Asteroid

Abstract

An asteroid is an irregularly shaped, rocky body orbiting the Sun. In contrast to planets and dwarf planets, asteroids do not have sufficient mass for their self-gravity to form a spheroid. In contrast to comets, asteroids display neither a coma nor a tail. Asteroids and comets are considered to be remnant bodies from the epoch of planet formation. Planet embryos formed in the protoplanetary disk about 4.5 billion years ago via the accretion of dust grains and collisions with smaller bodies (planetesimals). A number of planet embryos succeeded in developing into the planets we observe today; the growth of other planet embryos and planetesimals was terminated by catastrophic collisions or a lack of material in their orbital zones to accrete. Most asteroids are thought to be the fragments of bodies that formed in the inner Solar System and were subsequently broken up in collisions. Comets and related icy bodies are thought to have accreted in the cold, outer regions of the protoplanetary disk where volatile material, such as water and carbon dioxide, were abundant as ices. The main asteroid belt between the orbits of Mars and Jupiter contains most of the known asteroids. An asteroid is assigned a permanent designation, i.e. a sequential number, once its orbit has become accurately established through a sufficient number of astrometric observations. There are over 200,000 numbered asteroids in the main belt, consisting largely of silicates and metals and having sizes up to about 1000 km in diameter.