4 Vesta

Template:Short description Template:About Template:Use dmy dates Template:Good article Template:Infobox planet Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of Template:Convert.<ref name="Russell2012"/> It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807<ref name="jpldata"/> and is named after Vesta, the virgin goddess of home and hearth from Roman mythology.<ref>Template:Cite web</ref>

Vesta is thought to be the second-largest asteroid, both by mass and by volume, after the dwarf planet Ceres.<ref name="NASADawnMission"/><ref name="CambridgeSS"/><ref name="Russell2011"/> Measurements give it a nominal volume only slightly larger than that of Pallas (about 5% greater), but it is 25% to 30% more massive. It constitutes an estimated 9% of the mass of the asteroid belt.<ref name="Pitjeva05"/> Vesta is the only known remaining rocky protoplanet of the kind that formed the terrestrial planets.<ref name="lastofkind"/> Numerous fragments of Vesta were ejected by collisions one and two billion years ago that left two enormous craters occupying much of Vesta's southern hemisphere.<ref name="Jutzi2013"/><ref name="Cook2014a"/> Debris from these events has fallen to Earth as howardite–eucrite–diogenite (HED) meteorites, which have been a rich source of information about Vesta.<ref name="McSween2013"/><ref name="Kelley2003"/><ref name="nasa2011-vesta"/>

Vesta is the brightest asteroid visible from Earth. It is regularly as bright as magnitude 5.1,<ref name="Pasachoff1983"/> at which times it is faintly visible to the naked eye. Its maximum distance from the Sun is slightly greater than the minimum distance of Ceres from the Sun,Template:Efn although its orbit lies entirely within that of Ceres.<ref name="GravitySim"/>

NASA's Dawn spacecraft entered orbit around Vesta on 16 July 2011 for a one-year exploration and left the orbit of Vesta on 5 September 2012<ref name="Dawnstatus"/> en route to its final destination, Ceres. Researchers continue to examine data collected by Dawn for additional insights into the formation and history of Vesta.<ref name="Ammannito2013"/><ref name="Cook2014b"/>

History

Discovery

Template:Stack Heinrich Olbers discovered Pallas in 1802, the year after the discovery of Ceres. He proposed that the two objects were the remnants of a destroyed planet. He sent a letter with his proposal to the British astronomer William Herschel, suggesting that a search near the locations where the orbits of Ceres and Pallas intersected might reveal more fragments. These orbital intersections were located in the constellations of Cetus and Virgo.<ref name="Littmann2004"/> Olbers commenced his search in 1802, and on 29 March 1807 he discovered Vesta in the constellation Virgo—a coincidence, because Ceres, Pallas, and Vesta are not fragments of a larger body. Because the asteroid Juno had been discovered in 1804, this made Vesta the fourth object to be identified in the region that is now known as the asteroid belt. The discovery was announced in a letter addressed to German astronomer Johann H. Schröter dated 31 March.<ref name="Lynn1907"/> Because Olbers already had credit for discovering a planet (Pallas; at the time, the asteroids were considered to be planets), he gave the honor of naming his new discovery to German mathematician Carl Friedrich Gauss, whose orbital calculations had enabled astronomers to confirm the existence of Ceres, the first asteroid, and who had computed the orbit of the new planet in the remarkably short time of 10 hours.<ref name="Dunnington2004"/><ref name="Rao2003"/> Gauss decided on the Roman virgin goddess of home and hearth, Vesta.<ref name="MPDictionary"/>

Name and symbol

Vesta was the fourth asteroid to be discovered, hence the number 4 in its formal designation. The name Vesta, or national variants thereof, is in international use with two exceptions: Greece and China. In Greek, the name adopted was the Hellenic equivalent of Vesta, Hestia Template:Nobr in English, that name is used for Template:Nobr (Greeks use the name "Hestia" for both, with the minor-planet numbers used for disambiguation). In Chinese, Vesta is called the 'hearth-god(dess) star', Template:Lang Template:Transliteration, naming the asteroid for Vesta's role, similar to the Chinese names of Uranus, Neptune, and Pluto.Template:Efn

Upon its discovery, Vesta was, like Ceres, Pallas, and Juno before it, classified as a planet and given a planetary symbol. The symbol represented the altar of Vesta with its sacred fire and was designed by Gauss.<ref name=mcbeh-v15/><ref name="effemeridi-1809"/> In Gauss's conception, now obsolete, this was drawn Template:Nobr. His form is in the pipeline for Unicode 17.0 as U+1F777 Template:Math.<ref name=astunicode>Template:Cite report</ref><ref name=pipeline>Template:Cite web</ref>Template:Efn The asteroid symbols were gradually retired from astronomical use after 1852, but the symbols for the first four asteroids were resurrected for astrology in the 1970s. The abbreviated modern astrological variant of the Vesta symbol is Template:Nobr.<ref name=astunicode/>Template:Efn

