Asteroid

An asteroid is a small rocky body that orbits the Sun. Most are leftover planetesimals from the early solar system that never accreted into a planet. They vary widely in size, from meters to hundreds of kilometers across; the largest object in the asteroid population is Ceres, about 940 kilometers in diameter, which is classified as a dwarf planet. Asteroids can be carbonaceous, silicaceous, or metallic, and their surfaces range from bright to very dark.

Most asteroids reside in the main belt between Mars and Jupiter, between about 2.1 and 3.3 astronomical units from the Sun. Others are near-Earth asteroids whose orbits cross or approach Earth's orbit, and a minority are Trojan asteroids sharing a planet's orbit. Objects can have highly inclined or eccentric orbits, and many have moons. Rotation periods typically range from several hours to a day or two.

The first asteroid discovered was Ceres in 1801 by Piazzi; for a time it was considered a planet, then reclassified as an asteroid as more bodies were found. The term asteroid means star-like, from their appearance in early telescopes. Today tens of thousands of asteroids are cataloged, identified by orbital elements and spectral type. Observational surveys and spectroscopy help classify their composition and surface properties.

Space missions have visited or sampled asteroids, including Dawn (Vesta and Ceres), Hayabusa2 (Ryugu) and OSIRIS-REx (Bennu). Studying asteroids informs models of solar system formation and evolution, and some near-Earth asteroids pose impact hazards; ongoing monitoring and potential deflection research aim to mitigate regional risks.