Asteroids IV

1. INTRODUCTION Few topics within the field of asteroid studies garner more widespread attention than the near-Earth population. Many reasons motivate this attention, including: (1) These are the objects that (by definition) are capable of impacting Earth, thereby delivering meteorites (many times annually) and civilization-threatening impacts (centuries to geologic time‑ scales). (2) These are the most accessible spaceflight destinations for both robotic and human missions, including foreseeable in-space resource utilization. (3) The population is dynamic in terms of its ongoing resupply from both mainbelt asteroid and comet sources, where such resupply must occur because planetary encounters have long ago depleted the initial population. (4) Individual objects, ranging from meter-scale to tens of kilometers (12 orders of magnitude in mass), find themselves susceptible to a diverse range of physical processes involving their response to external factors such as solar flux and impacts. (5) Their proximity to Earth observers allows a variety of observational techniques through which their resolved shapes, surface properties, and diverse physical configurations can be revealed. In the most general terms, we refer to this population as near-Earth objects (NEOs) rather than near-Earth asteroids (NEAs) to recognize that both asteroid and comet origins are possible. NEAs is an applicable term when considering them as inert bodies (not outgassing), recognizing that a dormant or extinct comet can masquerade as an “asteroid.” As discussed in the chapter by Jewitt et al. in this volume, the reality is likely a continuum of properties spanning from volatile-rich to volatile-poor objects for which defining their provenance may be extraordinarily complex (see the chapter by Morbidelli et al. in this volume). Thus the simple label of “asteroid” or “comet” is inadequate for the intricacy of the problem. Nevertheless, the opportunity remains wide open for ongoing descriptive investigations. The basics, what one might call the ABCs of NEOs, are described in many sources (e.g., Binzel et al., 2002; Morbidelli et al., 2002; see the chapter by Morbidelli et al. in this volume). Here we summarize for convenience that the population is often subdivided in terms of its orbital properties (Shoemaker et al., 1979), where these orbital categories are depicted in Fig. 1. (Each category is typically named for one of the earliest discovered and named objects 243 Binzel R. P., Reddy V., and Dunn T. (2015) The near-Earth object population: Connections to comets, main-belt asteroids, and meteorites. In Asteroids IV (P. Michel et al., eds.), pp. 243–256. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816532131-ch013. The Near-Earth Object Population: Connections to Comets, Main-Belt Asteroids, and Meteorites Richard P. Binzel Massachusetts Institute of Technology Vishnu Reddy Planetary Science Institute Tasha Dunn Colby College Near-Earth objects (NEOs) owe their origins to both the main-belt asteroids and comets. They include (by definition) precursors for all meteorite samples. Thus understanding NEO connections is central to the modern study of small bodies in our solar system and serves as the principal focus of this chapter. Herein we also briefly highlight how the proximity of near-Earth objects enables detailed study of the smallest known and most accessible natural objects in space, and we provide links to other chapters addressing these aspects more fully. The success of Japan’s Hayabusa mission sample return yields a definitive link between the most common class of near-Earth asteroids and one of the most common meteorites, a watershed whose ground truth enables a deeper level of understanding and new questions. We can now investigate the near-Earth population to pinpoint specific main-belt source regions for broad taxonomic classes and specific meteorite types in addition to estimating the extinct comet contribution. Spectral properties combined with long-term orbital modeling reveal a strong role played by planetary encounters to resurface (and likely reshape) many objects. Outstanding puzzles remain for many of the newly revealed details; their resolution will generate new insights to the basic physical processes governing small bodies. 244   Asteroids IV therein.) Earth’s orbit serves as the dividing line among the categories, taking into account its 1.7% noncircularity (eccentricity , e). Amor asteroids are those whose perihelia (q) are greater than 1.017 AU but less than 1.3 AU, thus at all times orbiting beyond Earth. Apollo asteroids do intersect Earth’s orbit by having semimajor axes (a) greater than 1 AU with q ≤ 1.017 AU, noting the relationship q = a [1–e]. About 90% of all discovered NEOs are in the Amor and Apollo...