As other objects were discovered in the area it was realised that Ceres represented the first of a class of many similar bodies. In 1802 Sir William Herschel coined the term asteroid ("star-like") for such bodies, writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes".Sony VAIO VGN-AW160J/Q Battery
As the first such body to be discovered, it was given the designation 1 Ceres under the modern system of asteroid numbering.
The 2006 debate surrounding Pluto and what constitutes a 'planet' led to Ceres being considered for reclassification as a planet.Sony VAIO VGN-AW170Y/Q Battery
A proposal before the International Astronomical Union for the definition of a planet would have defined a planet as "a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet".Sony VAIO VGN-AW180Y/Q Battery
Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun. It was not accepted, and in its place an alternate definition came into effect as of 24 August 2006, carrying the additional requirement that a "planet" must have "cleared the neighborhood around its orbit".Sony VAIO VGN-AW190JAH Battery
By this definition, Ceres is not a planet because it does not dominate its orbit, sharing it with the thousands of other asteroids in the asteroid belt and constituting only about a third of the total mass. It is instead now classified as adwarf planet.
It is sometimes assumed that Ceres has been reclassified as a dwarf planet, and that it is therefore no longer considered an asteroid. Sony VAIO VGN-AW190NAB Battery
For example, a news update at Space.com spoke of "Pallas, the largest asteroid, and Ceres, the dwarf planet formerly classified as an asteroid", while an IAU question-and-answer posting states, "Ceres is (or now we can say it was) the largest asteroid", though it then speaks of "other asteroids" crossing Ceres's path and otherwise implies that Ceres is still one of the asteroids. Sony VAIO VGN-AW190NBB Battery
The Minor Planet Center notes that such bodies may have dual designations. The 2006 IAU decision that classified Ceres as a dwarf planet never addressed whether it is or is not an asteroid, as indeed the IAU has never defined the word 'asteroid' at all, preferring the term 'minor planet' until 2006, and 'small Solar System body' and 'dwarf planet' after 2006. Sony VAIO VGN-AW190NCB Battery
Lang (2011) comments, "The [IAU has] added a new designation to Ceres, classifying it as a dwarf planet. [...] By [its] definition, Eris, Haumea, Makemake and Pluto, as well as the largest asteroid, 1 Ceres, are all dwarf planets", and describes it elsewhere as "the dwarf planet–asteroid 1 Ceres". NASA continues to refer to Ceres as an asteroid, saying in a 2011 press announcement that "Dawn will orbit two of the largest asteroids in the Main Belt", as do various academic textbooks. Sony VAIO VGN-AW190YAB Battery
Ceres is the largest object in the asteroid belt between Mars and Jupiter. The mass of Ceres has been determined by analysis of the influence it exerts on smaller asteroids. Results differ slightly between researchers. The average of the three most precise values as of 2008 is 9.4×1020 kg. Sony VAIO VGN-AW190YBB Battery
With this mass Ceres comprises about a third of the estimated total 3.0 ± 0.2×1021 kg mass of the asteroid belt, which is in turn about 4% of the mass of the Moon. The surface area is approximately equal to the land area of India or Argentina. The mass of Ceres is sufficient to give it a nearly spherical shape in hydrostatic equilibrium. Sony VAIO VGN-AW190YCB Battery
In contrast, other large asteroids such as2 Pallas, 3 Juno, and in particular 10 Hygiea are known to be somewhat irregular in shape.
Ceres's oblateness is inconsistent with an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle. Sony VAIO VGN-AW190YDB Battery
This 100 km-thick mantle (23%–28% of Ceres by mass; 50% by volume) contains 200 million cubic kilometres of water, which is more than the amount of fresh water on the Earth. This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modelling. Sony VAIO VGN-AW220J/B Battery
Also, some characteristics of its surface and history (such as its distance from the Sun, which weakened solar radiation enough to allow some fairly low-freezing-point components to be incorporated during its formation), point to the presence of volatile materials in the interior of Ceres.
Alternatively, the shape and dimensions of Ceres may be explained by an interior that is porous and either partially differentiated or completely undifferentiated. Sony VAIO VGN-AW230J/H Battery
The presence of a layer of rock on top of ice would be gravitationally unstable. If any of the rock deposits sank into a layer of differentiated ice, salt deposits would be formed. Such deposits have not been detected. Thus it is possible that Ceres does not contain a large ice shell, but was instead formed from low-density asteroids with an aqueous component. Sony VAIO VGN-AW235J/B Battery
The decay of radioactive isotopes may not have been sufficient to cause differentiation.
The surface composition of Ceres is broadly similar to that of C-type asteroids. Some differences do exist. The ubiquitous features of the Cererian IRspectra are those of hydrated materials, which indicate the presence of significant amounts of water in the interior. Sony VAIO VGN-AW290JFQ Battery
Other possible surface constituents include iron-rich clays (cronstedtite) and carbonate minerals (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites. The spectral features of carbonates and clay are usually absent in the spectra of other C-type asteroids. Sometimes Ceres is classified as a G-type asteroid. Sony VAIO VGN-AW41JF Battery
The Cererian surface is relatively warm. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (about −38 °C, −36 °F) on 5 May 1991.Only a few Cererian surface features have been unambiguously detected. High-resolution ultraviolet Hubble Space Telescopeimages taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres. Sony VAIO VGN-AW41JF/H Battery
This was thought to be a crater. Later near-infrared images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed several bright and dark features moving with the dwarf planet's rotation.Two dark features had circular shapes and are presumably craters; one of them was observed to have a bright central region, Sony VAIO VGN-AW41MF Battery
while another was identified as the "Piazzi" feature. More recent visible-lightHubble Space Telescope images of a full rotation taken in 2003 and 2004 showed 11 recognizable surface features, the natures of which are currently unknown. One of these features corresponds to the "Piazzi" feature observed earlier. Sony VAIO VGN-AW41MF/H Battery
These last observations also determined that the north pole of Ceres points in the direction of right ascension 19 h 24 min (291°), declination +59°, in the constellation Draco. This means that Ceres's axial tilt is very small—about 3°.