Hygiea is the fourth-largest object in the asteroid belt, a dark carbon-rich world that reflects barely 7% of the sunlight falling on it. Sharp adaptive-optics images taken in 2019 revealed a surprisingly round shape, hinting that an ancient impact shattered it completely and the debris re-assembled into a near-sphere.
Open the 3D Solar SystemA dwarf planet is defined partly by being massive enough for gravity to pull it round - and Hygiea appears nearly as round as Ceres, probably because it re-accreted from rubble after a shattering collision. It is a reminder that our neat categories - asteroid, dwarf planet - are labels stretched over a messy continuum of real objects.
| Diameter | 434 km |
| Mass | 8.700e+19 kg |
| Density | 1.9 g/cm³ |
| Rotation period | 13.8 h |
| Geometric albedo | 0.07 |
| Atmosphere | None. |
| Composition | C-type: primitive, carbon-rich rock with water-bearing minerals. |
| Discovered by | Annibale de Gasparis |
| Year | 1849 |
| How | Naples Observatory refractor |
Hygiea’s rotation period was revised from 27.6 h to 13.8 h by 2019 adaptive-optics imaging; its mass and density remain approximate.