4 Vesta

**4 Vesta** () is the second most massive asteroid in the solar system, with an average diameter of about 530 km (around 330 miles) and an estimated mass of 9% of the mass of the entire asteroid belt. Its size and unusually bright surface makes Vesta the brightest asteroid, and the only one ever visible to the naked eye from Earth. Vesta's astronomical symbol is .

## Discovery 
's Moon. Vesta is fourth from the left.]]
Vesta was discovered by the German astronomer Heinrich Wilhelm Olbers on March 29, 1807. He allowed the prominent mathematician Carl Friedrich Gauss to name the asteroid after the Roman virgin goddess of home and hearth, Vesta.

After the discovery of Vesta in 1807, no asteroids were discovered for the next 38 years. During this time the four known asteroids were counted among the planets, and each had its own planetary symbol. Vesta was represented by a stylized hearth, currently . Old graphic variants include ,  and Symbol of Vesta. The modern form is a simplification of the last.
## Physical characteristics 
 images of May 1996.]]

Vesta is the second-most massive body in the asteroid belt. Vesta does have a different interior compared to its surface. It is in the Inner Main Belt, at a distance of about 2.50 AU. It is similar to 2 Pallas in volume, but somewhat more massive.

Vesta's shape is close to the shape of a sphere by its own gravity, but the large concavity and lump at the pole (see 'Surface Features' below) did not fit the criteria for being considered a planet under the International Astronomical Union. It is possible that Vesta may be listed as a dwarf planet in the future, if it is convincingly determined that its shape is from hydrostatic equilibrium.

Its rotation is  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° with an uncertainty of about 10°. This gives an axial tilt of 29°.

## Geology 
For Vesta, there is a large collection of possible samples available 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 iron-nickel core, with a rocky olivine mantle, and 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 timeline is as follows:
* Accretion completed after about 2-3 million years.
* Complete or almost complete melting due to radioactive decay of 26Al, leading to separation of the metal core at about 4-5 million years.
* Progressive crystallization of a convecting molten mantle. Convection stopped when about 80% of the material had crystallized, at about 6-7 million years.
* 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.

Vesta is the only known intact asteroid that has been resurfaced in this way. However, the presence of iron meteorites and achondritic meteorite classes without identified parent bodies indicates that there once were other differentiated planetesimals with igneous histories, which have since been broken by impacts.

## Surface features 
 alongside Earth's Moon.]]

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

The most notable surface feature is a very large crater that is 460 km in diameter centered near the south pole. It would also be the original site of origin of the HED meteorites. In fact, 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 radiation pressure. Spectroscopic analyses of the Hubble images It serves as a reference point with the 0° longitude prime meridian defined to pass through its center.

The eastern and western hemispheres show markedly different terrains. From preliminary spectral analyses of the Hubble Space Telescope images,

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

Proof of HED meteorite origin  is based on data from the Dawn probe which orbited Vesta in the asteroid belt for 10 months.  Vesta is the source of the HED meteorites, which are about 6% of all the meteorites which fall to Earth. These meteorites have pyroxene, which is a mineral rich in iron and magnesium. This has been matched exactly to the mineral signatures on Vesta's surface captured by Dawn's instruments.

At that time, opposition and perihelion were only a few weeks apart. It was visible in the constellations Ophiuchus and Scorpius.

Less favourable oppositions during late autumn in the Northern Hemisphere still have Vesta at a magnitude of around +7.0. Even when in conjunction with the Sun, Vesta will have a magnitude around +8.5; from a pollution-free sky it can be observed with binoculars even at elongations much smaller than near opposition.

## References 
### Further information 

*  — Horizons can be used to obtain a current ephemeris
* K. Keil, *Geological History of Asteroid 4 Vesta: The Smallest Terrestrial Planet* in Asteroids III, William Bottke, Alberto Cellino, Paolo Paolicchi, and Richard P. Binzel, (Editors), Univ. of Arizona Press (2002), 

## Other websites 
* Views of the Solar System: Vesta
* HubbleSite: Hubble Maps the Asteroid Vesta
* Encyclopaedia Britannica, Vesta - full article
* HubbleSite: Hubble Reveals Huge Crater on the Surface of the Asteroid Vesta
* HubbleSite: short movie composed from Hubble Space Telescope images from November 1994.
* Adaptive optics views of Vesta from Keck Observatory
* Differentiated interior of Vesta
* Orbital simulation from JPL (Java)
* 4 Vesta images at ESA/Hubble 
* Hubble views of Vesta on the Planetary Society Weblog  *(includes animation)*
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