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Ceres (dwarf planet)

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Ceres
Discovery
Discovered byGiuseppe Piazzi
Discovery dateJanuary 1, 1801
Designations
Designation
1 Ceres
A899 OF; 1943 XB
dwarf planet
main belt
Symbol
Orbital characteristics
Epoch November 26, 2005
(JD 2453700.5)[1]
Aphelion447,838,164 km
(2.987 AU)
Perihelion381,419,582 km
(2.544 AU)
414,703,838 km
(2.766 AU)
Eccentricity0.080
17.882 km/s
108.509°
Inclination10.587°
80.410°
Physical characteristics
Dimensions975×909 km[2]
Mass9.46±0.04×1020 kg[3][4]
Mean density
2.08 g/cm³[2]
0.27 m/s²
0.51 km/s
0.3781 d (9.074 h)[5]
Albedo0.113[6]
Surface temp. min mean max
Kelvin ~167 K[7] 239 K[7]
Spectral type
G[8]
3.34[6]

Ceres (/ˈsɪr.iz/, Latin: Cerēs), also designated 1 Ceres (see minor planet names), is the smallest dwarf planet in the Solar System and the only one located in the main asteroid belt. Its name is derived from the Roman goddess Ceres — the goddess of growing plants, the harvest, and of motherly love. It was discovered on January 1 1801, by Giuseppe Piazzi.[9] 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 less gravity.

Name

Piazzi's Book "Della scoperta del nuovo pianeta Cerere Ferdinandea" outlining the discovery of Ceres

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.[10] "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. 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 (cf. its genitive, Cereris).

Symbol

Ceres' astronomical symbol is a sickle, (Sickle variant symbol of Ceres), similar to Venus' symbol (Astronomical symbol of Venus) which is the female gender symbol and Venus' hand mirror. There have been several variants of the sickle design, including Old symbol of Ceres, Mirror variant symbol of Ceres and Other sickle variant symbol of Ceres.

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. 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".[11] 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.

Status

Ceres (left) in comparison with the Moon (right).

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,[12] writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes".[13] 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.[14][15] 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".[16] Had this resolution been adopted, this would have made Ceres the fifth planet in order from the Sun.[17] 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.[18]

Orbit

Orbit of Ceres

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° to be compared with 7° for Mercury and 17° for Pluto) and moderately eccentric (e=0.08 to compare with 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. Interestingly, the perihelia of Ceres (as well as those of several of the largest main belt asteroids) and Mars are on the opposite sides of the Sun. 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.[19]

Physical characteristics

Size comparison: the first 10 minor planets profiled against Earth's Moon. Ceres is far left.
Hubble Space Telescope images of Ceres, taken in 2003/4 with a resolution of about 30 km. The nature of the bright spot is uncertain. A movie was also made.[20]
File:1 ceres.png
Hubble Space Telescope UV image of Ceres, taken in 1995 with a resolution of about 60 km. The "Piazzi" feature is the dark spot in the center.

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 Eris, Pluto, 50000 Quaoar, 90482 Orcus, and 90377 Sedna.

At certain points in its orbit, Ceres can reach a visual magnitude of 7.0. 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[21]

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,[3] 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.[7] Taking into account also the heliocentric distance at the time, this gives an estimated maximum of ~239 K at perihelion.

Diagram showing differentiated layers of Ceres

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.[22][2]

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.[23] 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°).[2]

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:

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.[3]

Exploration of Ceres

Artist's conception of Dawn visiting Ceres and Vesta.

To date no space probes have visited Ceres. However, NASA is currently preparing the Dawn Mission for a launch on 7 July 2007. According to the current mission profile, Dawn is expected to 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.[27] 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.[28]

Namesakes

  • The chemical element cerium (atomic number 58) was discovered in 1803 by Berzelius and Klaproth, working independently. Berzelius named the element after Ceres.[29]
  • William Hyde Wollaston discovered palladium (atomic number 46) as early as 1802 and at first called it Ceresium. By the time he openly published his discovery in 1805, the name was already taken (by Berzelius) and he switched it to palladium in honour of 2 Pallas.[30]

