Bruker:Murgh/Ceres (dvergplanet)

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Ceres er den minste dvergplaneten i vårt solsystem, og den eneste som befinner seg i Asteroidebeltet. Den ble oppdagt den 1. januar, 1801 av Giuseppe Piazzi. Med en diameter på ca. 950 km er Ceres på langt nær det største legemet i Asteroidebeltet, og inneholder en tredjedel av Asteroidebeltets samlede masse.

[rediger] Navn

Ceres het opprinnelig Ceres Ferdinandea (Italiensk, Cerere Ferdinandea) etter både den Romersk-mytologiske guden Ceres (beskytter av voksende planter og morskjærlighet) og Kong Ferdinand III av Sicilia]]. "Ferdinandea" var et uakseptablet navn i andre land og ble fjernet. Ceres het også Hera en stund i Tyskland. In Hellas heter den Δήμητρα (Demeter), etter Ceres' Greske motstykke, men i resten av verden betegner navnet Demeter et annet legeme (asteroiden 1108 Demeter).

[rediger] Symbol

Ceres' astronomiske symbol er en sigd, ʡ U+02A1 (), noe likt Venus' symbol (). Det har eksistert flere varianter av sigdsymbolet, blant annet , Bilde:1 Ceres (1).png og .

[rediger] Oppdagelse

Piazzi på jakt etter en stjerne beskrevet av Francis Wollaston som Mayer 87 fordi den ikke fantes i Mayers zodiak katalog i nedtegnede posisjon. I stedet fant Piazzi et sterne-lignende objekt i bevegelse, som han først trodde vart en komet.

Piazzi observerte Ceres 24 ganger i alt, sist den 11. februar da sykdom intraff. Den 24. januar 1801 erklærte Piazzi sin oppdagelse i brev til astronomkolleger, blant annet hans landsmann, Barnaba Oriani fra Milan. Han beskrev den som en komet, men "siden dets bevegelse er så sakte og uniform, slår det meg at det er noe bedre enn en komet." ^[1] I april sendte Piazzi komplette obvservasjoner til Oriani, Bode, og Lalande i Paris, og i september ble de publisert i Monatliche Correspondenz.

For å finne igjen Ceres utviklet den da 24 år gamle Carl Friedrich Gauss en metode for baneberegning ut i fra tre observasjoner. på få uker klarte han å kalkulere banen, og han sendte dokumentasjonen til Franz Xaver, Baron von Zach, Monatliche Correspondenz' forlegger. Den 31. desember 1801 erklærte von Zach og Heinrich W. M. Olbers Ceres for gjenfunnet.

[rediger] Status

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

Ceres klassifikasjon har blitt endret flere ganger. Ved oppdagelsen ble den regnet som en planet; i 150 var den klassifisert som en asteroide; den fikk først status som dvergplanet i 2006.

Johann Elert Bode anså Ceres som "den savnede planet" som Johann Daniel Titius hadde hevdet opprinnelig eksisterte mellom Mars og Jupiter på en 419 million km (2.8 AU) fra Solen. Ceres ble gitt et astronomisk symbol og ført opp på listen over planeter (sammen med 2 Pallas, 3 Juno og 4 Vesta) som sto i ca. 50 år inntil flere asteroider ble oppdaget. Men ettersom mange flere objekter ble funnet i området, ble det vedtatt at disse representerte de første av en annen klasse himmellegemer. Sir William Herschel fant i 1802 opp begrepet asteroide ("stjerneaktig") for slike legemer.^[2] Han skrev, "de ligner små stjerner slik at det er vanskelig å skille dem ut, selv med svært gode teleskoper."^[3]

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^[4] Som den første av sitt slag ble fikk den navnet 1 Ceres under det moderne nummersystemet for asteroider.

Debatten i 2006 rundt Pluto og hva som danner grunnlaget for klassifiseringen planet, førte til at Ceres' status ble revurdert. Et forslag ble lagt fremfor den internasjonale astronomiske union for redefinisjon av en planet som "et himmellegeme som: a) har tilstrekkelig masse til at objektets egen-gravitasjon er kraftigere enn egen fysisk moststand, slik ... at hydrostatisk equilibrium (nær rund) form kan dannes og har ryddet sitt "nabolag" i egen bane, og b) går i bane rundt en stjerne, og er selv ikke stjerne eller en planets satellitt."^[5]^[6] Om dette hadde blitt vedtatt ville Ceres blitt den 5. planet fra Solen.^[7] Forslaget ble ikke akseptert, og en alternativ 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."

