4 Vesta

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4 Vesta

Discovery A
Discoverer	Heinrich Wilhelm Olbers
Discovery date	March 29, 1807
Alternate designations B	none
Category	Main belt ( Vesta family)
Orbital elements C
Epoch November 26, 2005 ( JD 2453700.5)
Eccentricity (e)	0.08902
Semi-major axis (a)	353.268 G m (2.361 AU)
Perihelion (q)	321.82 Gm (2.151 AU)
Aphelion (Q)	384.72 Gm (2.572 AU)
Orbital period (P)	1325.46 d (3.63 a)
Mean orbital speed	19.34 km/ s
Inclination (i)	7.133 °
Longitude of the ascending node (Ω)	103.926°
Argument of perihelion (ω)	150.297°
Mean anomaly (M)	205.652°
Physical characteristics D
Dimensions	578×560×458 km
Mass	2.7×1020 kg
Density	3.4 g/ cm³
Surface gravity	0.22 m/s²
Escape velocity	0.35 km/s
Rotation period	0.2226 d
Spectral class	V-type asteroid
Absolute magnitude	3.20
Albedo ( geometric)	0.423
Mean surface temperature	min: 85 K (-188° C) max: 255 K (-18 °C)

4 Vesta ( IPA /ˈvɛstə/, ( key) ves'-tə) is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass 9% the mass of the entire asteroid belt. Its size and unusually bright surface make Vesta the brightest asteroid, and the only one ever visible to the naked eye from Earth besides 1 Ceres, which is visible under exceptional viewing conditions. Due to the availability of rock samples in the form of the HED meteorites, it has also been the most studied.

Discovery

Size comparison: the first 10 asteroids profiled against Earth's Moon. Vesta is fourth from the left. (The leftmost object, 1 Ceres, is now classified as a dwarf planet)

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 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's symbol is a stylized hearth (see at top right of article).

Symbol and Terminology

When designated by a symbol, Vesta is normally represented by , but sometimes by or . All are simplifications of the original .

With Ceres reclassified as a dwarf planet in 2006, it has been suggested that the IAU may no longer consider Ceres an asteroid, which would make Vesta the largest asteroid in the Solar System. It is possible that Vesta may be classified as a dwarf planet in the future, if it is proven that its shape is due to hydrostatic equilibrium.

Physical characteristics

Vesta is the second-most-massive body in the asteroid belt. Vesta does have a differentiated interior . It is in the Inner Main Belt, which lies interior to the Kirkwood gap at 2.50 AU. It is similar to 2 Pallas in volume (to within uncertainty), but significantly 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 planet under IAU Resolution XXVI 5. In any case, this resolution was rejected by the IAU membership and Vesta will continue to be called an asteroid.

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° 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 seasons .

Geology

For Vesta, there is a large collection of potential samples 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 iron-nickel 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 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.

Elevation diagram of 4 Vesta viewed from the south-east, showing the south pole crater. As determined from Hubble Space Telescope images of May 1996.

Vesta is the only known intact asteroid that has been resurfaced in this manner. 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 shattered by impacts.

Vesta's crust is reasoned to consist of (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.

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 10 km thick.

Surface features

Elevation map of 4 Vesta, as determined from Hubble Space Telescope images of May 1996.

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

The most prominent surface feature is an enormous crater 460 km in diameter centered near the south pole. Its width is 80% of the entire diameter of Vesta. The floor of this crater is about 13 km below, and its rim rises 4-12 km above the surrounding terrain, with total surface relief of about 25 km. A central peak rises 18 km 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 family and V-type asteroids are the products of this collision. If this is the case, then the fact that 10 km fragments of the Vesta family and V-type asteroids have 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 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 which is indicated by spectral signatures of olivine. Interestingly Vesta was not disrupted nor resurfaced by an impact of this magnitude.

Spectral and albedo maps of 4 Vesta, as determined from Hubble Space Telescope images from November 1994.

Several other large craters about 150 km wide and 7 km deep are also present. A dark albedo feature about 200 km across has been named Olbers in honour of Vesta's discoverer, but it does not appear in elevation maps as a fresh crater would, and its nature is presently unknown, perhaps an old basaltic surface. It serves as a reference point with the 0° longitude prime meridian defined to pass through its centre.

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

4 Vesta and 1 Ceres alongside Earth's Moon.

Various small solar system objects are believed to be fragments of Vesta caused by collisions. The Vestoid 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.

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.

Exploration of Vesta

The first space mission to Vesta will be NASA's Dawn probe, which will enter orbit around the asteroid for nine months in 2010-2011.

Aspects