Asteroids: Definition, History, and Risks

Asteroid terminology and classification basics

Asteroid terminology and classification basics distinguish asteroids from other small Solar System bodies using both naming conventions and physical behavior. In modern usage, the International Astronomical Union (IAU) prefers the broad term “small Solar System body” for objects that are not planets, dwarf planets, or natural satellites; this category includes asteroids, comets, centaurs, trojans, and trans-Neptunian objects. Historically, the first discovered asteroid (Ceres) was initially treated like a planet, and the term “asteroid” was proposed because these objects looked star-like (moving points of light) rather than showing a planetary disk. Physically, an object is classified as an asteroid rather than a comet if it does not develop a coma/tail when warmed by solar radiation. The distinction is not always perfectly sharp: some bodies show a continuum of properties, and some objects can transition between comet-like and asteroid-like behavior as their volatile materials are depleted. Broad compositional classes commonly used for asteroids include C-type (carbonaceous), M-type (metallic), and S-type (silicaceous/rocky), reflecting differences in typical spectra and inferred makeup. In practice, “asteroid” is often restricted to inner Solar System minor planets such as main-belt asteroids, Jupiter trojans, and near-Earth asteroids, while more distant populations are often treated separately (e.g., centaurs and trans-Neptunian objects).

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History of asteroid observations

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