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4 Vesta

4 Vesta (Vesta) is the second most massive object in the asteroid belt, with a mean diameter of about and an estimated mass of 9% of the mass of the entire asteroid belt. It was discovered by the German 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 HED 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 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 was normally represented by a stylized hearth (⚶). Other symbols are and . All are simplifications of the original .

Physical characteristics

Vesta, the second-most massive body in the asteroid belt, is in the Inner Main Belt which lies 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 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. 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. It is believed to have once been spherical.

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 the 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.

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 thick.

Surface features

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 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 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.

Several other large craters about wide and deep are also present. A dark albedo feature about 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 center.

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

Some 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

The first space mission to Vesta will be NASA's Dawn probe--launched on September 27, 2007--which will orbit the asteroid for nine months from August 2011 until May 2012. Dawn will then proceed to its other target, Ceres and will probably continue to explore the asteroid belt on an extended mission using remaining fuel. The spacecraft is the first that can enter and leave orbit around more than one body, thanks to its weight-efficient ion driven engines.

NASA attempted to cancel Dawn in 2006, citing budget pressures and technical issues, but scientists appealed and won an additional $100 million to continue the program. Total mission costs will now be about $450 million.

Visibility

Its size and unusually bright surface make Vesta the brightest asteroid, and it is occasionally visible to the naked eye from dark (non-light polluted) skies. In May and June 2007, Vesta reached a peak magnitude of +5.4, the brightest since 1989. 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; thus from a pollution-free sky it can be observed with binoculars even at elongations much smaller than near opposition.

Notes and references

Footnotes

General references

Yeomans, Donald K. Horizons system. NASA JPL. Retrieved on 2007-03-20.. – 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), ISBN 0-8165-2281-2