Overview: An S-type asteroid is a category of asteroid characterized by a predominately stony, or silicaceous, composition. These bodies contain significant amounts of silicate minerals and metal, which give them different spectral and reflective properties from carbon-rich or metallic asteroids. The name "S-type" derives from this silicate-rich makeup; they are commonly called stony asteroids and are a principal class used in asteroid taxonomy.

Characteristics

S-type asteroids typically show spectral features produced by silicate minerals such as olivine and pyroxene, often accompanied by nickel-iron metal. Their spectra commonly display broad absorption bands near infrared wavelengths due to these minerals. In visible light they tend to be brighter than carbonaceous types because their surfaces reflect more sunlight. Classification schemes such as the Tholen and SMASS systems separate S-types from other spectral groups based on these diagnostic features.

Distribution and examples

These asteroids are especially abundant in the inner region of the main asteroid belt and among certain near-Earth populations, but they become much rarer farther from the Sun. Overall they make up a significant fraction of known asteroids—often cited at about 17%—making them the second most common major type after carbon-rich (C-type) asteroids; compare the classes C-type and others for context. The predominance of stony asteroids inwards of the belt reflects compositional gradients established in the early solar system.

  • Notable members include 15 Eunomia, one of the largest S-types, and many medium-size inner-belt bodies.
  • Several near-Earth asteroids with stony spectra are targets for study and mission visits.

S-type asteroids are central to understanding planetesimal evolution because many are linked to ordinary chondrite meteorites that fall on Earth. Sample-return and rendezvous missions to stony asteroids have confirmed compositional ties between surface spectra and meteorite types. Studies of S-types help reconstruct thermal history, collisional evolution, and the distribution of rock and metal in the early solar nebula.

Missions, research and practical notes

Robotic missions have visited S-type targets to study their geology and composition directly, returning high-value data and samples that constrain models of solar system formation. Their relative abundance in the inner belt and accessible orbits make some S-types attractive for scientific exploration and, in future contexts, resource assessment. For additional introductory material about their silicaceous nature see sources describing "silicon" and stony asteroids: silicaceous, and for distribution details beyond the inner belt consult summaries about the outer main belt.