restricted access Identification and Dynamical Properties of Asteroid Families

From: Asteroids IV

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1. INTRODUCTION As witnessed by the heavily cratered surfaces imaged by spacecrafts, the chief geophysical process affecting asteroids is impacts. On rare occasions, the impact of a large projectile can be so energetic that the target asteroid is violently torn apart, and the pieces are thrown into space. The sites of such cosmic accidents are filled with debris that gravitationally accumulate into larger conglomerates, and drift away at speeds that are roughly commensurate with the escape speed from the original target body (Vesc). Initially, all orbits are similar, because Vesc  Vorb, where Vorb = 15–20 km s–1 is the orbital speed of main-belt asteroids. On longer timescales, however, the orbits are altered by gravitational perturbations from planets, and the orbital elements of individual bodies start to diverge. It may therefore seem challenging to identify fragments of a catastrophic collision that happened eons ago. Fortunately , starting with the pioneering work of K. Hirayama (Hirayama, 1918; see also Cimrman, 1917), astronomers have developed various methods to deal with this issue (section 2). Roughly speaking, these methods consist in a transformation that brings the orbital elements at the observed epoch to a standard, called the proper elements (Knežević et al., 2002), that is unchanging in time (or, at least, would be unchanging if chaotic dynamics, nongravitational forces, and other perturbations could be ignored). Thus, ideally, daughter fragments produced by breakup of a parent asteroid will appear as a group in space of the proper elements even gigayears after the original collision. These groups are called asteroid families, or dynamical families, to emphasize that they have been identified from dynamical considerations. Telescopic surveys such as the Sloan Digital Sky Survey (SDSS), Wide-field Infrared Survey Explorer (WISE), and AKARI All-Sky Survey provide a wealth of data on physical properties of the main-belt asteroids (Ivezić et al., 2001; Mainzer et al., 2011; Usui et al., 2013). They have been used to cross-link the color and albedo measurements with the lists of dynamical families, in much the same way that spectroscopic and taxonomic data have previously been applied to this purpose (for a review, see Cellino et al., 2002). This work is useful to physically characterize the asteroid families (see the chapter by Masiero et al. in this volume), including cases where two or more dynamical families overlap, and identify distant “halo” family members that would otherwise be confused with the local background (e.g., Brož and Morbidelli, 2013). Given that the SDSS and WISE catalogs now contain data for more than 100,000 unique asteroids, it has also become practical to conduct search for families in extended space, where the color and/ or albedo data are taken into account simultaneously with the orbital elements (e.g., Parker et al., 2008; Masiero et al., 2013; Carruba et al., 2013a). The physical data can be used to identify interlopers. The problem of interlopers arises because the clustering criterion applied to identify the dynamical families is only a rough expression of the true membership. Unrelated asteroids that just happen to have nearby values of proper elements will 297 Identification and Dynamical Properties of Asteroid Families David Nesvorný Southwest Research Institute Miroslav Brož Charles University Valerio Carruba Universidade Estadual Paulista Asteroids formed in a dynamically quiescent disk but their orbits became gravitationally stirred enough by Jupiter to lead to high-speed collisions. As a result, many dozen large asteroids have been disrupted by impacts over the age of the solar system, producing groups of fragments known as asteroid families. Here we explain how the asteroid families are identified, review their current inventory, and discuss how they can be used to get insights into long-term dynamics of main-belt asteroids. Electronic tables of the membership for 122 notable families are reported on the Planetary Data System node. See related chapters in this volume for the significance of asteroid families for studies of physics of large-scale collisions, collisional history of the main belt, source regions of the near-Earth asteroids, meteorites and dust particles, and space weathering. Nesvorný D., Brož M., and Carruba V. (2015) Identification and dynamical properties of asteroid families. In Asteroids IV (P. Michel et al., eds.), pp. 297–321. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816532131-ch016. 298   Asteroids IV be grouped together with the true members, and will thus appear in the lists of dynamical families obtained from the proper elements (e.g., Migliorini et al., 1995). These interlopers, especially the large ones, introduce ambiguity in the...