This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics.

中文翻译：

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Sungrazers、Sunskir​​ters 和其他近太阳彗星的科学

这篇综述阐述了我们目前对靠近太阳的彗星的理解，因此它们暴露在比通常从地球上研究的彗星更极端的条件下。这些天体遇到的极端太阳加热和等离子体环境改变了它们行为的许多方面，从而产生了关于彗星本身的宝贵信息，补充了我们关于原始太阳系天体的其他数据，以及它们所处的近太阳环境。遍历。我们为这些彗星提出了明确的定义：我们使用“近太阳彗星”一词来涵盖所有从水星近日点距离 (0.307 AU) 向阳的物体。太阳裙被定义为通过太阳中心 33 个太阳半径范围内的物体，相当于水星近日点距离的一半，常用短语 sungrazers 是在 3.45 个太阳半径范围内到达近日点的物体，即流体罗氏极限。最后，轨道与太阳光球层相交的彗星被称为太阳潜水者。我们总结了过去对这些物体的研究，以及用于研究它们的仪器和设施，包括导致相关观测数量和质量最近发生革命的天基平台。描述了相关的彗星群，包括 Kreutz、Marsden、Kracht 和 Meyer 群、近太阳小行星，并简要讨论了它们的起源。强调了光变曲线的重要性及其提供的关于彗星成分的线索，以及收集到的关于核参数的信息，包括物体及其家族的大小和质量，以及它们的抗拉强度。对这些物体发生的物理过程进行了一些详细的考虑，包括原子核的破坏、升华和电离，我们还考虑了日冕中彗星和那些冒险到较低高度的彗星的质量、动量和能量损失。描述了彗尾和彗尾的不同成分，包括尘埃、中性和电离气体、它们的化学反应以及它们对近太阳环境的贡献。讨论了彗星-太阳风的相互作用，包括使用彗星作为其附近太阳风和日冕条件的探测器。我们解决了太阳附近彗星的工作与围绕其他恒星运行的类似物体的相关性，