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 18% of its mass. The data from the space probe also suggest that Vesta is a remnant protoplanet.
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