A88. The largest asteroid used to be Ceres. Although it is in the asteroid belt, it is a sphere shape and is now a dwarf planet not an asteroid. It is 620 miles in diameter and contains nearly a third of all the mass of the asteroid belt combined. The new largest asteroid is Vesta at 530 miles mean diameter it has only 9% of the mass in the asteroid belt. While being considered for dwarf planet-hood, a mass below guidelines for hydrostatic equilibrium, and a large crater on the south pole breaking its spheroid shape have so far kept it off the list.