After the discovery of Vesta, no further objects were discovered for 38 years, and during this time the Solar System was thought to have eleven planets.<ref name=18planets/> However, in 1845, new asteroids started being discovered at a rapid pace, and by 1851 there were fifteen, each with its own symbol, in addition to the eight major planets (Neptune had been discovered in 1846). It soon became clear that it would be impractical to continue inventing new planetary symbols indefinitely, and some of the existing ones proved difficult to draw quickly. That year, the problem was addressed by Benjamin Apthorp Gould, who suggested numbering asteroids in their order of discovery, and placing this number in a disk (circle) as the generic symbol of an asteroid. Thus, the fourth asteroid, Vesta, acquired the generic symbol Template:Big. This was soon coupled with the name into an official number–name designation, Template:Nobr as the number of minor planets increased. By 1858, the circle had been simplified to parentheses, Template:Nobr which were easier to typeset. Other punctuation, such as Template:Nobr and Template:Nobr was also briefly used, but had more or less completely died out by 1949.<ref name=WhenbecameMPs/>

Early measurements

Photometric observations of Vesta were made at the Harvard College Observatory in 1880–1882 and at the Observatoire de Toulouse in 1909. These and other observations allowed the rotation rate of Vesta to be determined by the 1950s. However, the early estimates of the rotation rate came into question because the light curve included variations in both shape and albedo.<ref name="McFadden2008"/>

Early estimates of the diameter of Vesta ranged from Template:Convert in 1825, to Template:Convert. E.C. Pickering produced an estimated diameter of Template:Convert in 1879, which is close to the modern value for the mean diameter, but the subsequent estimates ranged from a low of Template:Convert up to a high of Template:Convert during the next century. The measured estimates were based on photometry. In 1989, speckle interferometry was used to measure a dimension that varied between Template:Convert during the rotational period.<ref name="Hughes1994"/> In 1991, an occultation of the star SAO 93228 by Vesta was observed from multiple locations in the eastern United States and Canada. Based on observations from 14 different sites, the best fit to the data was an elliptical profile with dimensions of about Template:Convert.<ref name="Povenmire2001"/> Dawn confirmed this measurement.Template:Efn These measurements will help determine the thermal history, size of the core, role of water in asteroid evolution and what meteorites found on Earth come from these bodies, with the ultimate goal of understanding the conditions and processes present at the solar system's earliest epoch and the role of water content and size in planetary evolution.<ref name="nssdca 2007_043a"/>

Vesta became the first asteroid to have its mass determined. Every 18 years, the asteroid 197 Arete approaches within Template:Val of Vesta. In 1966, based upon observations of Vesta's gravitational perturbations of Arete, Hans G. Hertz estimated the mass of Vesta at Template:Val (solar masses).<ref name="science160_3825"/> More refined estimates followed, and in 2001 the perturbations of 17 Thetis were used to calculate the mass of Vesta to be Template:Val.<ref name="aaa430"/> Dawn determined it to be Template:Val.

Orbit

Vesta orbits the Sun between Mars and Jupiter, within the asteroid belt, with a period of 3.6 Earth years,<ref name="jpldata"/> specifically in the inner asteroid belt, interior to the Kirkwood gap at 2.50 AU. Its orbit is moderately inclined (i = 7.1°, compared to 7° for Mercury and 17° for Pluto) and moderately eccentric (e = 0.09, about the same as for Mars).<ref name="jpldata"/>

True orbital resonances between asteroids are considered unlikely. Because of their small masses relative to their large separations, such relationships should be very rare.<ref name="Christou"/> Nevertheless, Vesta is able to capture other asteroids into temporary 1:1 resonant orbital relationships (for periods up to 2 million years or more) and about forty such objects have been identified.<ref name="Christou2012"/> Decameter-sized objects detected in the vicinity of Vesta by Dawn may be such quasi-satellites rather than proper satellites.<ref name="Christou2012"/>

Rotation

Template:Multiple image Vesta's rotation is relatively fast for an asteroid (5.342 h) and prograde, with the north pole pointing in the direction of right ascension 20 h 32 min, declination +48° (in the constellation Cygnus) with an uncertainty of about 10°. This gives an axial tilt of 29°.<ref name="Thomas1997b"/>

