Asteroids

An asteroid is a minor planet of the inner Solar System. They are metallic or rocky bodies without atmospheres. The size and shapes of asteroids vary significantly, including even dwarf planets, but not planets.

Of the roughly one million known asteroids[1] the great number of them are located between the orbits of Mars and Jupiter, approximately 2 to 4 AU from the Sun, in the main asteroid belt. Asteroids are generally classified to be of three types: C-type, M-type, and S-type. These were named after and are generally identified with carbonaceous, metallic, and silicaceous compositions, respectively. The sizes of asteroids varies greatly; the largest, Ceres, is almost 1,000 km (600 mi) across and qualifies as a dwarf planet. The total mass of all the asteroids combined is less than that of Earth's Moon. The majority of main belt asteroids follow slightly elliptical, stable orbits, revolving in the same direction as the Earth and taking from three to six years to complete a full circuit of the Sun.[2]

Asteroids were historically observed from Earth; Galileo spacecraft was the first one to observe an asteroid from a small distance. After it, several dedicated missions to asteroids were launched by NASA and JAXA, and several missions are planned now. NASA's NEAR Shoemaker studied Eros, and Dawn observed Vesta and Ceres. JAXA's missions Hayabusa and Hayabusa2 studied and returned samples of Itokawa and Ryugu, respectively; NASA's OSIRIS-REx studied Bennu and would return a sample in 2023. Lucy, launched in 2021 would visit eight different asteroids, one from the main belt and seven Jupiter trojans. Psyche would be launched in August 2022, and would study asteroid of the same name in 2026.

Near-Earth asteroids can threaten all life on the planet; an asteroid impact event resulted in the Cretaceous–Paleogene extinction. Different asteroid deflection strategies were proposed; Double Asteroid Redirection Test was launched in 2021 and is currently underway to Dimorphos, and would attempt to alter the asteroid's orbit by crashing into it in September 2022.