 = 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 and gray ) . The segments of orbits below the ecliptic are plotted in darker colors , 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 <unk> ( q ) and <unk> ( Q ) of Ceres and Mars . In this diagram ( but not in general ) , 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 <unk> . The bottom diagram is a side view showing the inclination of the orbit of Ceres compared to the orbits of Mars and Jupiter . 
 Ceres was once thought to be a member of an asteroid family . The asteroids of this family share similar proper orbital elements , which may indicate a common origin through an asteroid collision some time in the past . Ceres was later found to have spectral properties different from other members of the family , which is now called the Gefion family after the next @-@ lowest @-@ numbered family member , <unk> Gefion . Ceres appears to be merely an <unk> in the Gefion family , <unk> having similar orbital elements but not a common origin . 
 
 = = = <unk> = = = 
 
 Ceres is in a near @-@ 1 : 1 mean @-@ motion orbital resonance with Pallas ( their proper orbital periods differ by 0 @.@ 2 % ) . However , a true resonance between the two would be unlikely ; due to their small masses relative to their large <unk> , such relationships among asteroids are very rare . Nevertheless , Ceres is able to capture other asteroids into temporary 1 : 1 <unk> orbital relationships ( making them temporary <unk> ) for periods up to 2 million years or more ; fifty such objects have been identified . 
 
 = = = Transits of planets from Ceres = = = 
 
 Mercury , Venus , Earth , and Mars can all appear to cross the Sun , or transit it , from a <unk> point on Ceres . The most common transits are those of Mercury , which usually happen every few years , most recently in 2006 and 2010 . The most recent transit of Venus was in 1953 , and the next will be in <unk> ; the corresponding dates are 1814 and <unk> for transits of Earth , and 767 and <unk> for transits of Mars . 
 
 = = <unk> and axial tilt = = 
 
 The rotation period of Ceres ( the Cererian day ) is 9 hours and 4 minutes . It has a small axial tilt of 4 ° . This is nevertheless large enough for Ceres 's polar regions to contain permanently shadowed craters that are expected to act as cold traps and accumulate water ice over time , similar to the situation on the Moon and Mercury . About 0 @.@ 14 % of water molecules released from the surface are expected to end up in the traps , hopping an average of 3 times before escaping or being trapped . 
 
 = = Geology = = 
 
 Ceres has a mass of 9 @.@ 39 × <unk> kg as determined from the Dawn spacecraft . With this mass Ceres comprises approximately a third of the estimated total 3 @.@ 0 ± 0 @.@ 2 × <unk> kg mass of the asteroid belt , which is in turn approximately 4 % of the mass of the Moon . Ceres is massive enough to give it a nearly spherical , equilibrium shape . Among Solar System bodies , Ceres is intermediate in size between the smaller Vesta and the larger <unk> . Its surface area is approximately the same as the land area of India or Argentina . 
 
 = = = Surface = = = 
 
 The surface composition of Ceres is broadly similar to that of C @-@ type asteroids . Some differences do exist . The ubiquitous features in Ceres ' IR spectrum are those of hydrated materials , which indicate the presence of significant amounts of water in its interior . Other possible surface constituents include iron @-@ rich clay minerals ( <unk> ) and carbonate minerals ( <unk> and <unk> ) , which are common minerals in <unk> <unk> meteorites . The spectral features of carbonates and clay minerals are usually absent in the spectra of other C @-@ type asteroids . Sometimes Ceres is classified as a G @-@ type asteroid . 
 Ceres ' surface is relatively warm . The maximum temperature with the Sun overhead was estimated from measurements to be 235 K ( approximately − 38 ° C , − 36 ° F ) on 5 May 1991 . Ice is unstable at this temperature . Material left behind by the sublimation of surface ice could explain the dark surface of Ceres compared to the icy moons of the outer Solar System . 
 
