4th Largest Asteroid, 10 Hygiea

10 Hygiea is the fourth largest asteroid by volume and mass. It is the largest of the class of dark C-type asteroids with a carbonaceous surface.

Discovery

Hygiea was discovered by Annibale de Gasparis on April 12, 1849, in Naples, Italy. It was the first of his nine asteroid discoveries.

Despite its size, due to its dark surface and larger-than-average distance from the Sun, it appears very dim when observed from Earth. For this reason several smaller asteroids were observed before Hygiea.

Naming

Hygiea, the asteroid, was named after Hygieia, the Greek goddess of health, daughter of Asclepius (God of Medicine and Healing in ancient Greek religion).

Stats

Diameter (mean): 407 km
Semi-major axis: 3.138 AU
Orbital Period: 5.56 years
Rotation period: 27.623 hrs
Date discovered: 1849.4.12
Class: C
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Physical characteristics

Hygiea's surface is composed of primitive carbonaceous material similar to the 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 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 largest of the class of dark C-type asteroids with a carbonaceous surface 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" — the dwarf planet Ceres and the asteroids 2 Pallas and 4 Vesta.

Generally Hygiea's properties are the most poorly known out of the "big four" objects in the asteroid belt.

It is an unusually slow rotator, taking 27 hours and 37 minutes for a revolution, whereas 6 to 12 hours are more typical for large asteroids.

Its direction of rotation is not certain at present, due to a twofold ambiguity in lightcurve data that is exacerbated by its long rotation period — which makes single-night telescope observations span at best only a fraction of a full rotation — but it is believed to be retrograde.