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Vesta, formal designation 4 Vesta, is an asteroid with a mean diameter of about 530 km.[1] Comprising an estimated 9% of the mass of the entire asteroid belt,[10] it is the second most massive object in the belt (the largest being the dwarf planet Ceres). It was discovered by the German astronomer Heinrich Wilhelm Olbers on March 29, 1807[1] and named after the Roman virgin goddess of home and hearth, Vesta. Vesta is the brightest asteroid. Its greatest distance from the Sun is slightly more than the minimum distance of Ceres from the Sun,[11] and its orbit is entirely within the orbit of Ceres.[12] 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 (HED) meteorites, a rich source of evidence about the asteroid.[13]
The discovery of Ceres in 1801 and Pallas in 1802 led German astronomer Heinrich Wilhelm Olbers to propose that the two objects were the remnants of a destroyed planet. In 1802 he sent a letter with his proposal to the English 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.[14] Olbers commenced his search in 1802, and on March 29, 1807 he coincidentally discovered Vesta in the constellation Virgo. As 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 main asteroid belt. This discovery was announced in a letter addressed to German astronomer Johann H. Schröter dated March 31.[15] Olbers allowed the prominent mathematician Carl Friedrich Gauss to name the asteroid after the Roman virgin goddess of home and hearth, Vesta.[16] The mathematician manually computed the first orbit for Vesta in the remarkably short time of 10 hours.[17][18] After the discovery of Vesta, no further objects were discovered for 38 years.[19] During this time Ceres, Pallas, Juno and Vesta were classified as planets and each had its own planetary symbol. Vesta was normally represented by a stylized hearth (Vesta symbol.svg, ⚶). Other symbols are Old symbol of Vesta and Old planetary symbol of Vesta. All are simplifications of the original 4 Vesta Unsimplified Symbol.svg.[20] Photometric observations of the asteroid Vesta were made at the Harvard College Observatory 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.[21] 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 km 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.[22] 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.[23] Vesta became the first asteroid to have its mass determined. Every 18 years, the asteroid 197 Arete approaches within 0.04 AU of Vesta. In 1966, based upon observations of Vesta's gravitational perturbations of Arete, Hans G. Hertz estimated the mass of Vesta as (1.20 ± 0.08) × 10-10 solar masses.[24] More refined estimates followed, and in 2001 the perturbations of 17 Thetis were used to estimate the mass of Vesta as (1.31 ± 0.02) × 10-10 solar masses.[25] Physical characteristics Vesta is the second-most massive body in the asteroid belt,[4] though only 28% as massive as Ceres.[10] It lies in the Inner Main Belt interior to the Kirkwood gap at 2.50 AU. It has a differentiated interior,[27] and is similar to 2 Pallas in volume (to within uncertainty) but about 25% more massive.[4] Vesta's shape is relatively close to a gravitationally relaxed oblate spheroid,[28] but the large concavity and protrusion at the pole (see 'Surface features' below) combined with a mass less than 5 × 1020 kg precluded Vesta from automatically being considered a dwarf planet under International Astronomical Union (IAU) Resolution XXVI 5.[29] Vesta may be listed as a dwarf planet in the future, if it is convincingly determined that its shape, other than the large impact basin at the southern pole, is due to hydrostatic equilibrium, as currently believed.[27] 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°.[28] 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.[7] 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 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 4,567 million years ago), a likely time line is as follows:[30][31][32][33][34] Timeline of the evolution of Vesta 2–3 million years Accretion completed Elevation diagram of 4 Vesta (as determined from Hubble Space Telescope images of May 1996) viewed from the south-east, showing the south pole crater. 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. Composition of the Vestan crust (in order of increasing depth)[35] A lithified regolith, the source of howardites and brecciated eucrites. 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 kilometres (6 mi) thick.[36] Surface features Some Vestian surface features have been resolved using the Hubble Space Telescope and ground based telescopes, e.g. the Keck Telescope.[37] The most prominent surface feature is an enormous crater 460 kilometres (290 mi) in diameter centered near the south pole.[28] Its width is 80% of the entire diameter of Vesta. The floor of this crater is about 13 kilometres (8.1 mi) 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 kilometres (11 mi) 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.[38] 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, as indicated by spectral signatures of olivine.[28] Several other large craters about 150 kilometres (93 mi) wide and 7 kilometres (4.3 mi) deep are also present. A dark albedo feature about 200 kilometres (120 mi) across has been named Olbers in honour of Vesta's discoverer, but it does not appear in elevation maps as a fresh crater would. Its nature is presently unknown; it may be an old basaltic surface.[39] 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,[38] 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.[38] 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.[13] 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 In 1981, a proposal for an asteroid mission was submitted to the 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 missions 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, Russia and the United States, but none were approved.[40] 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 as a high priority a mission to explore the asteroid belt using a spacecraft with an ion engine. Funding for this program remained problematic for several years, but by 2004 the Dawn vehicle had passed its critical design review.[41] NASA's Dawn probe—launched on September 27, 2007—is the first space mission to Vesta. It will orbit the asteroid for nine months from August 2011 until May 2012.[42] Dawn will then proceed to its other target, Ceres, and will possibly continue to explore the asteroid belt on an extended mission using any 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 driven engines.[42] Once Dawn arrives 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 perturbed by Vesta.[42] 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.[43] At that time, opposition and perihelion were only a few weeks apart. It was visible in the constellations Ophiuchus and Scorpius.[44] Less favorable 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.[45] 2010-2011 In 2010, Vesta reached opposition in the constellation of Leo on the night of February 17–18, when it was about magnitude 6.1,[46] a brightness that makes it visible in binocular range but probably not for the naked eye. However, 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 will next come to opposition on August 5, 2011, in the constellation of Capricornus at about magnitude 5.6.[46] See also * 3103 Eger
Footnotes 1. ^ a b c d e f "JPL Small-Body Database Browser: 4 Vesta". http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=4. Retrieved 2008-06-01.
* Yeomans, Donald K.. "Horizons system". NASA JPL. http://ssd.jpl.nasa.gov/?horizons. Retrieved 2007-03-20. – Horizons can be used to obtain a current ephemeris
* NASA's Dawn Spacecraft will reach orbit around Vesta in July 2011. Retrieved from "http://en.wikipedia.org/"
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