Coordinate systems

Two longitudinal coordinate systems are used for Vesta, with prime meridians separated by 150°. The IAU established a coordinate system in 1997 based on Hubble photos, with the prime meridian running through the center of Olbers Regio, a dark feature 200 km across. When Dawn arrived at Vesta, mission scientists found that the location of the pole assumed by the IAU was off by 10°, so that the IAU coordinate system drifted across the surface of Vesta at 0.06° per year, and also that Olbers Regio was not discernible from up close, and so was not adequate to define the prime meridian with the precision they needed. They corrected the pole, but also established a new prime meridian 4° from the center of Claudia, a sharply defined crater 700 meters across, which they say results in a more logical set of mapping quadrangles.<ref name="Claudian"/> All NASA publications, including images and maps of Vesta, use the Claudian meridian, which is unacceptable to the IAU. The IAU Working Group on Cartographic Coordinates and Rotational Elements recommended a coordinate system, correcting the pole but rotating the Claudian longitude by 150° to coincide with Olbers Regio.<ref name="IAUcoordinates"/> It was accepted by the IAU, although it disrupts the maps prepared by the Dawn team, which had been positioned so they would not bisect any major surface features.<ref name=Claudian/><ref name="Dawncoordinates"/>

Physical characteristics

Vesta is the second most massive body in the asteroid belt, although it is only 28% as massive as Ceres, the most massive body.<ref name="Baer2007"/><ref name="Pitjeva05"/> Vesta is however the most massive body that formed in the asteroid belt, as Ceres is believed to have formed between Jupiter and Saturn. Vesta's density is lower than those of the four terrestrial planets but is higher than those of most asteroids, as well as all of the moons in the Solar System except Io. Vesta's surface area is about the same as the land area of Pakistan, Venezuela, Tanzania, or Nigeria; slightly under Template:Convert. It has an only partially differentiated interior.<ref name="park"/> Vesta is only slightly larger (Template:Val<ref name="Russell2012"/>) than 2 Pallas (Template:Val) in mean diameter,<ref name="Carry2009"/> but is about 25% more massive.

Vesta's shape is close to a gravitationally relaxed oblate spheroid,<ref name="Thomas1997b"/> but the large concavity and protrusion at the southern pole (see 'Surface features' below) combined with a mass less than Template:Val precluded Vesta from automatically being considered a dwarf planet under International Astronomical Union (IAU) Resolution XXVI 5.<ref name="IAU0601"/> A 2012 analysis of Vesta's shape<ref name="Fu 2013"/> and gravity field using data gathered by the Dawn spacecraft has shown that Vesta is currently not in hydrostatic equilibrium.<ref name="Russell2012"/><ref name="Asmar 2012"/>

Temperatures on the surface have been estimated to lie between about Template:Convert with the Sun overhead, dropping to about Template:Convert at the winter pole. Typical daytime and nighttime temperatures are Template:Convert and Template:Convert, respectively. This estimate is for 6 May 1996, very close to perihelion, although details vary somewhat with the seasons.<ref name="Mueller01"/>

Surface features

Template:Further Before the arrival of the Dawn spacecraft, some Vestan surface features had already been resolved using the Hubble Space Telescope and ground-based telescopes (e.g., the Keck Observatory).<ref name="Zellner2005"/> The arrival of Dawn in July 2011 revealed the complex surface of Vesta in detail.<ref name="Jaumann2012"/>

File:PIA18788-VestaAsteroid-GeologicMap-DawnMission-20141117.jpg

Geologic map of Vesta (Mollweide projection).<ref name="Williams2014"/> The most ancient and heavily cratered regions are brown; areas modified by the Veneneia and Rheasilvia impacts are purple (the Saturnalia Fossae Formation, in the north)<ref name="Scully2014"/> and light cyan (the Divalia Fossae Formation, equatorial),<ref name="Williams2014"/> respectively; the Rheasilvia impact basin interior (in the south) is dark blue, and neighboring areas of Rheasilvia ejecta (including an area within Veneneia) are light purple-blue;<ref name="Schäfer2014"/><ref name="Kneissl2014"/> areas modified by more recent impacts or mass wasting are yellow/orange or green, respectively.

