The image above includes a light-curve derived 3D model of the C-type asteroid designated 10 Hygiea, the fourth largest asteroid in the solar system by both volume and mass, the latter of which is estimated to account for about 2.9% of all the mass in the asteroid belt. It is also the largest of the C-type asteroids with surfaces that largely consist of carbon.
10 Hygiea was discovered by Annibale de Gasparis from Naples on 12 April 1849, and it was originally named Igea Borbonica (“Bourbon Hygieia”) by the director of the Naples observatory in honor of the royal family of the Kingdom of the Two Sicilies, in which the city of Naples was located at the time.
• Aphelion: 3.5024 AU
• Perihelion: 2.7817 AU
• Eccentricity: 0.1146
• Orbital period: 5.57 years (2,034.3 days)
• Equatorial rotation velocity: 27.623 hours
• Average orbital speed: 16.76 km/sec
• Mean proper motion: Undetermined
• Dimensions: 530 × 407 × 370 kilometres
• Mass: 8.67 ± 0.15 × 1019 kilograms
• Surface area: 837,080.744 square kilometres
• Mean density: 2.08 ± 0.10 gram/cubic centimetre
• Escape velocity: 0.091 m/sec
• Apparent magnitude: Variable from 9.0 to 11.97
• Satellites: None
Even though Hygiea is relatively large, it is also very dim when viewed from Earth due to its dusky surface and above average distance from the Sun. In fact, of the first 23 asteroids to be discovered, Hygiea is the 3rd dimmest, and only 13 Egeria and 17 Thetis have lower magnitudes when they are in opposition. At best, Hygiea reaches a magnitude of +10.2 at opposition, which is at least four orders of magnitude dimmer than Vesta ever get from a similar position. Nonetheless, Hygiea can be observed with small and medium sized telescopes, as well as with 10 x 50 binoculars from dark sites during times when the asteroid is at perihelic opposition, i.e. closest to the Sun.
In general, the physical properties of Hygiea are the least understood of the four biggest objects in the asteroid belt. The usual practice of measuring the shape and diameter of objects during stellar occultations has produced few results: on at least five occasions, there were not enough independent observers available to determine meaningful parameters. Nonetheless, the Hubble Space Telescope has observed Hygiea directly, which has ruled out the possibility that the asteroid may have one or more satellites that are bigger than 16 kilometres (9.9 miles) in diameter.
Based solely on evidence gathered through spectral analysis, most investigators believe that Hygiea’s surface is composed of primitive, if not primordial carbon-rich material that is similar to those commonly found in carbonaceous chondrite meteorites. However, the fact that several aqueous alteration products have been detected in spectra of the asteroid’s surface suggests that there must have been water ice present at some point in the asteroid’s past.
Although the water ice must have melted to produce the observed substances on the asteroid’s surface, the overall primitive nature of Hygiea’s surface rules out the possibility that the entire asteroid had melted during its formation, unlike other large bodies in the asteroid belt such as Vesta, and others.
Size and Mass
Hygiea is the smallest of the “Big Four” objects in the asteroid belt (which includes the asteroids Vesta and Ceres), but it does accounts for well over 90% of all the mass of the C-type asteroids that inhabit the region beyond the Kirkwood gap in the asteroid belt that occurs at 2.82 AU from the Sun. Collectively, this group of asteroids are known as the Hygiea family of asteroids, with Hygiea its main member.
Based on computer models, Hygiea has a distinctly oblate spheroid shape with a mean diameter of 444 ± 35 kilometres, giving it a semi-major axis ratio of 1.11, which is more pronounced than the ratios for 4 Vesta, 2 Pallas, and Ceres. Moreover, Hygiea also has a mean density that is much lower than the average for most asteroids, and like Ceres its density is closer to that of the icy moons of Saturn and Jupiter than it is to stony meteorites or the rocky planets.
Unlike most other asteroids that rotate in the range of between 6 and 12 hours, Hygiea takes 27 hours and 37 minutes to complete one rotation, although the direction of rotation has not been established. Due to its long rotation period (that makes it impossible to observe one full rotation in a single night), and an ambiguity in light curve data that is made worse by the long rotation period, the asteroid’s direction of rotation can only be guessed at, although most investigators are of the opinion that Hygiea rotates in a retrograde fashion.