See also

Notes

  1. ^ Ted Bowell, Bruce v (January 2, 2003). "Asteroid Observing Services". Lowell Observatory. Retrieved 2007-01-17.
  2. ^ a b c d P. C. Thomas et al Differentiation of the asteroid Ceres as revealed by its shape, Nature, Vol. 437, pp. 224 (2005).
  3. ^ a b c Pitjeva, E. V. (2005). "High-Precision Ephemerides of Planets — EPM and Determination of Some Astronomical Constants" (PDF). Solar System Research. 39 (3): 176. doi:10.1007/s11208-005-0033-2.
  4. ^ D. T. Britt et al Asteroid density, porosity, and structure, pp. 488 in Asteroids III, University of Arizona Press (2002).
  5. ^ Harris, A. W. (2006). "Asteroid Lightcurve Derived Data. EAR-A-5-DDR-DERIVED-LIGHTCURVE-V8.0". NASA Planetary Data System. Retrieved 2007-03-15. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ a b Tedesco, E.F. (2004). "IRAS Minor Planet Survey. IRAS-A-FPA-3-RDR-IMPS-V6.0". NASA Planetary Data System. Retrieved 2007-03-15. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ a b c O. Saint-Pé Ceres surface properties by high-resolution imaging from earth, Icarus, vol. 105 pp. 271 (1993).
  8. ^ Neese, C. (2005). "Asteroid Taxonomy.EAR-A-5-DDR-TAXONOMY-V5.0". NASA Planetary Data System. Retrieved 2007-03-15. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  9. ^ Piazzi, Giuseppe (1801). Risultati delle osservazioni della nuova Stella scoperta il dì 1 gennajo all'Osservatorio Reale di Palermo (in Italian). Palermo.
  10. ^ Hoskin, Michael. "Bodes' Law and the Discovery of Ceres". Osservatorio Astronomico Di Palermo Giuseppe S. Vaiana. Retrieved 2007-01-17.
  11. ^ Hoskin, Michael. "8. Piazzi and the Discovery of Ceres". Bode's Law and the Disovery of Ceres. Osservatorio Astronomico di Palermo, Giuseppe S. Vaiana. Retrieved 2006-08-16.
  12. ^ Hilton, Dr. J. L. (September 17 2001). "When Did the Asteroids Become Minor Planets?". Retrieved 2006-08-16. {{cite web}}: Check date values in: |year= (help)CS1 maint: year (link)
  13. ^ Herschel, William (May 6 1802). "Observations on the two lately discovered celestial Bodies.". {{cite web}}: Check date values in: |year= (help)CS1 maint: year (link)
  14. ^ Battersby, Stephen (August 16, 2006). "Planet debate: Proposed new definitions". New Scientist. Retrieved 2007-04-27.
  15. ^ Connor, Steve (August 16, 2006). "Solar system to welcome three new planets". NZ Herald. Retrieved 2007-04-27.
  16. ^ Owen Gingerich; et al. (August 16, 2006). "The IAU draft definition of "Planet" and "Plutons"". IAU. Retrieved 2007-04-27. {{cite web}}: Explicit use of et al. in: |author= (help)
  17. ^ Staff Writers (August 16, 2006). "The IAU Draft Definition Of Planets And Plutons". Space Daily. Retrieved 2007-04-27.
  18. ^ Richard Binzel; et al. (August 24, 2006). "IAU 2006 General Assembly: Result of the IAU resolution votes". IAU. Retrieved 2007-04-27. {{cite web}}: Explicit use of et al. in: |author= (help) — "Ceres was an asteroid" - but note it then talks about "other asteroids" crossing Ceres' path.
  19. ^ Kelley, M. S.; Gaffey, M. J. (1996). "A Genetic Study of the Ceres (Williams #67) Asteroid Family". Bulletin of the American Astronomical Society. 28: 1097. Retrieved 2007-04-27.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  20. ^ Cooke, Bill (September 12 2005). "An icy interior for Ceres?". Astronomy. {{cite news}}: Check date values in: |date= (help) movie credit J. Parker, Southwest Research Institute
  21. ^ Martinez, Patrick, The Observer's Guide to Astronomy, page 298. Published 1994 by Cambridge University Press
  22. ^ "Largest Asteroid Might Contain More Fresh Water than Earth". SPACE.com. 07 September 2005. Retrieved 2006-08-16. {{cite news}}: |first= missing |last= (help); Check date values in: |date= (help); Unknown parameter |lat= ignored (help)
  23. ^ a b "Largest Asteroid May Be 'Mini Planet' with Water Ice". HubbleSite. September 7 2005. Retrieved 2006-08-16. {{cite news}}: Check date values in: |date= (help)
  24. ^ "Observations reveal curiosities on the surface of asteroid Ceres". Retrieved 2006-08-16.
  25. ^ J. W. Parker et al Analysis of the first disk-resolved images of Ceres from ultraviolet observations with the Hubble Space Telescope, The Astronomical Journal, Vol. 123 pp. 549 (2002).
  26. ^ Staff (October 11, 2006). "Keck Adaptive Optics Images the Dwarf Planet Ceres". Adaptive Optics. Retrieved 2007-04-27.
  27. ^ Rayman, Marc (2006-07-13). "Dawn: mission description". UCLA — IGPP Space Physics Center. Retrieved 2007-04-27.
  28. ^ Ambrosiano, Nancy (April 10, 2007). "GRaND science instrument moves closer to launch from Cape". Los Alamos National Laboratory. Retrieved 2007-04-27.
  29. ^ Staff. "Cerium: historical information". Adaptive Optics. Retrieved 2007-04-27.
  30. ^ "Amalgamator Features 2003: 200 Years Ago". Retrieved 2006-08-21.

References

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