Rheasilvia and Veneneia

The most prominent of these surface features are two enormous impact basins, the Template:Convert Rheasilvia, centered near the south pole; and the Template:Convert wide Veneneia. The Rheasilvia impact basin is younger and overlies the Veneneia.<ref name="Schenk2012"/> The Dawn science team named the younger, more prominent crater Rheasilvia, after the mother of Romulus and Remus and a mythical vestal virgin.<ref name="Rheasilvianamed"/> Its width is 95% of the mean diameter of Vesta. The crater is about Template:Convert deep. A central peak rises Template:Convert above the lowest measured part of the crater floor and the highest measured part of the crater rim is Template:Convert above the crater floor low point. It is estimated that the impact responsible excavated about 1% of the volume of Vesta, and it is likely that the Vesta family and V-type asteroids are the products of this collision. If this is the case, then the fact that Template:Convert fragments have survived bombardment until the present indicates that the crater is at most only about 1 billion years old.<ref name="Binzel1997"/> It would also be the site of origin of the HED meteorites. All the known V-type asteroids taken together account for only about 6% of the ejected volume, with the rest presumably either in small fragments, ejected by approaching the 3:1 Kirkwood gap, or perturbed away by the Yarkovsky effect or radiation pressure. Spectroscopic analyses of the Hubble images have shown that this crater has penetrated deep through several distinct layers of the crust, and possibly into the mantle, as indicated by spectral signatures of olivine.<ref name="Thomas1997b"/>

Subsequent analysis of data from the Dawn mission provided much greater detail on Rheasilvia's structure and composition, confirming it as one of the largest impact structures known relative to its parent body size.<ref name="Schenk2012"/> The impact clearly modified the pre-existing very large, Veneneia structure, indicating Rheasilvia's younger age.<ref name="Schenk2012"/> Rheasilvia's size makes Vesta's southern topography unique, creating a flattened southern hemisphere and contributing significantly to the asteroid's overall oblate shape.<ref name="Jaumann2012"/> Rheasilvia's ~22 km central peak stands as one of the tallest mountains identified in the Solar System.<ref name="Schenk2012"/> Its base width of roughly 180 km and complex morphology distinguishes it from the simpler central peaks seen in smaller craters.<ref name="Ivanov2013">Template:Cite journal</ref> Numerical modeling indicates that such a large central structure within a ~505 km diameter basin requires formation on a differentiated body with significant gravity. Scaling laws for craters on smaller asteroids fail to predict such a feature; instead, impact dynamics involving transient crater collapse and rebound of the underlying material (potentially upper mantle) are needed to explain its formation.<ref name="Ivanov2013"/> Hydrocode simulations suggest the impactor responsible was likely 60–70 km across, impacting at roughly 5.4 km/s.<ref name="Bowling2013">Template:Cite journal</ref> Models of impact angle (around 30-45 degrees from vertical) better match the detailed morphology of the basin and its prominent peak.<ref name="Ivanov2013"/> Crater density measurements on Rheasilvia's relatively unmodified floor materials and surrounding ejecta deposits, calibrated using standard lunar chronology functions adapted for Vesta's location, place the impact event at approximately 1 billion years ago.<ref name="Marchi2012">Template:Cite journal</ref><ref name="Williams2014"/> This age makes Rheasilvia a relatively young feature on a protoplanetary body formed early in Solar System history. The estimated excavation of ~1% of Vesta's volume<ref name="Schenk2012"/> provides a direct link to the Vesta family of asteroids (Vestoids) and the HED meteorites. Since Vesta's spectral signature matches that of the Vestoids and HEDs, this strongly indicates they are fragments ejected from Vesta most likely during the Rheasilvia impact.<ref name="McSween2013"/><ref name="Marchi2012"/>

The Dawn mission's VIR instrument helped to confirm the basin's deep excavation and compositional diversity. VIR mapping revealed spectral variations across the basin consistent with the mixing of different crustal layers expected in the HED meteorites. Signatures matching eucrites (shallow crustal basalts) and diogenites (deeper crustal orthopyroxenites) were identified, which usually correlate with specific morphological features like crater walls or slump blocks.<ref name="DeSanctis2012">Template:Cite journal</ref><ref name="McSween2013" /> The confirmed signature of olivine-rich material, which were first hinted at by Hubble observations is strongest on the flanks of the central peak and in specific patches along the basin rim and walls, suggesting it is not uniformly distributed but rather exposed in distinct outcrops.<ref name="Clenet2014">Template:Cite journal</ref><ref name="DeSanctis2012" /> As the dominant mineral expected in Vesta's mantle beneath the HED-like crust,<ref name="Russell2012" /> the presence of olivine indicates the Rheasilvia impact penetrated Vesta's entire crust (~20-40 km thick in the region) and excavated material from the upper mantle.<ref name="Clenet2014" /> Furthermore, the global stresses resulting from this massive impact are considered the likely trigger for the formation of the large trough systems, like Divalia Fossa, that encircle Vesta's equatorial regions.<ref name="Buczkowski2012" /><ref name="Jaumann2012" />

