10 Hygiea

Hygiea (minor-planet designation: 10 Hygiea) is a major asteroid located in the main asteroid belt. With a mean diameter of between $450 km$ km and a mass estimated to be 3% of the total mass of the belt, it is the fourth-largest asteroid in the Solar System by both volume and mass, and is the largest of the C-type asteroids (dark asteroids with a carbonaceous surface) in classifications that use G type for 1 Ceres. It is very close to spherical, apparently because it had re-accreted after the disruptive impact that produced the large Hygiean family of asteroids.

Observation
Despite its size, Hygiea appears very dim when observed from Earth. This is due to its dark surface and its position in the outer main belt. For this reason, six smaller asteroids were observed before Annibale de Gasparis discovered Hygiea on 12 April 1849. At most oppositions, Hygiea has a magnitude that is four magnitudes dimmer than Vesta's, and observing it typically requires at least a 100 mm telescope. However, while at a perihelic opposition, it can be observed just with 10×50 binoculars as Hygiea would have a magnitude of +9.1.

Discovery and name
On 12 April 1849, in Naples, Italy, astronomer Annibale de Gasparis (age 29) discovered Hygiea. It was the first of his nine asteroid discoveries. The director of the Naples observatory, Ernesto Capocci, named the asteroid. He chose to call it Igea Borbonica ("Bourbon Hygieia"), after the Greek goddess of health, daughter of Asclepius, and in honor of the ruling family of the Kingdom of the Two Sicilies where Naples was located.

In 1852, John Russell Hind wrote that "it is universally termed Hygiea, the unnecessary appendage 'Borbonica' being dropped." The English form is an irregular spelling of Greek Hygieia or Hygeia (Latin Hygea or Hygia).

Symbol
The intended astronomical symbol for Hygiea was a zeta-shaped serpent crowned with a star, in the pipeline for Unicode 17.0 as U+1F779 🝹 (Hygiea symbol (original, fixed width).svg); the serpent and serpent drinking from a bowl are traditional symbols of the goddess Hygieia (cf. U+1F54F 🕏). In later years it was substituted with a rod of Asclepius: (a serpent twined around a staff, U+2695 ⚕), confusing Hygieia with her masculine counterpart. These symbols are now both largely obsolete. In this century, 10 Hygiea has seen some minor astrological use, and its symbol was confused once again, with Asclepsius's rod replaced by Mercury's caduceus:, though in a more elaborate form (U+2BDA ⯚) than the symbol of the planet Mercury. The caduceus has long been mistaken for the rod of Asclepius.

Physical characteristics


Observations taken with the Very Large Telescope's SPHERE imager in 2017 and 2018 revealed that Hygiea is nearly spherical and is close to a hydrostatic equilibrium shape. Based on spectral evidence, Hygiea's surface is thought to consist of primitive carbonaceous materials similar to those found in 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 indicates that Hygiea had not melted during the early period of Solar System formation. However, observations suggest Hygiea suffered a major collision early in its history that completely disrupted it, with its present spherical shape due to re-accretion of the disrupted material. No deep basins are visible in VLT images, indicating that any large craters that formed after re-accretion must have flat floors, consistent with an icy C-type composition.

In images taken with the VLT in 2017, a bright surface feature is visible, as well as at least two dark craters, which have been informally named Serpens and Calix after the Latin words for 'snake' and 'cup', respectively. Serpens has a diameter of 180 km, Calix of 90 km.

Hygiea is the largest of the class of dark C-type asteroids that are dominant in the outer asteroid belt, beyond the Kirkwood gap at 2.82 AU. Its mean diameter $10 km$. Hygiea is close to spherical, with an axis ratio of $433 km$ that is consistent with a MacLaurin ellipsoid. Aside from being the smallest of the "big four", Hygiea has a relatively low density of $434 km$, comparable to Ceres (2.16) and the larger icy satellites of the Solar System (Ganymede 1.94, Callisto 1.83, Titan 1.88, Triton 2.06) rather than to Pallas ($0.94$) or Vesta (3.45).

Although it is the largest body in its region, due to its dark surface and farther-than-average distance from the Sun, Hygiea appears very dim when observed from Earth. In fact, it is the third dimmest of the first twenty-three asteroids discovered, with 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 observation calls for at least a 4 in telescope to resolve. However, at a perihelic opposition, Hygiea can reach +9.1 magnitude and may just be resolvable with 10 × 50 binoculars, unlike the next two largest asteroids in the asteroid belt, 704 Interamnia and 511 Davida, which are always beyond binocular visibility.

A total of 17 stellar occultations by Hygiea have been tracked by Earth-based astronomers, including two (in 2002 and 2014) that were seen by a large number of observers. The observations have been used to constrain Hygiea's size, shape and rotation axis. The Hubble Space Telescope has resolved the asteroid and ruled out the presence of any orbiting companions larger than about 16 km in diameter.

Orbit and rotation
Orbiting at an average of 3.14 AU from the Sun, Hygiea is the most distant of the "big four" asteroids. It lies closer to the ecliptic as well, with an orbital inclination of 4°. Its orbit is less circular than those of Ceres or Vesta, with an eccentricity of around 0.12. Its perihelion is at a quite similar longitude to those of Vesta and Ceres, though its ascending and descending nodes are opposite to the corresponding ones for those objects. Although its perihelion is extremely close to the mean distance of Ceres and Pallas, a collision between Hygiea and its larger companions is impossible because at that distance they are always on opposite sides of the ecliptic. In 2056, Hygiea will pass 0.025 AU from Ceres, and then in 2063, Hygiea will pass 0.020 AU from Pallas. At aphelion Hygiea reaches out to the extreme edge of the asteroid belt at the perihelia of the Hilda family, which is in a 3:2 orbital resonance with Jupiter.

As one of the most massive asteroids, Hygiea is used by the Minor Planet Center to calculate perturbations.

Hygiea is in an unstable three-body mean motion resonance with Jupiter and Saturn. The computed Lyapunov time for this asteroid is 30,000 years, indicating that it occupies a chaotic orbit that will change randomly over time because of gravitational perturbations by the planets. It is the lowest numbered asteroid in such a resonance (the next lowest numbered being 70 Panopaea).

Hygiea has a rotation period of 13.83 hours. Its single-peaked light curve has an amplitude of 0.27 mag, which is largely attributed to albedo variations. Hygiea's north pole points towards ecliptic longitude $8.74 kg$ and ecliptic latitude $8.32 kg$, which gives an axial tilt of 119° with respect to the ecliptic.

Hygiea family


Hygiea is the main member of the Hygiean asteroid family that constitutes about 1% of asteroids in the main belt. The family was formed when an object with a diameter of about 100 km collided with proto-Hygiea about 2 billion years ago. Because the impact craters on Hygiea today are too small to contain the volume of ejected material, it is thought that Hygiea was completely disrupted by the impact and that the majority of the debris recoalesced after the pieces that formed the rest of the family had escaped. Hygiea contains almost all the mass (over 98%) of the family.