Ceres  

Ceres
Discovery
Discovered by:	Giuseppe Piazzi
Discovery date:	January 1, 1801
Orbital characteristics
Epoch November 26, 2005 (JD 2453700.5)[1]
Aphelion distance:	447,838,164 km 2.987 AU
Perihelion distance:	381,419,582 km 2.544 AU
Semi-major axis:	414,703,838 km 2.765 956 424 AU[2]
Eccentricity:	0.07976017<ref name="jpl_sbdb" />
Orbital period:	1679.819 days 4.599 years
Avg. orbital speed:	17.882 km/s
Mean anomaly:	108.509°
Inclination:	10.586712°<ref name="jpl_sbdb" />
Longitude of ascending node:	80.40696°<ref name="jpl_sbdb" />
Argument of perihelion:	73.15073°<ref name="jpl_sbdb" />
Physical characteristics
Equatorial radius:	487.3 Â± 1.8 km[3]
Polar radius:	454.7 Â± 1.6 km[3]
Mass:	9.46 ± 0.041020 kg[4][5]
Mean density:	2.08 g/cm³[3]
Equatorial surface gravity:	0.27 m/s² 0.028 g
Escape velocity:	0.51 km/s
Sidereal rotation period:	0.3781 d 9.074 h[6]
Albedo:	0.113[7]
Surface temp.:    Kelvin	min mean max ~167 K[8] 239 K<ref name="Saint-Pe1993" />
Spectral type:	G[9]
Apparent magnitude:	6.7 <ref name="Pasachoff1983" />
Absolute magnitude:	3.34<ref name="iras" />
Angular size:	0.84"[10]

Ceres (IPA: /ˈsɪəriz/, Latin: Cerēs), also designated 1 Ceres (see minor planet names), is the smallest dwarf planet in the Solar System and the only one in the main asteroid belt. It was discovered on January 1 1801, by Giuseppe Piazzi,^[11] and is named after the Roman goddess Ceres—the goddess of growing plants, the harvest, and of motherly love.

With a diameter of about 950 km, Ceres is by far the largest and most massive body in the asteroid belt, and contains approximately a third of the belt's total mass. Recent observations have revealed that it is spherical, unlike the irregular shapes of smaller bodies with lower gravity. At its brightest it is still too dim to be seen with the naked eye.^[12]

On September 27, 2007, NASA launched the Dawn Mission space probe to explore Ceres and Vesta.

Discovery

Piazzi was searching for a star listed by Francis Wollaston as Mayer 87 because it was not in Mayer's zodiacal catalogue in the position given.^[13] Instead, Piazzi found a moving star-like object, which he thought at first was a comet.

Piazzi observed Ceres a total of 24 times, the final time on February 11, when illness interrupted. On January 24 1801, Piazzi announced his discovery in letters to fellow astronomers, among them his fellow countryman, Barnaba Oriani of Milan. He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet".<ref name="hoskin" /> In April, Piazzi sent his complete observations to Oriani, Bode, and Lalande in Paris. The information was published in the September, 1801 issue of the Monatliche Correspondenz.

To recover Ceres, Carl Friedrich Gauss, then only 24 years old, developed a method of orbit determination from three observations. In only a few weeks, he predicted its path, and sent his results to Franz Xaver, Baron von Zach, the editor of the Monatliche Correspondenz. On December 31 1801, von Zach and Heinrich W. M. Olbers unambiguously confirmed the recovery of Ceres.

Name

Piazzi originally suggested the name Ceres Ferdinandea (Ital­ian, Cerere Ferdinan­dea) for this body, after both the mythological figure Ceres (Roman goddess of plants) and King Ferdinand III of Sicily.^[14] "Ferdinandea" was not acceptable to other nations of the world and was thus dropped. Ceres was also called Hera for a short time in Germany.^[15] In Greece, it is called Δήμητρα (Demeter), after the goddess Ceres' Greek equivalent; in English usage, Demeter is the name of an asteroid (1108 Demeter).

