10 Hygiea is the fourth biggest object in the asteroid belt , somewhat non-spherical with diameters of 350 â `` 500 km and a mass estimated to be 3 % of the total mass of the asteroid belt . 10 Hygiea is an asteroid located in the main asteroid belt . With somewhat oblong diameters of 350 â `` 500 km , and a mass estimated to be 2.9 % of the total mass of the belt , it is the fourth largest object in the region by volume and mass. .