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4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt. It was discovered by the Germanmarker astronomer Heinrich Wilhelm Olbers on March 29, 1807, and named after the Roman virgin goddess of home and hearth, Vesta.

Vesta lost some 1% of its mass in a collision less than one billion years ago. Many fragments of this event have fallen to Earth as Howardite-Eucrite-Diogenite meteorites, a rich source of evidence about the asteroid. Vesta is the brightest asteroid. Its greatest distance from the Sun is slightly more than the minimum distance of Ceres from the Sun, and its orbit is entirely within the orbit of Ceres.

Discovery

Vesta was discovered by the Germanmarker astronomer Heinrich Wilhelm Olbers on March 29, 1807. He announced the discovery in a letter addressed to Johann H. Schröter dated March 31, and reported the asteroid's location in the constellation Virgo. Olbers allowed the prominent mathematician Carl Friedrich Gauss to name the asteroid after the Roman virgin goddess of home and hearth, Vesta. The mathematician manually computed the first orbit for Vesta in the remarkably short time of 10 hours.

After the discovery of Vesta in 1807, no further asteroids were discovered for 38 years. During this time the four known asteroids were counted among the planets, and each had its own planetary symbol. Vesta was normally represented by a stylized hearth (, ⚶). Other symbols are and . All are simplifications of the original .


Photometric observations of the asteroid Vesta were made at the Harvard College Observatorymarker between 1880–82 and at the Observatoire de Toulouse in 1909. These and other observations allowed the rotation rate of the asteroid 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.

Early estimates of the diameter of Vesta ranged from 383 (in 1825) to 444 km. William H. Pickering produced a estimated diameter of 513 ± 17 in 1879, which is close to the modern value for the mean diameter, but the subsequent estimates ranged from a low of 390 km up to a high of 602 km during the next century. The measured estimates were first based on photometry, then later on micrometers and a device called a diskmeter. In 1989, speckle interferometery was used to measure a dimension that varied between 498 and 548 km during the rotational period. In 1991, an occultation of the star SAO 93228 by Vesta was observed from multiple locations in the eastern US and Canada. Based on observations from 14 different sites, the best fit to the data is an elliptical profile with dimensions of about 550 km × 462 km.

Physical characteristics

Vesta is the second-most massive body in the asteroid belt, though only 28% as massive as Ceres. It lies in the Inner Main Belt interior to the Kirkwood gap at 2.50 AU. It has a differentiated interior, and is similar to 2 Pallas in volume (to within uncertainty) but about 25% more massive.

Vesta's shape is relatively close to a gravitationally relaxed oblate spheroid, but the large concavity and protrusion at the pole (see 'Surface features' below) precluded it from being considered a dwarf planet under International Astronomical Union Resolution XXVI 5, which in any case was rejected by the IAU membership. However, Vesta may be listed as a dwarf planet in the future, if it is convincingly determined that its shape, other than the massive impact basin at the southern pole, is due to hydrostatic equilibrium, as currently believed.

Its 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°.

Temperatures on the surface have been estimated to lie between about −20 °C with the Sun overhead, dropping to about −190 °C at the winter pole. Typical day-time and night-time temperatures are −60 °C and −130 °C, respectively. This estimate is for May 6, 1996, very close to perihelion, while details vary somewhat with the seasons.

Geology

There is a large collection of potential samples from Vesta accessible to scientists, in the form of over 200 HED meteorites, giving insight into Vesta's geologic history and structure.

Vesta is thought to consist of a metallic ironnickel core, an overlying rocky olivine mantle, with a surface crust. From the first appearance of Ca-Al-rich inclusions (the first solid matter in the Solar System, forming about 4567 million years ago), a likely time line is as follows:

{|class=wikitable
Vesta is the only known intact asteroid that has been resurfaced in this manner. However, the presence of iron meteoritesand achondriticmeteorite classes without identified parent bodies indicates that there once were other differentiated planetesimalswith igneoushistories, which have since been shattered by impacts.