 = = = = Observations prior to Dawn = = = = 
 
 Prior to the Dawn mission , only a few surface features had been unambiguously detected on Ceres . High @-@ resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface , which was nicknamed " Piazzi " in honor of the <unk> of Ceres . 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 Ceres ' rotation . Two dark features had circular shapes and were presumed to be craters ; one of them was observed to have a bright central region , whereas another was identified as the " Piazzi " feature . Visible @-@ light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 showed eleven recognizable surface features , the natures of which were then undetermined . One of these features corresponds to the " Piazzi " feature observed earlier . 
 These last observations indicated that the north pole of Ceres pointed in the direction of right ascension 19 h 24 <unk> ( 291 ° ) , declination + 59 ° , in the constellation <unk> , resulting in an axial tilt of approximately 3 ° . Dawn later determined that the north polar axis actually points at right ascension 19 h 25 m 40 @.@ 3 s ( 291 @.@ <unk> ° ) , declination + 66 ° 45 ' 50 " ( about 1 @.@ 5 degrees from Delta <unk> ) , which means an axial tilt of 4 ° . 
 
 = = = = Observations by Dawn = = = = 
 
 Dawn revealed a large number of craters with low relief , indicating that they lie over a relatively soft surface , probably of water ice . One crater , with extremely low relief , is 270 km ( 170 mi ) in diameter , reminiscent of large , flat craters on <unk> and <unk> . An unexpectedly large number of Cererian craters have central pits , and many have central peaks . Several bright spots have been observed by Dawn , the brightest spot ( " Spot 5 " ) located in the middle of an 80 @-@ kilometer ( 50 mi ) crater called <unk> . From images taken of Ceres on 4 May 2015 , the secondary bright spot was revealed to actually be a group of scattered bright areas , possibly as many as ten . These bright features have an albedo of approximately 40 % that are caused by a substance on the surface , possibly ice or salts , reflecting sunlight . A haze periodically appears above Spot 5 , the best known bright spot , supporting the hypothesis that some sort of outgassing or <unk> ice formed the bright spots . In March 2016 , Dawn found definitive evidence of water molecules on the surface of Ceres at <unk> crater . <unk> states : " This water could be bound up in minerals or , alternatively , it could take the form of ice . " 
 On 9 December 2015 , NASA scientists reported that the bright spots on Ceres may be related to a type of salt , particularly a form of <unk> containing magnesium sulfate <unk> ( <unk> · <unk> ) ; the spots were also found to be associated with ammonia @-@ rich <unk> . Another team thinks the salts are sodium carbonate . 
 
 = = = Internal structure = = = 
 
 Ceres ' <unk> is consistent with a differentiated body , a rocky core <unk> with an icy mantle . This 100 @-@ kilometer @-@ thick mantle ( 23 % – 28 % of Ceres by mass ; 50 % by volume ) contains up to 200 million cubic kilometers of water , which would be more than the amount of fresh water on Earth . This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modeling . 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 . It has been suggested that a remnant layer of liquid water may have survived to the present under a layer of ice . Measurements taken by Dawn confirm that Ceres is both differentiated and has a shape consistent with hydrostatic equilibrium , which makes Ceres the smallest object confirmed to be in hydrostatic equilibrium , being 600 km smaller and less than half the mass of Saturn 's moon Rhea , the next smallest such object . <unk> has suggested Ceres could have a small metallic core from partial differentiation of its rocky fraction . 
 
 = = <unk> = = 
 
 There are indications that Ceres may have a tenuous water vapor atmosphere outgassing from water ice on the surface . 
 Surface water ice is unstable at distances less than 5 AU from the Sun , so it is expected to sublime if it is exposed directly to solar radiation . Water ice can migrate from the deep layers of Ceres to the surface , but escapes in a very short time . As a result , it is difficult to detect water <unk> . Water escaping from polar regions of Ceres was possibly observed in the early 1990s but this has not been unambiguously demonstrated . It may be possible to detect escaping water from the surroundings of a fresh impact crater or from cracks in the subsurface layers of Ceres . <unk> observations by the <unk> spacecraft detected statistically significant amounts of hydroxide ions near Ceres ' north pole , which is a product of water vapor <unk> by ultraviolet solar radiation . 
 In early 2014 , using data from the Herschel Space Observatory , it was discovered that there are several localized ( not more than 60 km in diameter ) mid @-@ latitude sources of water vapor on Ceres , which each give off approximately <unk> molecules ( or 3 kg ) of water per second . Two potential source regions , designated Piazzi ( 123 ° E , 21 ° N ) and Region A ( 231 ° E , 23 ° N ) , have been visualized in the near infrared as dark areas ( Region A also has a bright center ) by the W. M. Keck Observatory . Possible mechanisms for the vapor release are sublimation from approximately 0 @.@ 6 km2 of exposed surface ice , or <unk> eruptions resulting from <unk> internal heat or from <unk> of a subsurface ocean due to growth of an <unk> layer of ice . Surface sublimation would be expected to be lower when Ceres is farther from the Sun in its orbit , whereas internally powered emissions should not be affected by its orbital position . The limited data available are more consistent with <unk> @-@ style sublimation . 
 
