3rd Largest Asteroid, 4 Vesta

Vesta is thought to be a remnant protoplanet with a differentiated interior. It lost some 1% of its mass less than a billion years ago in a collision that left an enormous crater occupying much of its southern hemisphere. Debris from this event has fallen to Earth as Howardite–Eucrite–Diogenite (HED) meteorites, a rich source of information about the asteroid.

Discovery

Heinrich Wilhelm Matthäus Olbers discovered Pallas in 1802, the year after the discovery of Ceres. He proposed that the two objects were the remnants of a destroyed planet.

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.

Olbers commenced his search in 1802, and on March 29, 1807 he discovered Vesta in the constellation Virgo — a coincidence, as Ceres, Pallas, and Vesta are not fragments of a larger body.

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 asteroid belt.

Naming

As Olbers already had credit for discovering a planet, Pallas, he gave the honor of naming his new discovery to German mathematician Carl Friedrich Gauss. Gauss's orbital calculations had enabled astronomers to confirm the existence of Ceres, the first asteroid, and who had computed the orbit of the new planet in the remarkably short time of 10 hours.

Gauss decided on the Roman virgin goddess of home and hearth, Vesta.

Stats

Diameter: 530 km
Semi-major axis: 2.362 AU
Orbital Period: 3.63 years
Rotation period: 5.342 hrs
Date discovered: 1807.3.29
Class: V
Family: Vesta
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Planetary Status

When Vesta was discovered by Olbers, Ceres, Pallas, and Juno were classified as planets. Vesta was likewise classified as a planet, and the Solar System was thought to have eleven planets.

After the discovery of Vesta, no further objects were discovered for 38 years. However, in 1845 new asteroids started being discovered at a rapid pace. By 1851 there were fifteen asteroids, in addition to the seven major planets. That year Benjamin Apthorp Gould suggested numbering asteroids in their order of discovery.

Orbit

Vesta orbits in the inner asteroid belt interior to the Kirkwood gap at 2.50 AU. Vesta's orbit lies entirely within the Cererian orbit.

Physical characteristics

Vesta is the brightest asteroid visible from Earth.

Vesta's 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.

Exploration

NASA's Dawn spacecraft entered orbit around Vesta on July 16, 2011 for a planned one-year exploration, and what is known about Vesta will be refined and extended as data from Dawn is received, analyzed and published.

Future Status

Vesta's shape is relatively close to a gravitationally relaxed oblate spheroid, but the large concavity and protrusion at the pole 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.

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.