{|class=wikitable
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 the south polar crater (see below), the crust is thought to be roughly thick.

Surface features

Some Vestian surface features have been resolved using the Hubble Space Telescope and ground based telescopes, e.g. the Keck Telescopemarker.

The most prominent surface feature is an enormous crater in diameter centered near the south pole. Its width is 80% of the entire diameter of Vesta. The floor of this crater is about below, and its rim rises 4–12 km above the surrounding terrain, with total surface relief of about 25 km. A central peak rises above the crater floor. It is estimated that the impact responsible excavated about 1% of the entire volume of Vesta, and it is likely that the Vesta familyand V-type asteroidsare the products of this collision. If this is the case, then the fact that 10 km fragments of the Vesta familyand V-type asteroidshave survived bombardment until the present indicates that the crater is only about 1 billion years old or younger. It would also be the original site of origin of the HED meteorites. In fact, all the known V-type asteroidstaken 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 effector radiation pressure. Spectroscopicanalyses of the Hubbleimages 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.

Several other large craters about wide and deep are also present. A dark albedofeature about across has been named Olbersin honour of Vesta's discoverer, but it does not appear in elevationmaps as a fresh crater would, and its nature is presently unknown, perhaps an old basalticsurface. It serves as a reference point with the 0° longitudeprime meridiandefined to pass through its center.

The eastern and western hemispheres show markedly different terrains. From preliminary spectral analyses of the Hubble Space Telescopeimages, the eastern hemisphere appears to be some kind of high albedo, heavily cratered "highland" terrain with aged regolith, and craters probing into deeper plutonic layers of the crust. On the other hand, large regions of the western hemisphere are taken up by dark geologic units thought to be surface basalts, perhaps analogous to the lunar maria.

Fragments

Some small solar system objects are believed to be fragments of Vesta caused by collisions. The Vestoidasteroids and HED meteoritesare examples. The V-type asteroid1929 Kollaahas been determined to have a composition akin to cumulate eucritemeteorites, indicating its origin deep within Vesta's crust.

Because a number of meteorites are believed to be Vestian fragments, Vesta is currently one of only five identified Solar systembodies for which we have physical samples, the others being Mars, the Moon, comet Wild 2, and Earthitself.

Exploration

The first space mission to Vesta will be NASAmarker's Dawn probe—launched on September 27, 2007—which will orbit the asteroid for nine months from August 2011 until May 2012.Dawnwill then proceed to its other target, Ceres, and will probably continue to explore the asteroid belton an extended mission using remaining fuel. The spacecraft is the first that can enter and leave orbit around more than one body as a result of its weight-efficient ion drivenengines. Once Dawnarrives at Vesta, scientists will be able to calculate Vesta's precise mass based on gravitational interactions. This will allow scientists to refine the mass estimates of the asteroids that are in turn perturbedby Vesta.

Visibility

Its size and unusually bright surface make Vesta the brightest asteroid, and it is occasionally visible to the naked eyefrom dark (non-light polluted) skies. In May and June 2007, Vesta reached a peak magnitudeof +5.4, the brightest since 1989. At that time, opposition and perihelion were only a few weeks apart. It was visible in the constellations Ophiuchusand Scorpius.

Less favorable oppositions during late autumn in the Northern Hemispheremarker still have Vesta at a magnitude of around +7.0.Even when in conjunctionwith the Sun, Vesta will have a magnitude around +8.5; thus from a pollution-free sky it can be observed with binocularseven at elongationsmuch smaller than near opposition.

See also



Notes and references

Footnotes

General references

  • – Horizons can be used to obtain a current ephemeris
  • 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), ISBN 0-8165-2281-2


External links



Timeline in 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, while older basalts are metamorphosed due to the pressure of newer surface layers.
Slow cooling of the interior
Composition of the Vestan crust (in order of increasing depth)
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.

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