 = = Origin and evolution = = 
 
 Ceres is possibly a surviving <unk> ( planetary embryo ) , which formed 4 @.@ 57 billion years ago in the asteroid belt . Although the majority of inner Solar System <unk> ( including all <unk> to Mars @-@ sized bodies ) either merged with other <unk> to form terrestrial planets or were ejected from the Solar System by Jupiter , Ceres is thought to have survived relatively intact . An alternative theory proposes that Ceres formed in the <unk> belt and later migrated to the asteroid belt . The discovery of ammonia salts in <unk> crater supports an origin in the outer Solar System . Another possible <unk> , Vesta , is less than half the size of Ceres ; it suffered a major impact after <unk> , losing ~ 1 % of its mass . 
 The geological evolution of Ceres was dependent on the heat sources available during and after its formation : friction from <unk> <unk> , and decay of various <unk> ( possibly including short @-@ lived isotopes such as the <unk> <unk> aluminium @-@ 26 ) . These are thought to have been sufficient to allow Ceres to differentiate into a rocky core and icy mantle soon after its formation . This process may have caused resurfacing by water <unk> and <unk> , <unk> older geological features . Due to its small size , Ceres would have cooled early in its existence , causing all geological resurfacing processes to cease . Any ice on the surface would have gradually <unk> , leaving behind various hydrated minerals like clay minerals and carbonates . 
 Today , Ceres appears to be a <unk> inactive body , with a surface sculpted only by impacts . The presence of significant amounts of water ice in its composition raises the possibility that Ceres has or had a layer of liquid water in its interior . This hypothetical layer is often called an ocean . If such a layer of liquid water exists , it is hypothesized to be located between the rocky core and ice mantle like that of the theorized ocean on Europa . The existence of an ocean is more likely if <unk> ( i.e. salts ) , ammonia , sulfuric acid or other <unk> compounds are dissolved in the water . 
 
 = = Potential <unk> = = 
 
 Although not as actively discussed as a potential home for <unk> extraterrestrial life as Mars , Titan , Europa or <unk> , there is evidence that Ceres ' icy mantle was once a watery subterranean ocean , and that has led to speculations that life could have existed there , and that hypothesized <unk> bearing microorganisms could have come from Ceres to Earth . 
 
 = = Observation and exploration = = 
 
 
 = = = Observation = = = 
 
 When Ceres has an opposition near the perihelion , it can reach a visual magnitude of + 6 @.@ 7 . This is generally regarded as too dim to be seen with the naked eye , but under exceptional viewing conditions a very sharp @-@ sighted person may be able to see it . Ceres was at its brightest ( 6 @.@ 73 ) on 18 December 2012 . The only other asteroids that can reach a similarly bright magnitude are 4 Vesta , and , during rare <unk> near perihelion , 2 Pallas and 7 <unk> . At a conjunction Ceres has a magnitude of around + 9 @.@ 3 , which corresponds to the faintest objects visible with 10 × 50 <unk> . It can thus be seen with <unk> whenever it is above the horizon of a fully dark sky . 
 Some notable observations and milestones for Ceres include : 
 1984 November 13 : An <unk> of a star by Ceres observed in Mexico , Florida and across the Caribbean . 
 1995 June 25 : <unk> Hubble Space Telescope images with 50 @-@ kilometer resolution . 
 2002 : <unk> images with 30 km resolution taken with the Keck telescope using adaptive optics . 
 2003 and 2004 : Visible light images with 30 km resolution ( the best prior to the Dawn mission ) taken using Hubble . 
 2012 December 22 : Ceres <unk> the star <unk> 1865 @-@ <unk> @-@ 1 over parts of Japan , Russia , and China . Ceres ' brightness was magnitude 6 @.@ 9 and the star , 12 @.@ 2 . 
 2014 : Ceres was found to have an atmosphere with water vapor , confirmed by the Herschel space telescope . 
 2015 : The NASA Dawn spacecraft approached and orbited Ceres , sending detailed images and scientific data back to Earth . 
 
