4 Vesta is the second most massive asteroid in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% the mass of the entire asteroid belt. Its size and unusually bright surface make Vesta the brightest asteroid, and the only one ever visible to the naked eye from Earth, apart from 1 Ceres under exceptional viewing conditions. Due to the availability of rock samples in the form of the HED meteorites, it has also been the most studied. NASA's Dawn spacecraft entered orbit around Vesta on July 16, 2011 for a one year visit.
Millbillillie w:HED meteorite