Asteroid

For other uses, see Asteroid (disambiguation).
Not to be confused with Astroid.

433 Eros photographed by NEAR Shoemaker
Galileo image of 243 Ida (the dot to the right is its moon Dactyl)
Dawn image of the dwarf planet Ceres
OSIRIS-REx image of 101955 Bennu, a rubble-pile 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 an identified comet— that orbits within the inner Solar System. They are rocky, metallic, or icy bodies with no atmosphere, classified as C-type (carbonaceous), M-type (metallic), or S-type (silicaceous). The size and shape of asteroids vary significantly, ranging from small rubble piles under a kilometer across and larger than meteoroids, to Ceres, a dwarf planet almost 1000 km in diameter. A body is classified as a comet, not an asteroid, if it shows a coma (tail) when warmed by solar radiation, although recent observations suggest a continuum between these types of bodies.[1][2]

Of the roughly one million known asteroids,[3] 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. 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.[4]

Asteroids have historically been observed from Earth. The first close-up observation of an asteroid was made by the Galileo spacecraft. 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 October 2023, aims to study the metallic asteroid Psyche.

Near-Earth asteroids have the potential for catastrophic consequences if they strike Earth, with a notable example being the Chicxulub impact, widely thought to have induced the Cretaceous–Paleogene mass extinction. As an experiment to meet this danger, in September 2022 the Double Asteroid Redirection Test spacecraft successfully altered the orbit of the non-threatening asteroid Dimorphos by crashing into it.

  1. ^ Cite error: The named reference :0 was invoked but never defined (see the help page).
  2. ^ Cite error: The named reference :1 was invoked but never defined (see the help page).
  3. ^ Public Domain This article incorporates text from this source, which is in the public domain: "Asteroids". NASA Solar System Exploration. Retrieved 29 March 2022.
  4. ^ Public Domain This article incorporates text from this source, which is in the public domain: "Asteroids (from the NEAR press kit)". nssdc.gsfc.nasa.gov. Retrieved 29 March 2022.