土星环后方的卫星
Moons Beyond Rings at Saturn
Image Credit: NASA, ESA, JPL, Cassini Imaging Team
Explanation: What’s happened to that moon of Saturn? Nothing — Saturn’s moon Rhea is just partly hidden behind Saturn’s rings. In 2010, the robotic Cassini spacecraft then orbiting Saturn took this narrow-angle view looking across the Solar System’s most famous rings. Rings visible in the foreground include the thin F ring on the outside and the much wider A and B rings just interior to it. Although it seems to be hovering over the rings, Saturn’s moon Janus is actually far behind them. Janus is one of Saturn’s smaller moons and measures only about 180 kilometers across. Farther out from the camera is the heavily cratered Rhea, a much larger moon measuring 1,500 kilometers across. The top of Rhea is visible only through gaps in the rings. After more than a decade of exploration and discovery, the Cassini spacecraft ran low on fuel in 2017 and was directed to enter Saturn’s atmosphere, where it surely melted.
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土星环后方的卫星
影像提供: NASA, ESA, JPL, Cassini Imaging Team
说明: 土星的卫星发生了什么事？没事，土星的土卫五（Rhea）只是被土星环给遮掉一部分。在2010年，当时在绕行土星的卡西尼号太空船拍下了这幅窄视角影像，直视太阳系最著名的行星环系。在影像前景清楚可辨的土星环之中，有纤细的F环，以及紧贴在它内侧、远更宽广的A环与B环。而乍看之下，像是飘浮在这些行星环上方的土卫十（Janus），其实远在它们的后方。宽度约只有180公里的土卫十，是土星的小卫星之一。离相机更远且满布撞击坑的土卫五，则宽达1,500公里。在这幅影像里，透过环缝依稀可见土卫五的顶部。在探索土星系统十多年并完成诸多发现之后，2017年时卡西尼号太空船的燃料几乎耗尽，在接获指令后，纵身切入土星大气并熔毁于其内。
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明日的图片: a year of sunrises
