10 Hygiea

10 Hygiea

Animated orbit of Hygiea relative to the orbits of the terrestrial planets and Jupiter.
Discovery
Discovered by	A. de Gasparis
Discovery date	April 12, 1849
Designations
Pronunciation	/haɪˈdʒiːə/ hy-JEE-ə
Named after	Hygieia
Alternative names	none
Minor planet category	Main belt (Hygiea family)
Adjective	Hygiean
Orbital characteristics[2]
Epoch November 30, 2008 (JD 2454800.5)
Aphelion	3.507 AU (524.63 Gm) (Q)
Perihelion	2.770 AU (414.38 Gm) (q)
Semi-major axis	3.139 AU (469.58 Gm)
Eccentricity	0.117
Orbital period	5.56 yr (2031.01 d)
Average orbital speed	16.76 km/s
Mean anomaly	197.96° (M)
Inclination	3.842°
Longitude of ascending node	283.45°
Argument of perihelion	313.19°
Proper orbital elements[3]
Proper semi-major axis	3.1417827 AU
Proper eccentricity	0.1356315
Proper inclination	5.1039060°
Proper mean motion	64.621768 deg / yr
Proper orbital period	5.57088 yr (2034.763 d)
Precession of perihelion	128.543611 arcsec / yr
Precession of the ascending node	−96.902395 arcsec / yr
Physical characteristics
Dimensions	530×407×370 ±7 km[4] 431 km (mean)
Surface area	837,080.744 km2 (323,198.682 sq mi)
Mass	(8.67 ±0.15)×1019 kg[4]
Mean density	2.08 ± 0.10 g/cm³[4]
Equatorial surface gravity	0.091 m/s²
Escape velocity	0.21 km/s
Rotation period	27.623 h (1.15 d)[2]
Albedo	0.0717 (geometric)[2]
Temperature	≈164 K max: 247 K (−26°C) [5]
Spectral type	C-type asteroid[2]
Apparent magnitude	9.0[6] to 11.97
Absolute magnitude (H)	5.43[2]
Angular diameter	0.321″ to 0.133″

10 Hygiea is the fourth largest asteroid in the Solar System by volume and mass and is located in the asteroid belt. With somewhat oblong diameters of 350–500 km, and a mass estimated to be 2.9% of the total mass of the belt,^[7] it is the largest of the class of dark C-type asteroids with a carbonaceous surface.

Despite its size, it appears very dim when observed from Earth. This is due to its dark surface and larger-than-average distance from the Sun. For this reason, several smaller asteroids were observed before Annibale de Gasparis discovered Hygiea on April 12, 1849. At most oppositions, Hygiea has a magnitude that is four orders lower than Vesta's, and observing it will require at least a 100 mm (4-inch) telescope, while at a perihelic opposition, it may be observable with 10x50 binoculars.

Discovery and name

Hygiea was discovered by Annibale de Gasparis on April 12, 1849, in Naples, Italy.^[8] 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") in honor of the ruling family of the Kingdom of the Two Sicilies where Naples was located.^[9]

However, in 1852, John Russell Hind wrote that "it is universally termed Hygeia, the unnecessary appendage 'Borbonica' being dropped."^[9] The name comes from Hygieia, the Greek goddess of health, daughter of Asclepius (Aesculapius for the Romans).^[10] The name was often spelled Hygeia in the nineteenth century, for example in the Monthly Notices of the Royal Astronomical Society.^[11]

Physical characteristics

Sizes of the first 10 asteroids discovered profiled against Earth's Moon. Hygiea is furthest right.

The IAU 2006 draft proposal listed Hygiea as a candidate planet^[1]

Hygiea's surface is composed of primitive carbonaceous material with a spectrum similar to those of 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.^[10] The primitive present surface composition would indicate that Hygiea had not been melted during the early period of Solar System formation,^[10] in contrast to other large planetesimals like 4 Vesta.

It is the main member of the Hygiea family and contains almost all the mass in this family (well over 90%). It is the largest of the class of dark C-type asteroids 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.^[10] This is much more than for the other objects in the "big four"—2 Pallas, 4 Vesta and the dwarf planet Ceres. Aside from being the smallest of the four, Hygiea, like Ceres, has a relatively low density, which is more comparable to the icy satellites of Jupiter or Saturn than to the terrestrial planets or the stony asteroids.

While it is the largest body in its region, due to its dark surface and larger-than-average distance from the Sun it appears very dim when observed from Earth. In fact, it is the third dimmest of the first twenty-three asteroids discovered, only 13 Egeria and 17 Thetis having lower mean opposition magnitudes.^[12] At most oppositions, Hygiea has a magnitude of around +10.2,^[12] which is as much as four orders fainter than Vesta, and will require at least a 4-inch (100 mm) telescope to resolve.^[13] At a perihelic opposition, however, Hygiea can reach +9.1 and may just be resolvable with 10x50 binoculars, unlike the next two largest asteroids in the asteroid belt, 704 Interamnia and 511 Davida, which are always beyond binocular visibility.

At least 5 stellar occultations by Hygiea have been tracked by Earth-based astronomers,^[14] but all with few independent observers so that little was learned of its shape. The Hubble Space Telescope has resolved the asteroid, and ruled out the presence of any orbiting companions larger than about 16 km in diameter.^[15]

Orbit and rotation

A rotating frame depiction of asteroid Hygeia's orbital motion relative to Jupiter; the latter (purple loop at upper right) is held nearly stationary (click twice to see animation)

Generally, Hygiea's properties are the most poorly known out of the "big four" objects in the asteroid belt. Its orbit is much closer to the plane of the ecliptic than those of Ceres, Pallas or Interamnia,^[10] but is less circular than Ceres or Vesta with an eccentricity of around 12%.^[2] 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 0.025AU from Ceres, and then in 2063, Hygiea will pass 0.020AU from Pallas.^[16] At aphelion Hygiea reaches out to the extreme edge of the asteroid belt at the perihelia of the Hilda family which is in 3:2 resonance with Jupiter.^[17] Hygiea is used by the Minor Planet Center to calculate perturbations.^[18]

It is an unusually slow rotator, taking 27 hours and 37 minutes for a revolution,^[2] 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.^[10] Lightcurve analysis indicates that Hygiea's pole points towards either ecliptic coordinates (β, λ) = (30°, 115°) or (30°, 300°) with a 10° uncertainty.^[19] This gives an axial tilt of about 60° in both cases.

References

External links

• Shape model deduced from lightcurve
• A simulation of the orbit of Hygiea
• JPL Ephemeris
• "Elements and Ephemeris for (10) Hygiea". Minor Planet Center. Retrieved 26 May 2011.  (displays Elong from Sun and V mag for 2011)