inferred the amplitude of rotational librational motion in longitude at the orbital period by fitting a shape model to the recent Cassini ISS images. By a quasiperiodic approximation of the orbital motion, we describe how the orbital swap impacts the rotation of the satellites. To that purpose, we have developed a formalism based on quasi-periodic series with long and short-period librations. In this framework, the amplitude of the libration at the orbital period is found proportional to a term accounting for the orbital swap. We checked the analytical quasi-periodic development by performing a numerical simulation and find both results in good agreement. To complete this study, the results regarding the short-period librations are studied with the help of an adiabatic-like approach. Show Chinese abstract

Abstract: Janus和Epimetheus以其由于1:1轨道共振而产生的独特马蹄形轨道而闻名。 每四年，这两颗卫星由于近距离接触而交换它们的轨道，交换距离约为几十公里。 最近，Tiscareno等人（2009）基于卡西尼轨道器的图像提出了一个旋转模型。 这些作者通过将形状模型拟合到最近的卡西尼ISS图像，推断出在轨道周期时经度方向的旋转摆动振幅。 通过轨道运动的准周期近似，我们描述了轨道交换如何影响卫星的旋转。 为此，我们开发了一个基于包含长周期和短周期摆动的准周期级数的形式理论。 在此框架中，发现轨道周期处的摆动振幅与反映轨道交换的项成正比。 我们通过数值模拟验证了分析性的准周期展开，发现两种结果有良好一致性。 为了完成这项研究，借助类似于绝热的方法，研究了关于短周期摆动的结果。