Ceres

Ceres is the very first and largest asteroid to be discovered. It was discovered on the 1^st of January, 1801 by the Italian astronomer Giuseppe Piazzi. Ceres, a main belt asteroid is about 940 km in diameter, although it is not precisely spherical; Ceres’ mass, 8.7 x 10²⁰ kg, represents about 30% of the bulk of the entire main belt, or about 1.2% of the mass of the Moon. This asteroid has a density of 2.0 g/cm3 (2,000 kg/m3), is less than that of most meteorites; it has an acceleration of gravity (on its surface) of 0.26 m/s2, and an escape velocity of 0.5 km/s.

Ceres rotates in about nine hours, its brightness showing little variation, which is indicative of a fairly uniform surface, thought to be powdery in nature. It lies close to the middle of the main belt, at an average heliocentric distance of 2.77 AU. Ceres has an orbital period of 4.60 years (1,680 days), an inclination of 10.6°, and an eccentricity of 0.079. The large asteroid orbits the Sun at an average distance of 414 million km (2.77 AU), with a periapsis of 381 million km (2.55 AU) and an apoapsis of 447 million km (2.98 AU). Ceres has an estimated mean surface temperature of –105°C. It has a low albedo (0.10) and cannot be observed with the naked eye. Since Ceres has a reflectance spectrum that suggests its composition is similar to carbonaceous chondrite meteorites, astronomers classify it as a G-type asteroid, which is a subdivision of the C-type (carbonaceous) asteroids that is distinguished spectroscopically by virtue of differences in the absorption of ultraviolet radiation. Since it was the first to be discovered, it is also called Ceres-1.

The possibility of a planet between Mars and Jupiter had been suggested in the early 1600s by Johannes Kepler, and in the late 18th century Bode’s law was interpreted as implying the likely existence of such a body. In 1800 a group of European astronomers formed the so-called Celestial police, having the aim of discovering this purported planet. Before they could begin their search, however, Ceres was discovered by Giuseppe Piazzi from Palermo, Sicily. Piazzi was checking Nicolas Lacaille’s catalogue of zodiacal stars when he found an uncharted body that moved over the subsequent nights. Piazzi wanted to call the object Ceres Ferdinandea (Ceres is the goddess of fertility, the patron of Sicily, while Ferdinand was the name of the Italian king), but only the first part of that name was accepted by astronomers in other countries. Although he was prevented by illness from following it for an extended period, Piazzi’s observations allowed Ceres to be recovered late in 1801. It soon became apparent that it was not large enough to be considered a major planet, and the rapid discovery of several other such objects in the following years, namely Pallas, Juno and Vesta, added to this view. William Herschel coined the term asteroid for these new objects; they are also termed minor planets.

Questions to Ponder

• How far is Ceres from the Earth?
• Does Ceres have an atmosphere?
• Are Eris and Ceres dwarf planets?