Hygiea
Key facts
- What it is
- Large main-belt asteroid
- Size
- About 430 km across
- Discovered
- 1849, by Annibale de Gasparis
- Spacecraft
- Not yet visited
- Orbit group
- Main-belt Asteroid
- Average distance from the Sun
- 3.15 au (about 470 million km)
- Time to go round the Sun
- 5.6 years
- Closest and farthest from the Sun
- 2.81 to 3.49 au
- Tilt of its orbit
- 3.8°
- Spins once every
- 13.8 hours
An au (astronomical unit) is Earth's average distance from the Sun, about 150 million km. Orbit figures are from open tracking data, figures last updated 1 Nov 2026.
Hygiea is the fourth largest body in the asteroid belt, about 430 kilometres across, yet it is far less famous than Ceres or Vesta. That is partly because its surface is very dark, reflecting only a small fraction of the sunlight that falls on it, so it looks faint even through a telescope. It was discovered by Annibale de Gasparis at Naples in 1849.
Hygiea is a C-type asteroid, the dark, carbon-rich kind thought to be related to the carbonaceous chondrite meteorites. It orbits in the outer part of the main belt, where the young Solar System was cool enough for ice to be part of the building material.
In 2019 astronomers imaged Hygiea with a large telescope fitted with adaptive optics, which removes much of the blurring caused by Earth's atmosphere. The pictures showed that it is almost perfectly round, more so than many had expected. A round shape means that its own gravity has pulled it into balance, and that is one of the conditions for being classed as a dwarf planet, so some scientists have argued that Hygiea qualifies. Its surface also lacks the giant impact basin that might have been expected, because Hygiea is the largest member of a big family of asteroids that seem to have come from a major collision.
One explanation is that the impact shattered Hygiea completely, and the pieces then fell back together into a sphere. Hygiea has not been visited, so its full story is still to be told.