Ceres

Ceres is a dwarf planet and the largest object in the asteroid belt, a region between Mars and Jupiter. It has a diameter of about 940 kilometers and is the closest dwarf planet to the Sun. Discovered by Giuseppe Piazzi on January 1, 1801, Ceres was initially considered a planet, then reclassified as an asteroid, and in 2006 was designated a dwarf planet by the International Astronomical Union.

Ceres orbits the Sun at an average distance of about 2.77 astronomical units and completes one orbit roughly every 4.6 Earth years. It rotates on its axis about once every 9 hours, with a small axial tilt of about 4 degrees.

Physically, Ceres is thought to be differentiated, consisting of a rocky core surrounded by an icy mantle. Its mean density is about 2.16 g/cm3, and its surface shows abundant water ice and hydrated minerals. Evidence from observations and spacecraft data suggests the presence of a subsurface ocean in the past, and possibly today, beneath the crust.

Surface features include bright spots in the Occator crater, interpreted as deposits of salts such as sodium carbonate formed by briny water reaching the surface. The dwarf planet also hosts Ahuna Mons, a prominent cryovolcanic dome, indicating past or present cryovolcanic activity.

Ceres has a tenuous atmosphere, or exosphere, composed mainly of water vapor detected on occasion. Surface temperatures range from about 105 K on the night side to about 235 K in direct sunlight.

The NASA Dawn spacecraft studied Ceres from 2015 to 2018, providing high-resolution imagery and data on its composition, geology, and gravity field. No another dedicated mission has visited since, but Ceres remains a key object for understanding water-rich bodies in the inner solar system.