Due to the rarity of the usage, there is no consensus as to the proper adjectival form of the name, although the nonce forms Cerian and Cerean have been used in fiction. Grammatically, the form Cererean would be correct, derived from its Latin genitive, Cereris.

The element Cerium was named after Ceres.^[16] The element Palladium was originally also named after Ceres, but the discoverer changed its name after Cerium was named. Palladium is named after asteroid 2 Pallas.^[17]

Ceres' astronomical symbol is a sickle, (), similar to Venus' symbol () which is the female gender symbol and Venus' hand mirror.^[18] There have been several variants of the sickle design, including , and .

Status

The classification of Ceres has changed more than once and has been the subject of some disagreement. At the time of its discovery it was considered a planet; but within forty-nine years it was reclassified by many astronomers as an asteroid. This lasted for over 150 years; and was first classified a dwarf planet in 2006.

Johann Elert Bode believed Ceres to be the "missing planet" that Johann Daniel Titius had proposed to exist between Mars and Jupiter, at a distance of 419 million km (2.8 AU) from the Sun. Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno and 4 Vesta) for about half a century until further asteroids were discovered. However as further objects were discovered in the area it was realised that it represented the first of a class of many similar bodies. Sir William Herschel coined in 1802 the term asteroid ("star-like") for such bodies,^[19] writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes".^[20] 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.^[21][22] An unsuccessful 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".^[23] Had this resolution been adopted, this would have made Ceres the fifth planet in order from the Sun.^[24] This draft definition was not accepted, and in its place an alternate definition of "planet" came into effect as of August 24, 2006. Under this definition, a 'planet' is "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit." By this definition, Ceres (along with Pluto) cannot be classified as a planet, and both are now classified as "dwarf planets", although it remains unclear as to whether or not it is also classified as an asteroid.^[25]

Physical characteristics

Ceres is the largest object in the asteroid belt, which mostly lies between Mars and Jupiter. However, it is not the largest object besides the Sun, planets and their satellites, in the solar system: the Kuiper belt is known to contain larger objects, including Pluto, 50000 Quaoar, and 90482 Orcus, while more distant Eris, in the scattered disc, is the largest of all these objects. The still more distant 90377 Sedna, variously described as a member of the 'inner Oort cloud' or 'extended scattered disc', is also larger.

At certain points in its orbit, Ceres can reach a visual magnitude of +6.7.^[27] This is generally regarded as being just barely too dim to be seen with the naked eye, but under exceptional viewing conditions a very sharp-sighted person may be able to see this dwarf planet with the naked eye. The only asteroids that can reach so bright a magnitude are 4 Vesta, and, during rare oppositions near perihelion, 2 Pallas and 7 Iris.^[28] At conjunction Ceres has a magnitude of around +9.5, which corresponds to the faintest objects visible with 10×50 binoculars. It can thus be seen with binoculars whenever it is above the horizon of a fully dark sky. Pallas and Iris, though brighter at a near-perihelion opposition, are beyond binocular visibility at small elongations.

Ceres' size and mass are sufficient to give it a nearly spherical shape. That is, it is close to hydrostatic equilibrium. Large asteroids such as 2 Pallas, 3 Juno, and 4 Vesta are known to be quite irregular, while lightcurve analysis of 10 Hygiea indicates it is oblong although it appears spheroidal in low-resolution images (presumably due to viewing angle).

With a mass of 9.5 10²⁰ kg, Ceres comprises about a third of the estimated total 3.0 ± 0.2 10²¹ kg mass of the asteroids in the solar system,<ref name="Pitjeva2005" /> together totalling about 4% of the mass of the Moon.

There are some indications that the surface of Ceres is relatively warm and that it may have a tenuous atmosphere and frost. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (about -38°C) on May 5 1991.^[8] Taking into account also the heliocentric distance at the time, this gives an estimated maximum of ~239 K at perihelion.



