Meteorites constitute the most abundant source of extraterrestrial material. They formed under a wide range of redox conditions and contain many minerals not found on Earth. Their study extends the range of known petrological and geochemical processes; they serve as concrete examples of shock metamorphism of natural materials. They contain the most ancient examples of organic compounds and aqueously altered minerals that can be studied in the lab. Calcium-aluminium-rich inclusions (CAIs) yield the age of the Solar System and CI chondrites provide the cosmic abundances of most elements. Meteorites can provide information about the interactions between cosmic rays and solid materials. They likely delivered raw materials to the early Earth, possibly facilitating the origin of life. Impact-crater formation by asteroids is the main geomorphological process in the Solar System; it changed the course of biological evolution on Earth. Meteorites provide clues to the geological history of asteroids, the Moon, and Mars, and many iron meteorites provide samples of planetesimal cores. Presolar grains permit the in situ examination of materials from other stars that existed long before the Solar System.

Meteorites are classified using a hierarchical scheme based on the degree of relatedness of samples. Chondrite groups are typically from a single parent body; clans and classes are clusters of related groups that accreted in similar regions of the solar nebula. Classification of a new meteorite requires visual observation of macroscopic characteristics, microscopic examination of textures, and analyses of minerals. Isotopic or bulk compositional data may also be acquired.

The number ratio of carbonaceous to ordinary chondrites (the CC/OC ratio) is mass dependent. It is somewhat high for large meteorites (0.20), very high for the largest fireball-producing meteoroids (30), low for most meteorite falls (0.04-0.05), and extremely high for micrometeorites (86) and interplanetary dust particles (IDPs) (>>100). The high CC/OC ratio among small particles reflects the predominance of C asteroids beyond 3 AU; these particles spiral into the Inner Solar System (and reach the Earth) via the Poynting-Robertson effect. The high CC/OC ratio among large objects results from the seasonal Yarkovsky effect, which transfers asteroids (mainly the abundant C asteroids from the Outer Solar System) into Near-Earth Asteroid (NEA) orbits.

Just as the purloined letter in Edgar Allan Poe's celebrated detective story was hidden in plain sight, so too can ordinary chondrites hold vital clues to the nature of the Solar System. Even highly weathered equilibrated samples, seemingly unworthy of a second look, may bear the markings of thermal metamorphism, shock metamorphism, and post-shock annealing. To study the heavens, we need only keep our eyes open; the rocks beneath our feet may conceal the secrets of the cosmos.

After a meteorite reaches the Earth’s surface, it is subject to terrestrial weathering. Metallic Fe-Ni grains develop thin red coatings of goethite; the goethite fills pores within the whole-rocks, eventually decreasing their porosity to zero. Other bulk parameters that change during terrestrial weathering of ordinary chondrites are magnetic susceptibility, thermal conductivity, compressive strength, and tensile strength. Evaporite minerals grow on the surfaces of Antarctic finds with phases including Mg carbonates, Mg sulfates, and Ca sulfate. OC whole rocks become contaminated with terrestrial C and water, affecting their bulk isotopic compositions. Frost wedging can cause rocks to expand and shatter as water seeps into fractures and freezes. There are a few OC ventifacts sculpted by wind erosion in arid environments; these rocks typically have three or four flat sides that meet at angular interfaces. A small number of ordinary chondrites are shatter cones, shocked rocks with striated surfaces that have a horsetail-like appearance. Such structures are produced beneath the floors of impact craters.

Meteorite falls can produce light phenomena (meteors, fireballs), sonic booms, and electrophonic sounds. Doppler radar can identify falls by their positions and velocity vectors. Incoming meteoroids lose mass during atmospheric passage; after slowing, the remaining pieces develop a fusion crust, typically a 1–2-mm thick melt-coating that solidifies in the air. Most meteoroids also develop regmaglypts during descent due to localized vortices of hot, turbulent gas sculpting the meteoroid’s surface. Some specimens maintain a fixed orientation during atmospheric passage and develop nose-cone shapes. The disruption of a meteoroid in the atmosphere can shatter it into thousands of fragments; when these individuals hit the ground, they form an elliptical pattern (strewn field) in which the largest fragments tend to occur at the terminus of the field along the line of the meteoroid’s trajectory. There are fossil ordinary chondrites recovered from Ordovician sedimentary rocks. Terrestrial impact craters associated with ordinary-chondrite remnants include Carancas (Peru) and Morokweng (South Africa). Meteorites have been concentrated on Earth in cold deserts (e.g., Antarctica) and hot deserts (e.g., the Sahara).

Precise definitions can aid in meteorite classification and reduce confusion during interactions across disciplines.

This chapter discusses how a classical universe arises out of the quantum wave function of the universe. The process of decoherence is described, first in general and then applied to cosmology. The classicalization of the background spacetime (and the associated reduction in interference between saddle points) as well as the classicalization of long-wavelength perturbation modes is discussed, by studying an example of interactions between background and fluctuations, as well as interaction between perturbation modes of different wavelengths. Comments on the interpretation of the wave function are included.