Asteroids IV

1. INTRODUCTION Well before the first asteroid was discovered, people unknowingly had asteroid samples in their hands. Could stones fall from the sky? For many centuries, the official answer was no. Only at the end of the eighteenth century and the beginning of the nineteenth century did the evidence that rocks did fall from the sky become so overwhelming that this fact was accepted by the scientific community. It was the German scientist Ernst Chladni who first recognized their extraterrestrial origin in 1794. Nevertheless, it took some time before his idea became accepted (see, e.g., Lauretta and McSween, 2006). The stones that fell from the sky are distinct from all terres‑ trial rocks. They are called meteorites. Meteorites are classified into many different types according to their mineralogical composition. The two basic classes are stony and iron meteorites , although a mixture of these chemistries (stony-iron meteorites) exists as well. Iron meteorites are composed of metallic iron with an admixture of nickel and some traces of other elements. Stony meteorites are mostly composed of silicates such as olivine and pyroxene, with some metallic iron also present in many cases. The elements and most of minerals found in meteorites are known from terrestrial rocks, but their ratios in meteorites are different. The most common type of stony meteorites, called chondrites, contains millimeter-sized spherical structures called chondrules. Such structures are not present in any terrestrial rock. More detailed classification of meteorites is given in the chapter by Binzel et al. in this volume. The study of meteorites is a well-developed scientific field and rich literature is available (e.g., Dodd, 1981; Papike, 1998; Hutchison, 2004; Lauretta and McSween, 2006). According to the circumstances of their recovery, meteorites can be divided into finds and falls. Finds are meteorites found on ground (by chance or by dedicated searches), but the date of their fall is unknown. Falls are meteorites, whose fall was witnessed — either the meteorite was seen (or heard) to hit the ground or the fireball caused by the ablation of the parent meteoroid in the atmosphere was observed and the meteorites were found later. Meteoroid is a term for the original body that dropped the meteorite(s). In fact, there is no strictly defined boundary between large meteoroids and small asteroids. For the purpose of this chapter, we will call meteoroids all bodies smaller than 10 m and larger than 50 µm. Bodies smaller than 50 µm are called dust particles. A dust particle, when entering the atmosphere, may not give rise to the phenomenon called a meteor, depending on its initial speed. Meteor is the radiation, and associated phenomenon (heat, shock, ionization), caused by the entry of a meteoroid in the atmosphere. Dust particles are sufficiently small that they may decelerate before they start to evaporate, so they do not produce meteors. They gradually sediment through the atmosphere and finally reach the ground. The vast majority of meteoroids are destroyed in the atmosphere and do not produce any macroscopic meteorites. Parts of them may not be evaporated completely and can reach the ground as dust or micrometeorites (objects smaller 257 Borovička J., Spurný P., and Brown P. (2015) Small near-Earth asteroids as a source of meteorites. In Asteroids IV (P. Michel et al., eds.), pp. 257–280. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816532131-ch014. Small Near-Earth Asteroids as a Source of Meteorites Jiří Borovička and Pavel Spurný Astronomical Institute of the Czech Academy of Sciences Peter Brown University of Western Ontario Small asteroids intersecting Earth’s orbit can deliver extraterrestrial rocks to Earth, called meteorites. This process is accompanied by a luminous phenomena in the atmosphere called bolides or fireballs. Observations of bolides provide pre-atmospheric orbits of meteorites, physical and chemical properties of small asteroids, and the flux (i.e., frequency of impacts) of bodies at Earth in the centimeter to decameter size range. In this chapter we explain the processes occurring during the penetration of cosmic bodies through the atmosphere and review the methods of bolide observations. We compile available data on the fireballs associated with 22 instrumentally observed meteorite falls. Among them are the heterogeneous falls Almahata Sitta (2008 TC3) and Benešov, which revolutionized our view on the structure and composition of small asteroids; the Příbram-Neuschwanstein orbital pair, carbonaceous chondrite meteorites with orbits on the asteroid-comet boundary; and the Chelyabinsk fall, which produced a damaging blast wave. While most meteoroids disrupt into fragments during atmospheric...