Other craters

Template:Stack Several old, degraded craters approach Rheasilvia and Veneneia in size, although none are quite so large. They include Feralia Planitia, shown at right, which is Template:Convert across.<ref name="Av-10"/> More-recent, sharper craters range up to Template:Convert Varronilla and Template:Convert Postumia.<ref name="Planetary Names"/>

Dust fills up some craters, creating so-called dust ponds. They are a phenomenon where pockets of dust are seen in celestial bodies without a significant atmosphere. These are smooth deposits of dust accumulated in depressions on the surface of the body (like craters), contrasting from the Rocky terrain around them.<ref>Template:Cite news</ref> On the surface of Vesta, we have identified both type 1 (formed from impact melt) and type 2 (electrostatically made) dust ponds within 0˚–30°N/S, that is, Equatorial region. 10 craters have been identified with such formations.<ref>Template:Cite journal</ref>

"Snowman craters"

The "snowman craters" are a group of three adjacent craters in Vesta's northern hemisphere. Their official names, from largest to smallest (west to east), are Marcia, Calpurnia, and Minucia. Marcia is the youngest and cross-cuts Calpurnia. Minucia is the oldest.<ref name="Williams2014"/> Template:Multiple image

Troughs

The majority of the equatorial region of Vesta is sculpted by a series of parallel troughs designated Divalia Fossae; its longest trough is Template:Convert wide and Template:Convert long. Despite the fact that Vesta is a one-seventh the size of the Moon, Divalia Fossae dwarfs the Grand Canyon. A second series, inclined to the equator, is found further north. This northern trough system is named Saturnalia Fossae, with its largest trough being roughly 40 km wide and over 370 km long. These troughs are thought to be large-scale graben resulting from the impacts that created Rheasilvia and Veneneia craters, respectively. They are some of the longest chasms in the Solar System, nearly as long as Ithaca Chasma on Tethys. The troughs may be graben that formed after another asteroid collided with Vesta, a process that can happen only in a body that is differentiated,<ref name="Buczkowski2012"/> which Vesta may not fully be. Alternatively, it is proposed that the troughs may be radial sculptures created by secondary cratering from Rheasilvia.<ref name = "Hirata et al. 2023">Template:Cite journal</ref> Template:Multiple image Template:Clear

Surface composition

Compositional information from the visible and infrared spectrometer (VIR), gamma-ray and neutron detector (GRaND), and framing camera (FC), all indicate that the majority of the surface composition of Vesta is consistent with the composition of the howardite, eucrite, and diogenite meteorites.<ref name="DeSanctis2012a"/><ref name="Prettyman2012"/><ref name="Reddy2012"/> The Rheasilvia region is richest in diogenite, consistent with the Rheasilvia-forming impact excavating material from deeper within Vesta. The presence of olivine within the Rheasilvia region would also be consistent with excavation of mantle material. However, olivine has only been detected in localized regions of the northern hemisphere, not within Rheasilvia.<ref name="Ammannito2013"/> The origin of this olivine is currently unclear. Though olivine was expected by astronomers to have originated from Vesta's mantle prior to the arrival of the Dawn orbiter, the lack of olivine within the Rheasilvia and Veneneia impact basins complicates this view. Both impact basins excavated Vestian material down to 60–100 km, far deeper than the expected thickness of ~30–40 km for Vesta's crust. Vesta's crust may be far thicker than expected or the violent impact events that created Rheasilvia and Veneneia may have mixed material enough to obscure olivine from observations. Alternatively, Dawn observations of olivine could instead be due to delivery by olivine-rich impactors, unrelated to Vesta's internal structure.<ref name="Palomba2015"/>

Features associated with volatiles

Pitted terrain has been observed in four craters on Vesta: Marcia, Cornelia, Numisia and Licinia.<ref name="Denevi2012"/> The formation of the pitted terrain is proposed to be degassing of impact-heated volatile-bearing material. Along with the pitted terrain, curvilinear gullies are found in Marcia and Cornelia craters. The curvilinear gullies end in lobate deposits, which are sometimes covered by pitted terrain, and are proposed to form by the transient flow of liquid water after buried deposits of ice were melted by the heat of the impacts.<ref name="Scully2014"/> Hydrated materials have also been detected, many of which are associated with areas of dark material.<ref name="DeSanctis2012b"/> Consequently, dark material is thought to be largely composed of carbonaceous chondrite, which was deposited on the surface by impacts. Carbonaceous chondrites are comparatively rich in mineralogically bound OH.<ref name="Reddy2012"/>

Geology

A large collection of potential samples from Vesta is accessible to scientists, in the form of over 1200 HED meteorites (Vestan achondrites), giving insight into Vesta's geologic history and structure. NASA Infrared Telescope Facility (NASA IRTF) studies of asteroid Template:Mpl suggest that it originated from deeper within Vesta than the HED meteorites.<ref name="Vestainterior"/>

Vesta is thought to consist of a metallic iron–nickel core, variously estimated to be 90 km<ref name=park>Template:Cite journal</ref> to 220 km<ref name="Russell2012"/> in diameter, an overlying rocky olivine mantle, with a surface crust of similar composition to HED meteorites.

