Ejecta cloud from the AIDA space project kinetic impact on the secondary of a binary asteroid: II. Fates and evolutionary dependencies
Ejecta cloud from the AIDA space project kinetic impact on the secondary of a binary asteroid: II. Fates and evolutionary dependencies
复制标题
来自 AIDA 空间的喷射云预计对双星小行星的次级行星产生动力影响：II。
DOI：
10.1016/j.icarus.2018.04.017
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发表时间：
2017-08
期刊：
影响因子：
3.2
通讯作者：
Michel P.
中科院分区：
文献类型：
--
作者：
Yu Y.;Michel P.
本文对一颗双星小行星的超高速撞击所释放的抛射物云的演化进行了定量研究。美国宇航局和欧洲航天局合作的小行星撞击和偏转评估（AIDA）任务项目旨在执行小行星偏转演示，使用半吨的弹射体，将对双近地小行星（65803）Didymos（以下称为Didymoon）的次级表面进行超高速撞击。我们在AIDA当前任务场景的框架下对喷射云的撞击后动力学进行了数值模拟。我们的分析依赖于将轨道分类为抛射命运的函数，例如，与其中一个二元组分的碰撞或从系统的影响区域逃逸。在Didymoon的全球范围内定义了弹射发射地点的网格搜索，并考虑了弹射速度的广泛范围，确定了弹射命运与发射地点（弹丸撞击地点）和速度的依赖关系。这个范围使我们能够跟踪包含不同类型动态命运的所有复杂情况。结果揭示了在考虑的时间尺度结束时，在主/次星上绕轨道运行、逃逸或重新吸积的喷出物的详细比例，作为喷射速度的函数，这使我们能够探索喷出物动力学命运的全局特征。在抛射云的抛射后演化过程中，两种主要机制被发现广泛地起作用：1)抛射在与Didymoon平均运动共振轨道上的抛射在抛射物撞击后至少几周内在Didymoon上产生长期的准周期性雨，2)非共振轨道上的抛射产生快速和高的再吸积通量。这种快速而高的通量只会发生一次，因为在这样的轨道上的抛射物会离开星系，除非它们在第一次相遇时经历碰撞。对于这两种机制，都发现会发生Didymoon的摆动。由于抛射初始条件对抛射的结果极为敏感，这些抛射是混沌运动的一个来源。此外，对于所有的抛射速度，在抛射体撞击大约两个月后，在天球的中纬度区域周围出现了一个无抛射区。此外，这个区域的范围取决于喷射速度。在本研究的第二部分，我们对6个假设的撞击点释放的喷射云进行了全尺寸模拟。为了根据弹坑标度规律确定抛射物的初始条件，我们考虑了构成Didymoon地下的两种物质，然后将抛射物的幂律大小分布与抛射速度分布结合起来。我们发现喷出云的演化可分为两个阶段。它开始于第一个剧烈时期（<10小时），伴随着从系统中喷出的物质的快速再吸积或喷射。发现第二个周期比第一个周期对发射场更敏感。在第二阶段，抛射物要么重新聚集，要么被抛射出系统，这取决于它们的大小和它们在二元成分附近的平均生存时间。因此，存在由太阳辐射压力决定的尺寸分类效应，事实证明，对于所有考虑的发射场和材料类型，将尘埃大小的喷射物（<1毫米）从系统中移出是有效的。另一方面，较大的抛射物受太阳辐射压力的影响较小或不受影响，可以在系统中存活更长时间。
This paper presents a quantitative study of the evolution of the ejecta cloud released from a hypervelocity impact on a binary asteroid. The Asteroid Impact & Deflection Assessment (AIDA) mission project in collaboration between NASA and ESA aims to perform an asteroid deflection demonstration, using a half-ton projectile that will perform a hypervelocity impact on the surface of the secondary of the binary near-Earth asteroid (65803) Didymos, called hereafter Didymoon. We performed numerical simulations of the post-impact dynamics of the ejecta cloud in the framework of the current mission scenario of AIDA. Our analysis relies on a classification of the orbits as a function of the ejecta fates, e.g., a collision with one of the binary components or the escape from the region of influence of the system. A grid search of launching sites of ejecta was defined over the globe of Didymoon, and considering a wide range of possible ejection speeds, we determined the dependency of ejecta fate on launching sites (projectile impact sites) and speeds. This range enables us to track all the complex cases that include different types of dynamical fates. The results reveal the detailed proportions of the ejecta that are either orbiting, escaping or re-accreting on the primary/secondary at the end of the considered timescale, as a function of the ejection speed, which allows us to explore the global characteristics of the ejecta dynamical fates. Two major mechanisms are found to be working broadly during the post-ejection evolution of the ejecta cloud: 1) ejecta on mean motion resonance orbits with Didymoon produce long-term quasi-periodic showers onto Didymoon over at least a couple of weeks after the projectile impact, 2) ejecta on non-resonant orbits produce a rapid and high re-accretion flux. This rapid and high flux occurs just once because ejecta on such orbits leave the system unless they experience a collision during their first encounter. For both mechanisms, swing-bys of Didymoon are found to occur. These swing-bys are a source of chaotic motion because the outcome of the swing-by is extremely sensitive to the ejecta initial conditions. Moreover, for all ejecta speeds, a zone free of ejecta is noticed to emerge around the mid-latitude zone of the celestial sphere about two months after the projectile impact. Also, the extent of this zone depends on the ejecta speed. For the second part of this study, we performed full-scale simulations of the ejecta cloud released from 6 hypothetical impact sites. To define the initial conditions of the ejecta based on cratering scaling laws, we considered two kinds of material composing Didymoon’s subsurface and then combined a power-law size distribution of the ejecta with an ejection speed distribution. We find that the ejecta cloud evolution can be divided in two periods. It starts with a first violent period (<10 h) with fast re-accretion or ejection of the ejecta from the system. A second period is found to be more sensitive to the launching site than the first one. During this second period, ejecta will either re-accrete or being ejected from the system, depending both on their sizes and on their average survival time in close proximity of the binary components. There is thus a size-sorting effect dictated by the solar radiation pressure, which proves to be efficient to move out of the system the dust-size ejecta (<1 mm) for all considered launching sites and material types. On the other hand, the larger ejecta, being less or not affected by the solar radiation pressure, can survive longer in the system.
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