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1. INTRODUCTION 1.1. Motivation Multiple-asteroid systems are important because they represent a sizable fraction of the asteroid population and because they enable investigations of a number of properties and processes that are often difficult to probe by other means. The binaries, triples, and pairs inform us about a great variety of asteroid attributes, including physical, mechanical, and thermal properties, composition, interior structure, formation processes, and evolutionary processes. Observations of binaries and triples provide the most powerful way of deriving reliable masses and densities for a large number of objects. The density measurements help us understand the composition and internal structure of minor planets. Binary systems offer opportunities to measure thermal and mechanical properties, which are generally poorly known. The binary and triple systems within near-Earth asteroids (NEAs), main-belt asteroids (MBAs), and transneptunian objects (TNOs) exhibit a variety of formation mechanisms (Merline et al., 2002c; Noll et al., 2008). As such, they provide an invaluable window on accretional, collisional, tidal, and radiative processes that are critical in planet formation. The distribution and configurations of the multiple-asteroid systems also provide a rich array of constraints on their environment , their formation, and their evolutionary pathways. Observations rely primarily on groundbased telescopes and the Hubble Space Telescope (HST). For an up-to-date list of binaries and triples in the solar system, see Johnston (2014). We describe observational techniques only briefly because this material is available elsewhere (e.g., Merline et al., 2002c). A few emerging techniques will be described in more detail. Likewise, we refer the reader to other texts for an extensive history of the field (e.g., Merline et al., 2002c) and highlight only a few of the developments here. 1.2. History Early search programs for asteroid satellites were unsuccessful , returning negative or dubious results, such that the authors of the Asteroids II review chapter chose the prudent title “Do asteroids have satellites?” (Weidenschilling et al., 355 Asteroid Systems: Binaries, Triples, and Pairs Jean-Luc Margot University of California, Los Angeles Petr Pravec Astronomical Institute of the Czech Republic Academy of Sciences Patrick Taylor Arecibo Observatory Benoît Carry Institut de Mécanique Céleste et de Calcul des Éphémérides Seth Jacobson Côte d’Azur Observatory In the past decade, the number of known binary near-Earth asteroids has more than quadrupled and the number of known large main-belt asteroids with satellites has doubled. Half a dozen triple asteroids have been discovered, and the previously unrecognized populations of asteroid pairs and small main-belt binaries have been identified. The current observational evidence confirms that small (20 km) binaries with small satellites are most likely created during large collisions. Margot J.-L., Pravec P., Taylor P., Carry B., and Jacobson S. (2015) Asteroid systems: Binaries, triples, and pairs. In Asteroids IV (P. Michel et al., eds.), pp. 355–374. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816532131-ch019. 356   Asteroids IV 1989). The chapter provides an excellent discussion of the physics of several formation mechanisms that were postulated at the time. The perspective changed with the flyby of (243) Ida by the Galileo spacecraft in 1993 and the discovery of its small satellite Dactyl (Chapman et al., 1995; Belton et al., 1995). Groundbased efforts intensified and resulted in the discovery of a satellite around (45) Eugenia by Merline et al. (1999). Several other discoveries followed in rapid succession. The relatively small sizes of the MBA satellites suggested formation in subcatastrophic or catastrophic collisions (Durda, 1996; Doressoundiram et al., 1997). The discovery of MBA satellites, coupled with analysis of terrestrial doublet craters (Bottke and Melosh, 1996a,b) and anomalous lightcurve observations (Pravec and Hahn, 1997), suggested the existence of binary asteroids in the nearEarth population as well. The unambiguous detection of five NEA binaries by radar cemented this finding and indicated that NEA satellites form by a spin-up and rotational fission process (Margot et al., 2002). Lightcurve observers reached the same conclusion independently (Pravec and Harris, 2007). Both radar and lightcurve observations revealed that, far from being rare, binary asteroids are relatively common (Pravec et al., 1999, 2006; Margot et al., 2002). By the time the Asteroids III review chapter was written, a more decisive title (“Asteroids do have satellites”) had become appropriate (Merline et al., 2002c). This review focuses on the developments that followed the publication of Asteroids III. 1.3. Terminology Two- and three-component asteroids that are gravitationally bound will be referred to as binary asteroids (or binaries ) and...


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