From the first appearance of calcium–aluminium-rich inclusions (the first solid matter in the Solar System, forming about 4.567 billion years ago), a likely time line is as follows:<ref name="Ghosh1998" /><ref name="Righter1997" /><ref name="Drake2001" /><ref name="Sahijpal2007" /><ref name="Gupta2010" />

Timeline of the evolution of Vesta

2–3 million years	Accretion completed
4–5 million years	Complete or almost complete melting due to radioactive decay of 26Al, leading to separation of the metal core
6–7 million years	Progressive crystallization of a convecting molten mantle. Convection stopped when about 80% of the material had crystallized
Extrusion of the remaining molten material to form the crust, either as basaltic lavas in progressive eruptions, or possibly forming a short-lived magma ocean.
The deeper layers of the crust crystallize to form plutonic rocks, whereas older basalts are metamorphosed due to the pressure of newer surface layers.
Slow cooling of the interior

Vesta is the only known intact asteroid that has been resurfaced in this manner. Because of this, some scientists refer to Vesta as a protoplanet.<ref name="nasa-dawn20110329"/>

Composition of the Vestan crust (by depth)<ref name="Takeda1997"/>

A lithified regolith, the source of howardites and brecciated eucrites.

Basaltic lava flows, a source of non-cumulate eucrites.

Plutonic rocks consisting of pyroxene, pigeonite and plagioclase, the source of cumulate eucrites.

Plutonic rocks rich in orthopyroxene with large grain sizes, the source of diogenites.

On the basis of the sizes of V-type asteroids (thought to be pieces of Vesta's crust ejected during large impacts), and the depth of Rheasilvia crater (see below), the crust is thought to be roughly Template:Convert thick.<ref name="Yamaguchi1995"/> Findings from the Dawn spacecraft have found evidence that the troughs that wrap around Vesta could be graben formed by impact-induced faulting (see Troughs section above), meaning that Vesta has more complex geology than other asteroids. The impacts that created the Rheasilvia and Veneneia craters occurred when Vesta was no longer warm and plastic enough to return to an equilibrium shape, distorting its once rounded shape and prohibiting it from being classified as a dwarf planet today.Template:Citation needed

Regolith

Vesta's surface is covered by regolith distinct from that found on the Moon or asteroids such as Itokawa. This is because space weathering acts differently. Vesta's surface shows no significant trace of nanophase iron because the impact speeds on Vesta are too low to make rock melting and vaporization an appreciable process. Instead, regolith evolution is dominated by brecciation and subsequent mixing of bright and dark components.<ref name="Pieters2012"/> The dark component is probably due to the infall of carbonaceous material, whereas the bright component is the original Vesta basaltic soil.<ref name="McCord2012"/>

Fragments

Some small Solar System bodies are suspected to be fragments of Vesta caused by impacts. The Vestian asteroids and HED meteorites are examples. The V-type asteroid 1929 Kollaa has been determined to have a composition akin to cumulate eucrite meteorites, indicating its origin deep within Vesta's crust.<ref name="Kelley2003"/>

Vesta is currently one of only eight identified Solar System bodies of which we have physical samples, coming from a number of meteorites suspected to be Vestan fragments. It is estimated that 1 out of 16 meteorites originated from Vesta.<ref name="siliconvalleyastrolecture"/> The other identified Solar System samples are from Earth itself, meteorites from Mars, meteorites from the Moon, and samples returned from the Moon, the comet Wild 2, and the asteroids 25143 Itokawa, 162173 Ryugu, and 101955 Bennu.<ref name="nasa2011-vesta"/>Template:Efn

Exploration

Template:Stack In 1981, a proposal for an asteroid mission was submitted to the European Space Agency (ESA). Named the Asteroidal Gravity Optical and Radar Analysis (AGORA), 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. AGORA would reach the asteroid belt either by a gravitational slingshot trajectory past Mars or by means of a small ion engine. However, the proposal was refused by the ESA. A joint NASA–ESA asteroid mission was then drawn up for a Multiple Asteroid Orbiter with Solar Electric Propulsion (MAOSEP), with one of the mission profiles including an orbit of Vesta. NASA indicated they were not interested in an asteroid mission. Instead, the 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.<ref name="ulivi_harland08"/> Exploration of Vesta by fly-by and impacting penetrator was the second main target of the first plan of the multi-aimed Soviet Vesta mission, developed in cooperation with European countries for realisation in 1991–1994 but canceled due to the dissolution of the Soviet Union.