 = = = Exploration = = = 
 
 In 1981 , a proposal for an asteroid mission was submitted to the European Space Agency ( ESA ) . Named the <unk> Gravity Optical and Radar Analysis ( <unk> ) , this spacecraft was to launch some time in 1990 – 1994 and perform two flybys of large asteroids . The preferred target for this mission was Vesta . <unk> would reach the asteroid belt either by a gravitational <unk> trajectory past Mars or by means of a small ion engine . However , the proposal was refused by ESA . A joint NASA – ESA asteroid mission was then drawn up for a Multiple <unk> Orbiter with Solar Electric <unk> ( <unk> ) , with one of the mission <unk> including an orbit of Vesta . NASA indicated they were not interested in an asteroid mission . Instead , ESA set up a technological study of a spacecraft with an ion drive . Other missions to the asteroid belt were proposed in the 1980s by France , Germany , Italy , and the United States , but none were approved . Exploration of Ceres by fly @-@ by and impacting <unk> was the second main target of the second plan of the <unk> Soviet Vesta mission , developed in cooperation with European countries for realisation in 1991 – 1994 but canceled due to the Soviet Union disbanding . 
 In the early 1990s , NASA initiated the Discovery Program , which was intended to be a series of low @-@ cost scientific missions . In 1996 , the program 's study team recommended as a high priority a mission to explore the asteroid belt using a spacecraft with an ion engine . Funding for this program remained problematic for several years , but by 2004 the Dawn vehicle had passed its critical design review . 
 It was launched on 27 September 2007 , as the space mission to make the first visits to both Vesta and Ceres . On 3 May 2011 , Dawn acquired its first targeting image 1 @.@ 2 million kilometers from Vesta . After orbiting Vesta for 13 months , Dawn used its ion engine to depart for Ceres , with gravitational capture occurring on 6 March 2015 at a separation of 61 @,@ 000 km , four months prior to the New Horizons flyby of Pluto . 
 Dawn 's mission profile calls for it to study Ceres from a series of circular polar orbits at successively lower altitudes . It entered its first <unk> orbit ( " <unk> " ) around Ceres at an altitude of 13 @,@ 500 km on 23 April 2015 , staying for only approximately one orbit ( fifteen days ) . The spacecraft will subsequently reduce its orbital distance to 4 @,@ 400 km for its second <unk> orbit ( " survey " ) for three weeks , then down to 1 @,@ 470 km ( " <unk> " ) for two months and then down to its final orbit at 375 km ( " <unk> " ) for at least three months . The spacecraft instrumentation includes a framing camera , a visual and infrared <unk> , and a gamma @-@ ray and neutron detector . These instruments will examine Ceres ' shape and elemental composition . On 13 January 2015 , Dawn took the first images of Ceres at near @-@ Hubble resolution , revealing impact craters and a small high @-@ albedo spot on the surface , near the same location as that observed previously . Additional imaging sessions , at increasingly better resolution took place on 25 January , 4 , 12 , 19 , and 25 February , 1 March , and 10 and 15 April . 
 Dawn 's arrival in a stable orbit around Ceres was delayed after , close to reaching Ceres , it was hit by a cosmic ray , making it take another , longer route around Ceres in back , instead of a direct spiral towards it . 
 The Chinese Space Agency is designing a sample <unk> mission from Ceres that would take place during the <unk> . 
 
 = = Maps = = 
 
 
 = = = Map of <unk> = = = 
 
 The following <unk> of the dwarf planet Ceres is divided into 15 <unk> . They are named after the first craters whose names the IAU approved in July 2015 . The map image ( s ) were taken by the Dawn space probe . 
 
 = = Gallery = = 
 
 
 = = = True @-@ color images = = = 
 
 
 = = = <unk> = = = 
 
 
 