Vesta (vĕsˈtə) [key], in astronomy, the fourth asteroid to be discovered. It was found in 1807 by H. Olbers. It is the third largest asteroid, with a diameter of c.326 mi (525 km). Its average distance from the sun is 2.36 astronomical units, and the period of its orbit is 1,325 days. Vesta is the only asteroid that can be seen with the naked eye; it can be seen only when it is in the right position in the sky relative to the earth and sun, namely, when it is at opposition and perihelion simultaneously.
Vesta is geologically different from the other large asteroids. Images taken by the Hubble Space Telescope show a basaltic surface indicative of ancient lava flows originating from a molten interior and two large, overlapping impact basins at the south pole; the largest, Rhea Silvia, about 295 mi (475 km) across and 8 mi (13 km) deep, may go all the way through the crust to expose the mantle. The crater is thought to have resulted from a collision with another celestial body, the impact tearing out large chunks of Vesta that formed a distinctive class of small asteroids, some of which have reached earth as meteorites. (Alternatively, they all may have originated from the breakup of a large parent body early in the history of the solar system.) These hypotheses were made more credible in 1999 when the space probe Deep Space 1 spectroscopically analyzed the small asteroid Braille and found it to be akin in composition to Vesta. In 2011 the space probe Dawn arrived at Vesta and began a year of study; it determined that Vesta has a metal-rich core constituting some 40% of its diameter and 80% of its mass.