Our Solar System comprises Sun, eight planets, their satellites, between 1 and 2 million asteroids larger than 1 km, and millions of smaller bodies. A surprisingly small number of extraterrestrial bodies have been probed by sample return missions directly so far: Moon (by Apollo and Luna programs), solar wind (Genesis mission), coma of the comet Wild 2 (Stardust mission), and S-type asteroid 25143 Itokawa (Hayabusa mission). The bulk of existing knowledge about the composition and history of other bodies was produced by laboratory studies of meteorites – the remains of the rocks that were excavated from asteroids, sent to Earth-crossing orbits by collisions, ablated while passing through the Earth’s atmosphere, and landed at the Earth’s surface. Meteorite collections around the world contain about 55,000 rocks that are thought to be derived from >110 asteroids (~60 iron, ~35 achondritic, ~15 chondritic), and include a relatively large number of rocks from Vesta, Mars and the Moon.
Most known achondrites (extraterrestrial igneous rocks composed mostly of silicate minerals) are classified into groups on the basis of similarities in mineralogy and elemental and oxygen isotope abundances, and these similarities are often interpreted as an indication of the same parent body. The studies of several achondrites from the same group that formed at different times allow us to reconstruct the evolution of their parent asteroid. Many achondrites found in recent years have unique chemical and isotopic characteristics, and do not fit in the established groups, and so are referred to as ungrouped achondrites. They are thought to sample a large number of different asteroids. Discovery of these meteorites, together with recent advancements of analytical techniques, allows us for the first time to go beyond the case studies of a small number of select asteroids to a representative picture of the accreting protosolar nebula based on the data from diverse asteroids that formed and partially melted in different domains, at different times, from different source materials, and under different conditions.