10 Hygiea is the fourth biggest object in the asteroid belt. It is somewhat non-spherical in shape, with diameters of 350 – 500 km. Its mass is estimated to be 3% of the total mass of the asteroid belt.
It is the biggest of the class of dark C-type asteroids with a carbonaceous surface that are dominant in the farther part main belt, which lies beyond the Kirkwood gap at 2.82 AU, and is also the biggest body in this region. Its dark surface and bigger than average distance from the Sun makes it very dim for such a big asteroid when seen from Earth. In fact it is the third dimmest of the first twenty-three asteroids discovered, with only 13 Egeria and the quite small 17 Thetis having lower mean opposition magnitudes.
At most oppositions, Hygiea has a magnitude of around +10.2, which is as much as four orders fainter than Vesta, and will need at least a 4-inch telescope to resolve. At a perihelic opposition, however, Hygiea can reach +9.1 and may just be resolvable with 10x50 binoculars, unlike the fifth and sixth biggest asteroids 704 Interamnia and 511 Davida which are always beyond binocular visibility.