The Callisto satellite

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图注： Callisto is the outermost of the four large Galilean moons of Jupiter and the third largest in the whole solar system, falling behind only Ganymede and Saturn's moon Titan. It orbits Jupiter at an average distance of around 1.9 million kilometers, roughly six times the distance between Earth and the Moon. With a diameter of nearly 5,000 kilometers, it is almost as large as Mercury, the smallest planet in our solar system, but has only a third of its mass as it is composed of approximately equal amounts of rock and ice. Callisto orbits Jupiter in a tidally locked rotation, always showing the same side to Jupiter. In contrast to the other three Galilean satellites – Io, Europa, and Ganymede – it is not in orbital resonance due to its farther outward location and is thus not appreciably tidally heated. It does not show any signs of plate tectonics or volcanic activity. Its lack of geological processes means that surface features like craters remain unchanged for billions of years. It is thought to have evolved predominantly under the influence of impacts, as its surface is the oldest and most heavily cratered object in the solar system. In fact, it is so covered in craters that any new impact will likely erase a former one. Callisto's internal structure is still subject to discussion; it may consist of a deep salty ocean. It is thought to be composed mainly of a mix of ice and rock, which are present in various states throughout the inner layers of the moon. Callisto hosts a very thin atmosphere composed mainly of carbon dioxide and some traces of oxygen. The moon has been studied by numerous space probes, such as the Pioneer and Voyager missions and the Galileo orbiter, and will be subject to further explorations by NASA, ESA, and China’s National Space Administration (CNSA).
来源： NASA/JPL/DLR
来源链接

词汇表: 伽利略卫星 , 卫星

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卫星群与夜空之美，作者 Torsten Hansen，德国

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为 OAE 制作

图注： 2021 年国际天文学联合会 OAE 天文摄影比赛光污染类第三名。 这幅金星和昴星团的图像还显示了星链卫星的轨迹。这些卫星位于大约 550 公里的高空，是不断扩大的卫星群的一部分，旨在提供全球互联网接入。卫星的反射表面，以及它们围绕地球运行，意味着需要长时间曝光的天文观测会在图像中捕捉到卫星的 "轨迹"。由于测量结果和数据包含这些 "轨迹"，这导致用于科学研究的天文图像无法使用。卫星的数量预计会不断增加，在不久的将来，地球上可能再无任何地方能避开这些卫星划过天空的轨迹。这是一种新型的光污染，似乎正成为我们即将要面对的问题，因为这些卫星可能会阻碍对天空的最佳观测。
来源： Torsten Hansen/IAU OAE

词汇表: 光污染 , 人造卫星

授权许可： 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 图标

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Space debris orbiting the Earth

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图注： This animation shows objects orbiting the Earth. The data date from February 2024 and are taken from space-track.org which maintains a public catalogue of objects tracked by the United States Space Command (USSPACECOM). At first in this video we see green dots, each representing one of the 31,000 human-made objects orbiting Earth. These include both operational and decommissioned satellites, used rocket stages from space launches, and fragments larger than 10cm created by collisions or explosions. Later in the video, some of the green dots are replace by orange dots, each of these representing one of the 9,300 operational satellites orbiting the Earth. Note that since February 2024 more satellites and rockets have been launched in to space and some objects will have fallen back to Earth, so these numbers will have changed. We see two main groups of objects. The first are those objects orbiting close to the Earth, objects in low Earth orbit. These include communication satellites (many from the satellite constellations used to provide satellite internet services), Earth observation satellites, military satellites, two operational space stations and other objects such as rocket stages and other debris. More distant from Earth we see a thin ring of satellites. These are in geostationary orbit. This is a special orbit where the orbital period matches the rotation period of the Earth. This means that a satellite in geostationary orbit stays above the same point on the equator and, when observed from the side of the Earth facing the satellite, will appear to stay at a position on the sky that does not change over time. Because of this, geostationary orbit is used by many communications satellites as a satellite dish on Earth used to receive or transmit a signal to that satellite can point in a fixed direction and does not need to dynamically track the satellite. Geostationary orbit is also used by weather observation satellites.
来源： NASA's Scientific Visualization Studio
来源链接

词汇表: 人造卫星 , 太空碎片 , 航天器
分类： 地球 , 太空探索

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Ganymede seen by Juno spacecraft

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图注： Ganymede is the largest and most massive natural moon of Jupiter, and at the same time, the largest moon in the solar system. It also stands out as it is the only moon in the solar system which has an internally generated magnetic field. This is likely created by a liquid iron or iron sulfide core. With a diameter of over 5,000 kilometers, it exceeds Mercury in size. Although it has more than twice the mass of Earth's Moon, it hosts weaker surface gravity than Mercury, Io, and the Moon due to its lower mean density. Ganymede's appearance is primarily composed of darker, heavily cratered regions, and brighter, less bombarded ones with deep grooves and ridges indicating geological activity. Whether this might be caused by tidal heating is still subject to further analysis. The brighter regions make up around two-thirds of Ganymede's surface. The remaining area consists of dark regions, saturated with impact craters, which have been dated back to four billion years. It is assumed that the moon hosts a large saltwater ocean under its icy surface with extreme depths of up to 800 kilometers. Ganymede likely has a very tenuous oxygen atmosphere, comparable to that found on the Jupiter moon Europa. It cannot be interpreted as a sign of life. Instead, it is thought that hydrogen and oxygen are split up by radiation on the moon's surface. The first space probe to visit Ganymede was Pioneer 10, performing a high-speed flyby manoeuvre through the Jupiter system. Since then, it has been investigated by several other probes such as Galileo, New Horizons, and Juno. The Jupiter Icy Moons Explorer (JUICE) will be the first to actually enter orbit around Ganymede itself (in around 2034) to further study the moon's magnetic and icy surface. JUICE will impact Ganymede as the last act of its mission.
来源： NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill
来源链接

词汇表: 伽利略卫星 , 卫星

授权许可： 知识共享许可协议 Attribution 2.0 Generic 知识共享许可协议 Attribution 2.0 Generic 图标

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欧罗巴

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图注： 这张照片由美国国家航空航天局的伽利略探测器于1996年9月拍摄，展示了木星的被冰覆盖的卫星——欧罗巴，以近似自然色呈现。画面中长长的暗线是冰壳上的裂缝，其中一些裂缝长度超过3000公里（1850英里）。图像下方三分之一处，包含中央暗点的明亮区域是一个直径约50公里（31英里）新形成的撞击坑。欧罗巴的直径约为3160公里（1950英里），与地球的卫星月球大小相当。
来源： 美国国家航空航天局/喷气推进实验室/德国航空航天中心

词汇表: 伽利略卫星 , 卫星
分类： 太阳系

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