Ceres

Ceres /ˈsɪəriːz/ (minor-planet designation: 1 Ceres) is the largest object in the main asteroid belt that lies between the orbits of Mars and Jupiter. With a diameter of 940 km (580 mi), Ceres is both the largest of the asteroids and the only dwarf planet inside Neptune's orbit. It is the 25th-largest body in the Solar System within the orbit of Neptune.

Ceres was the first asteroid to be discovered by Giuseppe Piazzi at Palermo Astronomical Observatory on 1 January 1801. It was originally considered a planet, but was reclassified as an asteroid in the 1850s after many other objects in similar orbits were discovered.

Ceres is the only object in the asteroid belt rounded by its own gravity, although Vesta and perhaps other asteroids were so in the past. From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, peaking once at opposition every 15 to 16 months, which is its synodic period. Thus even at its brightest, it is too dim to be seen by the naked eye, except under extremely dark skies. Ceres has been classified as a C-type asteroid and, due to the presence of clay minerals, as a G-type asteroid.

Ceres appears to be partially differentiated into a muddy (ice-rock) mantle/core and a less-dense but stronger crust that is at most 30 percent ice. It probably no longer has an internal ocean of liquid water, but there is brine that can flow through the outer mantle and reach the surface. The surface is a mixture of water ice and various hydrated minerals such as carbonates and clay. Cryovolcanoes such as Ahuna Mons form at the rate of about one every fifty million years. In January 2014, emissions of water vapor were detected from several regions of Ceres. This was unexpected because large bodies in the asteroid belt typically do not emit vapor, a hallmark of comets. The atmosphere, however, is transient and of the minimal kind known as an exosphere.

The robotic NASA spacecraft Dawn entered orbit around Ceres on 6 March 2015.