asteroids

Asteroids are small rocky bodies that orbit the Sun, primarily in the asteroid belt between Mars and Jupiter, though many populate near-Earth space or share orbits as Trojans. They are remnants from the early solar system, composed of rock and metal, and range from meters to hundreds of kilometers across. Most asteroids are irregular in shape and rotate with periods from a few hours to days. The term 'asteroid' means 'star-like' in reference to their appearance as points of light in early telescopes.

Composition is broadly categorized into several types: C-type (carbonaceous) are dark and contain water-bearing minerals; S-type (silicaceous) are brighter and rich in silicates; M-type are relatively metal-rich. The largest objects include Ceres (a dwarf planet at about 940 km in diameter), Vesta, Pallas, and Hygiea. The asteroid belt is also home to many collisional families—groups of fragments resulting from past impacts.

Origins trace to the planetesimal population that never accreted into a planet due to Jupiter's gravity, as well as fragments from collisions. Most asteroids have fairly stable orbits, but near-Earth asteroids cross or approach the inner Solar System and can pose impact risks, which are monitored by space agencies.

Space missions have studied asteroids up close: NASA's Dawn explored Vesta and Ceres; OSIRIS-REx collected a sample from Bennu; Hayabusa2 returned samples from Ryugu. Today, thousands of asteroids have been identified, with tens of thousands of near-Earth objects cataloged.