A type of asteroid whose members are rare in the main belt but which are increasingly encountered beyond 3.3 au from the Sun, a distance at which an asteroid has an orbital period exactly half that of Jupiter (i.e. a 2 : 1 resonance). D-type asteroids have a very low albedo (0.02–0.05) and a generally featureless reflectance spectrum. They are very red at longer wavelengths, possibly due to carbon-rich material. Examples include (1256) Normannia (a member of the Hilda group), diameter 69 km, and many Trojan asteroids, including (911) Agamemnon, diameter 167 km.