The asteroid designated 4 Vesta is one of the largest objects in the asteroid belt with a diameter of 525 kilometres (326 miles). Discovered by Heinrich Wilhelm Olbers on 29 March 1807, and named after Vesta, the Virgin goddess of home and hearth in Roman mythology, the asteroid is the second most massive body in the asteroid belt. Asteroids can be difficult to spot from sites that are not fully dark, but northern hemisphere observers that are fortunate enough to observe from optimal locales can sometimes observe Vesta using small to medium telescopes.
• Aphelion: 2.57138 AU
• Perihelion: 2.15221 AU
• Orbital eccentricity: 0.08874
• Orbital period: 3.63 yr (1 325.75 days)
• Average orbital speed: 19.34 km/sec
• Equatorial rotation velocity: 257.5 m/s
• Mean proper motion: 99.1888 degrees / year
• Volume: 7.46 ± 0.3 × 107 cubic kilometres
• Mass: 2.59076 ± 0.00001 × 1020 kilograms
• Surface area: 8.66±0.2 × 105 square kilometres (Roughly the surface area of Pakistan)
• Mean density: 3.456 ± 0.035 gram/cubic centimetre
• Escape velocity: 0.36 km/sec
• Apparent magnitude: Variable from 5.1 to 8.48
• Satellites: None
Vesta is the second most massive body in the asteroid belt, although it is only 28% as massive as Ceres, which holds pride of place. While Vesta is less dense than the four terrestrial planets, it is more dense than most other asteroids and all of the solar system moons except for Jupiter’s moon, Io.
Although the asteroid has a largely differentiated interior that is overlaid by a 10-km thick crust, and approximates a gravitationally relaxed oblate spheroid in terms of its overall shape, its mass of less than 5×1020 kg and the large surface irregularities at its south pole preclude it from being classified as a dwarf planet. Moreover, data obtained from the Dawn spacecraft has shown that Vesta is not in hydrostatic equilibrium*, which is the primary requirement for classification as a dwarf planet according to Resolution XXVI 5 of the International Astronomical Union (IAU).
* “Hydrostatic equilibrium” refers to a condition in which a body is formed into a sphere or ellipsoid under the force of its own gravity, and where surface irregularities are the result of local features in a thin, but solid crust. It is currently also the major distinguishing criterion that determines whether or not a solar system body is classified as a dwarf planet or a small solar system body, the latter being a distinct classification category. Apart from the Sun, there are currently only 31 known solar system objects known that are said to be in hydrostatic equilibrium.
Temperatures on Vesta’s surface are estimated to be about -20C (-4F) when the Sun is at the zenith, reducing to about -190C (-310F) at the poles. Typical average temperatures during daytime are estimated to be about -60C (-76F), while nighttime temperatures are expected to be as low as -130C (-202F). Note though that these estimates apply to early May 1996, when the asteroid was almost at its closest point of approach to the Sun. Therefore, temperatures on Vesta can vary widely, depending where it is in its orbit relative to the Sun.
Although both the Hubble Space Telescope and several ground-based telescopes had resolved some surface detail on Vesta prior to the Dawn mission that arrived at the asteroid in July of 2011, these details were not clear. The Dawn mission on the other hand, showed fine details that included major crater and other geological features which explained the asteroid’s formation processes.
Two surface features in particular are noteworthy. These are enormous impact craters named Rheasilvia and Veneneia, respectively, with the crater Rheasilvia being the younger since it overlays the crater Veneneia. Rheasilvia is centred on the asteroid’s South Pole, and stretches across 500 km (310 miles), which distance accounts for 95% of the asteroids diameter. The crater is enormous by any standard; it is 19 km (12 miles) deep, the tip of the central peak stands 23 km (14 miles) above the lowest measured point of the crater floor, while the crater’s rim stands 31 km (19 miles) above the lowest point in the crater.
Investigators have estimated that the impact that created the crater had excavated about 1% of the asteroid’s total volume, which makes it very likely that the entire Vesta family asteroids and all the V-type asteroids as well as the HED (howardite-eucrite-diogenite) meteorites were created during the impact, which is thought to have occurred only about one billion years ago.
However, the total mass of all the known V-type asteroids account for approximately 6% of the material ejected during the impact, and it is thought that the remaining 94% consists of either small, undetectable fragments, or larger fragments that have been ejected from the asteroid belt. Nonetheless, Hubble images and spectroscopic studies have shown that the impactor was probably planet-sized, since the crater extends down through several layers of rock, and likely into the asteroid’s mantle, since the asteroid’s spectrum shows the spectral signature of the mineral olivine.
The asteroid Vesta is one of only six known solar system objects of which physical samples exist on Earth. These samples are either complete meteorites, or fragments of meteorites that are believed to have originated on Vesta. In fact, at least one out of every 16 known meteorites on Earth is believed to have come from Vesta. The other solar system bodies of which parts exist on Earth are meteorites from Mars, meteorites from the Moon, rocks and regolith brought back to Earth by the Apollo missions, particles from the comet Wild 2, and fragments of the asteroid 25143 Itokawa.