| Object type | Main-belt asteroid, C-type (carbonaceous); possible dwarf planet[3] |
|---|---|
| Discovered | April 12, 1849 — Capodimonte Observatory, Naples, by Annibale de Gasparis[3] |
| Named for | Hygieia, the Greek goddess of health[3] |
| Mean diameter | ≈ 433 km (269 mi)[3] |
| Dimensions | ≈ 450 × 430 × 424 km — nearly spherical[3] |
| Mass | ≈ 8.24 × 10¹⁹ kg — about 3% of the main belt's mass[3] |
| Rank | 4th-largest main-belt asteroid, after Ceres, Vesta and Pallas[1] |
| Distance from Sun | 2.81–3.49 AU (average ≈ 3.15 AU)[2] |
| Orbital period | ≈ 5.6 years[2] |
| Orbital inclination | ≈ 3.8°[2] |
| Rotation period | ≈ 13.8 hours (retrograde)[2] |
| Geometric albedo | ≈ 0.07 — a very dark surface[2] |
| Known craters | 2 large: informally “Serpens” (~180 km) and “Calix” (~100 km)[3] |
| Asteroid family | Parent of the Hygiea family, close to 7,000 members[1] |
| Brightness | Around magnitude +10.2 at most oppositions; up to +9.1 at its best[3] |
Discovery and name
Hygiea was discovered on the evening of April 12, 1849, by Italian astronomer Annibale de Gasparis at the Capodimonte Observatory in Naples. It was the tenth asteroid known.[3] Despite its size, six smaller asteroids were found before it, because its dark surface makes it relatively faint.[3]
The observatory's director, Ernesto Capocci, suggested Igea — the Italian spelling of Hygieia, the Greek goddess of health — and de Gasparis added Borbonica to honor King Ferdinand II, giving “Igea Borbonica.” The royal adjective had been dropped by 1852.[3]
Size and shape
Hygiea is the fourth-largest object in the asteroid belt, after Ceres, Vesta and Pallas.[1] Its mean diameter is about 433 km and its mass about 8.24 × 10¹⁹ kg — roughly 3% of the entire main belt's mass.[3]
It is nearly spherical, about 450 × 430 × 424 km, making it the second-most spherical main-belt asteroid after Ceres.[3] Its rotation axis is tilted about 120°, so it spins backward (retrograde) relative to its orbit.[3]
The smallest dwarf planet?
In 2019, astronomers using the SPHERE instrument on ESO's Very Large Telescope resolved Hygiea's shape for the first time and found it nearly spherical. Because it orbits the Sun, is not a moon and has not cleared its orbital neighborhood, a round shape could qualify it as a dwarf planet — potentially the smallest known.[1]
The International Astronomical Union has not officially classified Hygiea as a dwarf planet.[3]
A battered, dark, carbon-rich world
Hygiea's surface is dark, reflecting only about 6–7% of sunlight.[3] It is a carbonaceous (C-type) asteroid whose surface is rich in water-altered clay-like minerals (phyllosilicates) and carbonates; the James Webb Space Telescope found its clays resemble ammoniated saponite.[3]
The VLT images revealed only two clear large craters, too small to explain the huge Hygiea asteroid family. Simulations suggest a head-on collision with a 75–150 km projectile about 2 billion years ago shattered the parent body; the pieces that fell back together formed today's round Hygiea, while the rest became thousands of family members.[1]
Orbit
Hygiea orbits in the outer main belt, between about 2.8 and 3.5 AU from the Sun, taking about 5.6 years to go around once.[2] It is the largest asteroid in the outer main belt.[3]
Can you see Hygiea?
Yes — with modest equipment. At most oppositions Hygiea shines around magnitude +10.2, which calls for at least a 4-inch (100 mm) telescope. At a favorable perihelic opposition it can reach +9.1, when it may just be visible in 10×50 binoculars.[3] It looks like a faint star; you confirm it by seeing it move against the background stars from night to night. Generate a current position from JPL Horizons.
Missions
No spacecraft has visited Hygiea. Proposals have included the EVE concept submitted to NASA's Discovery Program and the INSIDER mission concept proposed to ESA in 2013.[3]
Read more
Is Hygiea the smallest dwarf planet?
How ESO's Very Large Telescope revealed a round Hygiea in 2019, and why the IAU still hasn't changed its label.
Read →Igea Borbonica: the 1849 discovery and naming of Hygiea
A Naples astronomer, a royal dedication, John Herschel's rival suggestion and a serpent-and-star symbol.
Read →How to see Hygiea with binoculars or a small telescope
What magnitude +9 to +10 really means, the gear you need, and how to confirm you've found a moving asteroid.
Read →Sources
- ESO Telescope Reveals What Could be the Smallest Dwarf Planet yet in the Solar System (eso1918) — European Southern Observatory
- Small-Body Database Lookup: 10 Hygiea — NASA JPL Solar System Dynamics
- 10 Hygiea — Wikipedia
- Horizons System — NASA JPL Solar System Dynamics
- Planetary Fact Sheet (orbit comparison) — NASA NSSDCA
Figures are rounded; where sources differ we note it. Spotted an error? Tell us.