Asteroids: Definition, History, and Exploration

Asteroid terminology and classification

Asteroid terminology and classification covers how these objects are named and how they are distinguished from related small bodies such as comets and other “small Solar System bodies.” The International Astronomical Union (IAU) introduced the broad term “small Solar System body” for objects that are not planets, dwarf planets, or natural satellites, which includes asteroids, comets, centaurs, trojans, and trans-Neptunian objects. Historically, the first discovered asteroid (Ceres) was initially treated as a planet, and the term “asteroid” was proposed by William Herschel; it comes from Greek roots meaning “star-like” or “star-shaped,” reflecting how early telescopic observations showed these objects as star-like points that moved. In classification, an object is generally treated as an asteroid (rather than a comet) if it does not develop a coma/tail when warmed by solar radiation. The main difference is that comets show activity due to sublimation of near-surface ices, while asteroids are typically rocky, metallic, or icy bodies without an atmosphere. However, observations suggest a continuum between asteroids and comets rather than a strict boundary. Asteroids are also commonly grouped by spectral/physical type—broadly C-type (carbonaceous), M-type (metallic), and S-type (silicaceous)—and are often restricted in usage to “classical asteroids” such as main-belt asteroids, Jupiter trojans, and near-Earth asteroids, rather than all minor planets beyond Jupiter.

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History of asteroid observations

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