The first asteroid to be discovered, by G. Piazzi on 1801 January 1, and hence given the number 1. Ceres is by far the largest asteroid, an oblate spheroid 975 × 909 km, mean diameter 952 km, mass 9 × 1020 kg (about 1% the mass of the Moon) and mean density about 2.0 g/cm3. Its rotation period is 9.075 hours. Ceres contains roughly one-third the mass of the entire asteroid belt. It is a member of the new class of dwarf planets, along with Eris and Pluto. Its mean magnitude at opposition is 7.4; only Vesta can ever become brighter. Its orbit has a semimajor axis of 2.765 AU, period 4.60 years, perihelion 2.55 AU, aphelion 2.98 AU, and inclination 10°.6. Ceres is of G class, with a reflectance spectrum that implies a composition resembling the carbonaceous chondrite meteorites. Ceres is the largest member of a small family of asteroids with a mean distance of 2.76–2.80 AU from the Sun and orbital inclinations of 9–11°. Other Ceres family members include (39) Laetitia, (264) Libussa, (374) Burgundia (all S-class), and (446) Aeternitas (A-class).
Subjects: Astronomy and Astrophysics.