Meteorites are pieces of rock and metal that fall to the Earth. Almost all are fragments broken from asteroids during collisions, taking between around 0.2 and 100 million years to journey from the asteroid belt to the Earth. There are also currently about 30 meteorites that come from the Moon, and a similar number from Mars. Meteorites are the only physical materials available on the Earth that allow direct study of the original dust from which the Solar System formed. Meteorites are named from their place of find or fall. Wherever possible, the name is taken from the nearest inhabited place to the actual site. In practice, the recovery of meteorites from desert regions has resulted in a name-number nomenclature that combines geographic and date information. The rules for naming newly recovered meteorites have been standardised by the Nomenclature Committee of the Meteoritical Society, and are summarised on the Society's web site (www.meteoriticalsociety.org). This section is intended to be a very general outline of the main features of meteorites and their classification.
Meteorites can be divided into two main types, according to the processes they have experienced: unmelted (unfractionated) and melted (or differentiated). The unmelted meteorites, or chondrites, are all stone, whilst the melted meteorites, cover a range of compositions from stone, through stony-iron to iron. Both unmelted and melted meteorites are further sub-divided into groups and classes; the inter-relationships between the different groups are shown in the "family tree". Classification of meteorites into groups is one way of identifying materials that might be associated in space and time, eg, through accretion in closely neighbouring regions of the solar nebula, or having suffered similar processes of heating, melting, differentiation and/or hydrothermal alteration. However, the classification scheme is incomplete, and there are many meteorites that do not fit comfortably into the framework. There is not always a clear cut distinction between types: eg, there are many iron meteorites that contain silicate inclusions related to chondritic meteorites. Clasts and inclusions within meteorites also frequently defy ready assignation to recognised meteorite groups.