A group of asteroids, often called the Hildas, marking the outer boundary of the main asteroid belt, concentrated close to the 3 : 2 resonance with Jupiter, at a mean distance of 4.0 au from the Sun. In common with Trojan asteroids, Hildas may have more elongated shapes than main-belt asteroids of the same sizes. The group is named after (153) Hilda, a P-type asteroid of diameter 171 km, discovered in 1875 by the Austrian astronomer Johann Palisa (1848–1925) in 1875. Hilda’s orbit has a semimajor axis of 3.98 au, period 7.94 years, perihelion 3.42 au, aphelion 4.53 au, and inclination 7°.8.