4 Vesta is the second most massive asteroid in the solar system , with an average diameter of about 530 km ( around 330 miles ) and an estimated mass 9 % the mass of the entire asteroid belt . Vesta , formal designation 4 Vesta , is an asteroid that is thought to be a remnant protoplanet with a differentiated interior and a mean diameter of about 530 km . Comprising an estimated 9 % of the mass of the entire asteroid belt , it is the second-most-massive object in the belt after the dwarf planet Ceres .