File:Dawn Flight Configuration 2.jpg

Artist's conception of Dawn orbiting Vesta

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 a mission to explore the asteroid belt using a spacecraft with an ion engine as a high priority. Funding for this program remained problematic for several years, but by 2004 the Dawn vehicle had passed its critical design review<ref name="Russell2007"/> and construction proceeded.Template:Citation needed

It launched on 27 September 2007 as the first space mission to Vesta. On 3 May 2011, Dawn acquired its first targeting image 1.2 million kilometers from Vesta.<ref name="pr2011-138"/> On 16 July 2011, NASA confirmed that it received telemetry from Dawn indicating that the spacecraft successfully entered Vesta's orbit.<ref name="Vega2011"/> It was scheduled to orbit Vesta for one year, until July 2012.<ref name="Dawntimeline"/> DawnTemplate:'s arrival coincided with late summer in the southern hemisphere of Vesta, with the large crater at Vesta's south pole (Rheasilvia) in sunlight. Because a season on Vesta lasts eleven months, the northern hemisphere, including anticipated compression fractures opposite the crater, would become visible to DawnTemplate:'s cameras before it left orbit.<ref name="DawnMission"/> Dawn left orbit around Vesta on 4 September 2012 Template:Nowrap to travel to Ceres.<ref name="Dawndeparture"/>

NASA/DLR released imagery and summary information from a survey orbit, two high-altitude orbits (60–70 m/pixel) and a low-altitude mapping orbit (20 m/pixel), including digital terrain models, videos and atlases.<ref name="Russell2013"/><ref name="Roatsch2012"/><ref name="Roatsch2013"/><ref name="journeyaboveVesta"/><ref name="dawn_gis"/><ref name="NASADawn"/> Scientists also used Dawn to calculate Vesta's precise mass and gravity field. The subsequent determination of the J₂ component yielded a core diameter estimate of about 220 km assuming a crustal density similar to that of the HED.<ref name="Russell2013"/>

Dawn data can be accessed by the public at the UCLA website.<ref name="Dawnpublic"/>

Observations from Earth orbit

• Albedo and spectral maps of 4 Vesta, as determined from Hubble Space Telescope images from November 1994
  Albedo and spectral maps of 4 Vesta, as determined from Hubble Space Telescope images from November 1994
• Elevation map of 4 Vesta, as determined from Hubble Space Telescope images of May 1996
  Elevation map of 4 Vesta, as determined from Hubble Space Telescope images of May 1996
• Elevation diagram of 4 Vesta (as determined from Hubble Space Telescope images of May 1996) viewed from the south-east, showing Rheasilvia crater at the south pole and Feralia Planitia near the equator
  Elevation diagram of 4 Vesta (as determined from Hubble Space Telescope images of May 1996) viewed from the south-east, showing Rheasilvia crater at the south pole and Feralia Planitia near the equator
• Vesta seen by the Hubble Space Telescope in May 2007
  Vesta seen by the Hubble Space Telescope in May 2007
• The 2006 IAU draft proposal on the definition of a planet listed Vesta as a candidate.<ref name="IAU-draft2006"/> Vesta is shown fourth from the left along the bottom row.
  The 2006 IAU draft proposal on the definition of a planet listed Vesta as a candidate.<ref name="IAU-draft2006"/> Vesta is shown fourth from the left along the bottom row.

Observations from Dawn

Vesta comes into view as the Dawn spacecraft approaches and enters orbit:

• Vesta from 100,000 km (1 July 2011)
  Vesta from 100,000 km
  (1 July 2011)
• Vesta from 41,000 km (9 July 2011)
  Vesta from 41,000 km
  (9 July 2011)
• In orbit at 16,000 km (17 July 2011)
  In orbit at 16,000 km
  (17 July 2011)
• In orbit from 10,500 km (18 July 2011)
  In orbit from 10,500 km
  (18 July 2011)
• The northern hemisphere from 5,200 km (23 July 2011)
  The northern hemisphere from 5,200 km
  (23 July 2011)
• In orbit from 5,200 km (24 July 2011)
  In orbit from 5,200 km
  (24 July 2011)
• In orbit from 3,700 km (31 July 2011)
  In orbit from 3,700 km
  (31 July 2011)
• Full rotation (1 August 2011)
  Full rotation
  (1 August 2011)
• Composite greyscale image
  Composite greyscale image
• Cratered terrain with hills and ridges (6 August 2011)
  Cratered terrain with hills and ridges
  (6 August 2011)
• Densely cratered terrain near terminator (6 August 2011)
  Densely cratered terrain near terminator
  (6 August 2011)
• Vestan craters in various states of degradation, with troughs at bottom (6 August 2011)
  Vestan craters in various states of degradation, with troughs at bottom
  (6 August 2011)
• Hill shaded central mound at the south pole of Vesta (2 February 2015)
  Hill shaded central mound at the south pole of Vesta
  (2 February 2015)

