Asteroid

Images of visited asteroids illustrating their differences: (top row) 433 Eros and 243 Ida with its moon Dactyl, (bottom row) Ceres and 101955 Bennu. Sizes are not to scale.

An asteroid is a minor planet—an object that is neither a true planet nor a comet—that orbits within the inner Solar System. They are rocky, metallic, or icy bodies with no atmosphere. The size and shape of asteroids vary significantly, ranging from small rubble piles under a kilometer across to Ceres, a dwarf planet almost 1000 km in diameter.

Of the roughly one million known asteroids,^[1] the greatest number are located between the orbits of Mars and Jupiter, approximately 2 to 4 AU from the Sun, in a region known as the main asteroid belt. Asteroids are generally classified to be of three types: C-type, M-type, and S-type. These describe asteroids with carbonaceous, metallic, and silicaceous compositions, respectively. The size 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 only 3% 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 have historically been observed from Earth; the Galileo spacecraft made the first close-up observation of an asteroid. Several dedicated missions to asteroids were subsequently launched by NASA and JAXA, with plans for other missions in progress. 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. OSIRIS-REx studied Bennu, collecting a sample in 2020 which was delivered back to Earth in 2023. NASA's Lucy, launched in 2021, is tasked with studying ten different asteroids, two from the main belt and eight Jupiter trojans. Psyche, launched in October 2023, aims to study a metallic asteroid of the same name.

Near-Earth asteroids can threaten all life on the planet; an asteroid impact event may have resulted in the Cretaceous–Paleogene extinction. Different asteroid deflection strategies have been proposed; the Double Asteroid Redirection Test spacecraft, or DART, was launched in 2021 and intentionally impacted Dimorphos in September 2022, successfully altering its orbit by crashing into it.