A study led by Peter Thomas of Cornell University suggests that Ceres has a differentiated interior: observations coupled with computer models suggest the presence of a rocky core overlain with an icy mantle. This mantle of thickness from 120 to 60 km could contain 200 million cubic kilometres of water, which is more than the amount of fresh water on the Earth.^[29][3]

There has been some ambiguity regarding surface features on Ceres. Low resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres. This was thought to be a crater. Later images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed no sign of "Piazzi". However, two dark features were seen to move with the dwarf planet's rotation, one with a bright central region. These are presumably craters. More recent visible light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 show an enigmatic white spot, the nature of which is currently unknown.^[30] The dark albedo features seen with Keck are, however, not immediately recognizable in these images.

These last observations also determined that Ceres' north pole points (give or take about 5°) in the direction of right ascension 19 h 24 min, declination +59°, in the constellation Draco. This means that Ceres' axial tilt is very small (about 4±5°).^[3]

Orbit

Ceres follows an orbit between Mars and Jupiter, within the main asteroid belt, with a period of 4.6 years. The orbit is moderately inclined (i=10.6° compared to 7° for Mercury and 17° for Pluto) and moderately eccentric (e=0.08 compared to 0.09 for Mars).

The diagram illustrates the orbits of Ceres (blue) and several planets (white/grey). The segments of orbits below the ecliptic are plotted in darker colours, and the orange plus sign is the Sun's location. The top left diagram is a polar view that shows the location of Ceres in the gap between Mars and Jupiter. The top right is a close-up demonstrating the locations of the perihelia (q) and aphelia (Q) of Ceres and Mars. The perihelion of Mars is on the opposite side of the Sun from those of Ceres and several of the large main belt asteroids, including 2 Pallas and 10 Hygiea. The bottom diagram is a perspective view showing the inclination of the orbit of Ceres compared to the orbits of Mars and Jupiter.

In the past, Ceres had been considered to be the largest member of an asteroid family. These groupings of asteroids share similar orbital elements, which can indicate a common origin through an asteroid collision some time in the past. In the case of Ceres, however, it has different spectral properties from the other members of the family, and so this grouping is now called the Gefion family, named after the lowest-numbered family member, 1272 Gefion. Ceres is merely an interloper in this family, coincidentally having similar orbital elements but not a common origin.^[31]

Observations

Some notable observation milestones for Ceres include:

An occultation of a star by Ceres was observed in Mexico, Florida and across the Caribbean on November 13 1984.

Features on Ceres' surface have been telescopically imaged several times in recent years.

These include:

• Ultraviolet Hubble Space Telescope images with 50 km resolution taken in 1995.^[32][33]
• Visible images with 60 km resolution taken with the Keck telescope in 2002 using adaptive optics.
• Infrared images with 30 km resolution also taken with the Keck telescope in 2002 using adaptive optics.^[34]
• The best resolution to date (30 km) visible light images using Hubble again in 2003 and 2004.^[30]

Radio signals from spacecraft in orbit around Mars and on its surface have been used to estimate the mass of Ceres from the perturbations induced by it onto the motion of Mars.^[4]

Exploration

To date no space probes have visited Ceres. However, NASA launched the Dawn Mission on 27 September 2007, which will explore the asteroid 4 Vesta in 2011 before arriving at Ceres in 2015.

The mission profile calls for the Dawn Spacecraft to enter orbit around Ceres at an altitude of 5,900 km. After five months of study the spacecraft will reduce the orbital distance to 1,300 km, then down to 700 km after another five months.^[35] The spacecraft instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutron detector. These will be used to examine the dwarf planet's shape and element abundance.^[36]

See also

• Ceres in fiction
• List of Solar System bodies formerly regarded as planets
• Colonization of Ceres
• Dwarf planet
• Asteroid Belt

Notes

References

External links

• Movie of one Ceres rotation (processed Hubble images)
• How Gauss determined the orbit of Ceres from keplersdiscovery.com
• An up-to-date summary of knowledge about Ceres, plus an Earth-Ceres size comparison (the Planetary Society)
• A simulation of the orbit of Ceres
• A website dedicated entirely to 1 Ceres

Minor planets
(asteroid navigator)	1 Ceres	Next minor planet
List of asteroids

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