10 Hygiea - Physical Characteristics

Physical Characteristics

Hygiea's surface is composed of primitive carbonaceous material with a spectrum similar to those of carbonaceous chondrite meteorites. Aqueous alteration products have been detected on its surface, which could indicate the presence of water ice in the past which was heated sufficiently to melt. The primitive present surface composition would indicate that Hygiea had not been melted during the early period of Solar System formation, in contrast to other large planetesimals like 4 Vesta.

It is the main member of the Hygiea family and contains almost all the mass in this family (well over 90%). It is the largest of the class of dark C-type asteroids that are dominant in the outer asteroid belt—which lie beyond the Kirkwood gap at 2.82 AU. Hygiea appears to have a noticeably oblate spheroid shape, with an average diameter of 444 ± 35 km and a semimajor axis ratio of 1.11. This is much more than for the other objects in the "big four"—2 Pallas, 4 Vesta and the dwarf planet Ceres. Aside from being the smallest of the four, Hygiea, like Ceres, has a relatively low density, which is more comparable to the icy satellites of Jupiter or Saturn than to the terrestrial planets or the stony asteroids.

While it is the largest body in its region, due to its dark surface and larger-than-average distance from the Sun it appears very dim when observed from Earth. In fact, it is the third dimmest of the first twenty-three asteroids discovered, only 13 Egeria and 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 require at least a 4-inch (100 mm) telescope to resolve. At a perihelic opposition, however, Hygiea can reach +9.1 and may just be resolvable with 10x50 binoculars, unlike the next two largest asteroids in the asteroid belt, 704 Interamnia and 511 Davida, which are always beyond binocular visibility.

At least 5 stellar occultations by Hygiea have been tracked by Earth-based astronomers, but all with few independent observers so that little was learned of its shape. The Hubble Space Telescope has resolved the asteroid, and ruled out the presence of any orbiting companions larger than about 16 km in diameter.