Had this resolution been adopted, this would have made Ceres the fifth planet in order from the Sun.^[8] 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.^[9]

[rediger] 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 other of the largest MBAs) 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.

[rediger] Physical characteristics

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.^[10]

Bilde: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 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 the asteroid with the naked eye. The only other asteroid that can be seen with the naked eye is 4 Vesta.

Ceres' size and mass are sufficient to give it a nearly spherical shape. That is, it is close to hydrostatic equilibrium. Other 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 all the asteroids in the solar system^[11] (note how all these amount to only 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.^[12] 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.^[13]^[14]

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 asteroid'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.^[15] 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°).^[14]

Ceres was long thought to be the parent body of the "Ceres asteroid family". However, that grouping is now defunct because Ceres has been shown to be an interloper in its "own" family, and physically unrelated. The bulk of that asteroid group is now called the Gefion family.

[rediger] 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.^[16]^[17]
Visible images with 60 km resolution taken with the Keck telescope in 2002 using adaptive optics.^[18]
Infrared images with 30 km resolution also taken with the Keck telescope in 2002 using adaptive optics.^[19]
The best resolution to date (30 km) visible light images using Hubble again in 2003 and 2004.^[15]

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.^[11]

[rediger] Exploration of Ceres

Artist's conception of Dawn visiting Ceres and Vesta.

To date no space probes have visited Ceres. However, NASA is currently developing the Dawn Mission, with a projected launch in 2007. According to the current mission profile, Dawn is expected to explore the asteroid 4 Vesta in 2011 before arriving at Ceres in 2015.

[rediger] Namesakes

The chemical element cerium (atomic number 58) was discovered in 1803 by Berzelius and Klaproth, working independently. Berzelius named the element after Ceres. ^[20]
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. ^[21]

[rediger] Aspects

Stationary, retrograde	Opposition	Distance to Earth (AU)	Maximum brightness (mag)	Stationary, prograde	Conjunction to Sun
March 21 2005	May 8 2005	1.68631	7.0	June 30 2005	December 28 2005
June 26 2006	August 12 2006	1.98278	7.6	November 27 2006	March 22 2007
September 20 2007	November 9 2007	1.83690	7.2	January 1 2008	June 28 2008
January 17 2009	February 24 2009	1.58526	6.9	April 16 2009	October 31 2009
April 28 2010	June 18 2010	1.81988	7.0	August 9 2010	January 30 2011
July 31 2011	September 16 2011	1.99211	7.7	November 12 2011	April 26 2012
October 30 2012	December 17 2012	1.68842	6.7	February 4 2013	August 17 2013
March 1 2014	April 15 2014	1.63294	7.0	June 7 2014	December 10 2014
June 6 2015	July 25 2015	1.94252	7.5	September 16 2015	March 3 2016
September 1 2016	October 20 2016	1.90844	7.4	December 15 2016	June 5 2017
December 21 2017	January 31 2018	1.59531	8.8	March 20 2018	October 7 2018
April 9 2019	May 29 2019	1.74756	7.0	July 20 2019	January 14 2020
July 13 2020	August 28 2020	1.99916	7.7	October 23 2020	April 7 2021

Den fikk denne betegnelsen i august 2006, etter å ha blitt regnet som den største av asteroidene.

Den er også den først oppdagede av asteroidene, og hadde derfor tallet 1 foran det offisielle navnet (1 Ceres). Den ble oppdaget i 1801 av den italienske munk og astronom Giuseppe Piazzi i Palermo. Den ligger i asteroidebeltet mellom Mars og Jupiter.

Da Ceres ble oppdaget i 1801, mente Piazzi at det måtte være en planet. Senere ble det funnet mange mindre objekter i samme område, og disse ble etterhvert klassifisert som asteroider. Ceres var imidlertid stor sammenlignet med andre asteroider, og er nesten kuleformet, så den falt innenfor den definisjonen av en dvergplanet som astronomer vedtok under en konferanse i Praha i august 2006.

Soloppgang på Ceres

Det var IAU, Den Internasjonale Astronomiske Union, som tok stilling til dette. Ved siden av Ceres ble da også Pluto, og Eris erklært å være dvergplaneter.