Mechanical properties, in particular, strength (tensile, shear, compressive) and porosity, are important parameters for understanding the evolution and activity of comets. However, they are notoriously difficult to measure. Unfortunately, neither Deep Impact nor other comet observations prior to Rosetta provided firm data on the strength of cometary material. This changed with the Rosetta mission and its detailed close observation data and with the landing(s) of Philae in 2014. There are already many articles and reviews in the literature that derive or compile many different strength values from various Rosetta and Philae data. In this paper, we attempt to provide an overview of the available direct and indirect data; we focus on comet Churyumov–Gerasimenko/67P but include a discussion on the Deep Impact strength results. As a prerequisite, we start by giving precise definitions of ‘strength’, discuss soil mechanics based on the Mohr–Coulomb ‘law’ of micro-gravity, and discuss bulk density and porosity, sintering, and the physics of the strength of a cohesive granular medium. We proceed by discussing the scaling of strength with the size and strain rate, which is needed to understand the observational data. We show how measured elastic properties and thermal (conductivity) data can be correlated with strength. Finally, a singular very high strength value is reviewed as well as some particularly small-strength values inferred from the bouncing motion of Philae, data from its collisions with the surface of the comet, and scratch marks it left, allegedly, on the surface close to its final resting site. The synthesis is presented as an overview figure of the tensile and compressive strength of cometary matter as a function of the size scale; conclusions about the size dependence and apparent natural variability of strength are drawn.

中文翻译：

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彗星表面的机械性能

机械性能，特别是强度（拉伸、剪切、压缩）和孔隙率，是了解彗星演化和活动的重要参数。然而，众所周知，它们很难测量。不幸的是，深度撞击和罗塞塔之前的其他彗星观测都没有提供关于彗星物质强度的可靠数据。随着罗塞塔任务及其详细的近距离观测数据以及 2014 年菲莱的着陆，这种情况发生了变化。文献中已经有许多文章和评论从各种罗塞塔和菲莱数据中推导出或汇编了许多不同的强度值。在本文中，我们试图概述可用的直接和间接数据；我们专注于 Churyumov–Gerasimenko/67P 彗星，但包括对深度冲击强度结果的讨论。作为先决条件，我们首先给出“强度”的精确定义，讨论基于微重力莫尔-库仑“定律”的土壤力学，并讨论体积密度和孔隙率、烧结以及粘性颗粒介质强度的物理特性。我们继续讨论强度与尺寸和应变率的比例，这是理解观察数据所必需的。我们展示了测量的弹性特性和热（传导性）数据如何与强度相关联。最后，回顾了一个非常高的奇异强度值，以及从菲莱的弹跳运动推断的一些特别小的强度值，它与彗星表面碰撞的数据，以及据称它在表面上留下的划痕。到它最后的安息地。该合成以彗星物质的拉伸和压缩强度作为尺寸尺度函数的概览图呈现；得出关于强度的尺寸依赖性和明显的自然变化的结论。