True-color images

• File:Vesta in natural color.jpg
• File:Vesta-SouthPole.png
• File:Vesta 5755-728.png

Detailed images retrieved during the high-altitude (60–70 m/pixel) and low-altitude (~20 m/pixel) mapping orbits are available on the Dawn Mission website of JPL/NASA.<ref>Template:Cite web</ref>

Visibility

Its size and unusually bright surface make Vesta the brightest asteroid, and it is occasionally visible to the naked eye from dark skies (without light pollution). In May and June 2007, Vesta reached a peak magnitude of +5.4, the brightest since 1989.<ref name="SkyTelescope2007a"/> At that time, opposition and perihelion were only a few weeks apart.<ref name="SkyTelescope2007b"/> It was brighter still at its 22 June 2018 opposition, reaching a magnitude of +5.3.<ref>Template:Cite book</ref> Less favorable oppositions during late autumn 2008 in the Northern Hemisphere still had Vesta at a magnitude of from +6.5 to +7.3.<ref name=James2008/> Even when in conjunction with the Sun, Vesta will have a magnitude around +8.5; thus from a pollution-free sky it can be observed with binoculars even at elongations much smaller than near opposition.<ref name="James2008"/>

2010–2011

In 2010, Vesta reached opposition in the constellation of Leo on the night of 17–18 February, at about magnitude 6.1,<ref name="Horizons"/> a brightness that makes it visible in binocular range but generally not for the naked eye. Under perfect dark sky conditions where all light pollution is absent it might be visible to an experienced observer without the use of a telescope or binoculars. Vesta came to opposition again on 5 August 2011, in the constellation of Capricornus at about magnitude 5.6.<ref name="Horizons"/><ref name="MPC-eph"/>

2012–2013

Vesta was at opposition again on 9 December 2012.<ref name="NightSkyOnline"/> According to Sky and Telescope magazine, this year Vesta came within about 6 degrees of 1 Ceres during the winter of 2012 and spring 2013.<ref name=flanders/> Vesta orbits the Sun in 3.63 years and Ceres in 4.6 years, so every 17.4 years Vesta overtakes Ceres (the previous overtaking was in April 1996).<ref name="flanders"/> On 1 December 2012, Vesta had a magnitude of 6.6, but it had decreased to 8.4 by 1 May 2013.<ref name=flanders/>

2014

Ceres and Vesta came within one degree of each other in the night sky in July 2014.<ref name=flanders/>

See also

• 3103 Eger
• 3551 Verenia
• 3908 Nyx
• 4055 Magellan
• Asteroids in fiction
• Diogenite
• Eucrite
• List of former planets
• Howardite
• Vesta family (vestoids)
• List of tallest mountains in the Solar System

Bibliography

• The Dawn Mission to Minor Planets 4 Vesta and 1 Ceres, Christopher T. Russell and Carol A. Raymond (Editors), Springer (2011), Template:ISBN
• Keil, K.; Geological History of Asteroid 4 Vesta: The Smallest Terrestrial Planet in Asteroids III, William Bottke, Alberto Cellino, Paolo Paolicchi, and Richard P. Binzel, (Editors), University of Arizona Press (2002), Template:ISBN

External links

File:The Smallest Planet.ogv

This video explores Vesta's landscape, history and planet-like characteristics.

• Interactive 3D gravity simulation of the Dawn spacecraft in orbit around Vesta Template:Webarchive
• Vesta Trek – An integrated map browser of datasets and maps for 4 Vesta
• JPL Ephemeris
• Views of the Solar System: Vesta
• HubbleSite: Hubble Maps the Asteroid Vesta
• Encyclopædia Britannica, Vesta – full article
• HubbleSite: short movie composed from Hubble Space Telescope images from November 1994.
• Adaptive optics views of Vesta from Keck Observatory
• 4 Vesta images at ESA/Hubble Template:Webarchive
• Dawn at Vesta (NASA press kit on Dawn's operations at Vesta)
• NASA video Template:Webarchive
• Vesta atlas
• Template:AstDys
• Template:JPL small body