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Hygiea ( minor-planet designation: 10 Hygiea) is a major
asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...
and possible dwarf planet located in the
main asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
. With a diameter of 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. In some spectral classifications it is the largest of the dark
C-type asteroid C-type (carbonaceous) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks ...
s with a
carbonaceous Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up o ...
surface, whereas in others it is second after
1 Ceres Ceres (; minor-planet designation: 1 Ceres) is a dwarf planet in the asteroid belt between the orbits of Mars and Jupiter. It was the first asteroid discovered, on 1 January 1801, by Giuseppe Piazzi at Palermo Astronomical Observatory in Sici ...
. Observations taken with the
Very Large Telescope The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each with a primary mirror 8.2 m across ...
's SPHERE imager in 2017 and 2018, and announced in late 2019, revealed that Hygiea is nearly spherical and is close to a hydrostatic equilibrium shape. The authors of the study therefore consider it a possible dwarf planet. However, Hygiea is conjectured to have been disrupted by an impact, with the subsequent debris reaccumulating, rather than being massive enough to be plastic. The disruptive impact produced the largest known
collisional family In astronomy, a collisional family is a group of objects that are thought to have a common origin in an impact (collision). They have similar compositions and most share similar orbital elements. Numerous asteroid families, most of the irregular ...
.


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 Annibale de Gasparis (9 November 1819, Bugnara – 21 March 1892, Naples; ) was an Italian astronomer, known for discovering asteroids and his contributions to theoretical astronomy. Biography De Gasparis was born in 1819 in Bugnara to A ...
discovered Hygiea on 12 April 1849. At most
oppositions ''Oppositions'' was an architectural journal produced by the Institute for Architecture and Urban Studies from 1973 to 1984. Many of its articles contributed to advancing architectural theory and many of its contributors became distinguished practi ...
, Hygiea has a
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
that is four magnitudes dimmer than Vesta's, and observing it typically requires at least a
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
. However, while at a
perihelic opposition Mars has an orbit with a semimajor axis of 1.524 astronomical units (228 million km), and an eccentricity of 0.0934.Jean Meeus, ''Astronomical Formulæ for Calculators''. (Richmond, VA: Willmann-Bell, 1988) 99. Elements by F. E. Ross The planet o ...
, it can be observed just with 10x50
binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes ( binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
as Hygiea would have a magnitude of +9.1.


Discovery and name

On 12 April 1849, in Naples, Italy, astronomer
Annibale de Gasparis Annibale de Gasparis (9 November 1819, Bugnara – 21 March 1892, Naples; ) was an Italian astronomer, known for discovering asteroids and his contributions to theoretical astronomy. Biography De Gasparis was born in 1819 in Bugnara to A ...
(age 29) discovered Hygiea. It was the first of his nine asteroid discoveries. The director of the Naples observatory,
Ernesto Capocci Ernesto Capocci Belmonte (Picinisco, 31 March 1798 – Naples, 6 January 1864) was an Italian mathematician, astronomer and politician. From 1815 he was a pupil at the Astronomical Observatory of Naples directed by his uncle Federigo Zuccar ...
, named the asteroid. He chose to call it ''Igea Borbonica'' ("
Bourbon Bourbon may refer to: Food and drink * Bourbon whiskey, an American whiskey made using a corn-based mash * Bourbon barrel aged beer, a type of beer aged in bourbon barrels * Bourbon biscuit, a chocolate sandwich biscuit * A beer produced by B ...
Hygieia"), after the Greek goddess of health, daughter of
Asclepius Asclepius (; grc-gre, Ἀσκληπιός ''Asklēpiós'' ; la, Aesculapius) is a hero and god of medicine in ancient Greek religion and mythology. He is the son of Apollo and Coronis, or Arsinoe, or of Apollo alone. Asclepius represen ...
, and in honor of the ruling family of the
Kingdom of the Two Sicilies The Kingdom of the Two Sicilies ( it, Regno delle Due Sicilie) was a kingdom in Southern Italy from 1816 to 1860. The kingdom was the largest sovereign state by population and size in Italy before Italian unification, comprising Sicily and all ...
where Naples was located. In 1852,
John Russell Hind John Russell Hind FRS FRSE LLD (12 May 1823 – 23 December 1895) was an English astronomer. Life and work John Russell Hind was born in 1823 in Nottingham, the son of lace manufacturer John Hind and Elizabeth Russell, and was educated at N ...
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 Zeta (, ; uppercase Ζ, lowercase ζ; grc, ζῆτα, el, ζήτα, label=Demotic Greek, classical or ''zē̂ta''; ''zíta'') is the sixth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 7. It was derived fr ...
-shaped serpent crowned with a star: 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 The caduceus (☤; ; la, cādūceus, from grc-gre, κηρύκειον "herald's wand, or staff") is the staff carried by Hermes in Greek mythology and consequently by Hermes Trismegistus in Greco-Egyptian mythology. The same staff was also ...
: , 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

Based on spectral evidence, Hygiea's surface is thought to consist of primitive
carbonaceous Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up o ...
materials similar to those found in
carbonaceous chondrite Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small prop ...
meteorites.
Aqueous An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be ...
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 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 Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
. However, observations in 2019 suggest Hygiea had suffered a head-on collision which had disrupted it, with its re-accretion resulting in its present spherical shape. No deep basins are visible in the VLT images, indicating that any large craters must have flat floors, consistent with an icy C-type composition. In images taken with the
Very Large Telescope The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each with a primary mirror 8.2 m across ...
imager 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 size of 180 km, Calix is about 90 km in diameter. Hygiea is the largest of the class of dark
C-type asteroid C-type (carbonaceous) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks ...
s that are dominant in the outer asteroid belt, beyond the Kirkwood gap at 2.82 AU. Hygiea has an average diameter of 434 ± 14 km. While early studies (Ragazzoni et al., 2000) have found a noticeably oblate shape with a semimajor axis ratio of 1.11 (much more than for the other objects in the " big four"—
2 Pallas Pallas (minor-planet designation: 2 Pallas) is the second asteroid to have been discovered, after Ceres. It is believed to have a mineral composition similar to carbonaceous chondrite meteorites, like Ceres, though significantly less hydr ...
,
4 Vesta Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
and the dwarf planet
Ceres Ceres most commonly refers to: * Ceres (dwarf planet), the largest asteroid * Ceres (mythology), the Roman goddess of agriculture Ceres may also refer to: Places Brazil * Ceres, Goiás, Brazil * Ceres Microregion, in north-central Goiás ...
), recent results indicate that Hygiea is more spherical, with an axis ratio of 1.06, consistent with a MacLaurin ellipsoid. Aside from being the smallest of the "big 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. 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 Egeria (minor planet designation: 13 Egeria) is a large main-belt G-type asteroid. It was discovered by Annibale de Gasparis on November 2, 1850. Egeria was named by Urbain Le Verrier, whose computations led to the discovery of Neptune, after th ...
and
17 Thetis Thetis, minor planet designation 17 Thetis, is a stony asteroid from the inner regions of the asteroid belt, approximately 90 kilometers in diameter. It was discovered on 17 April 1852, by German astronomer Robert Luther at Bilk Observatory in ...
having lower mean opposition
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
s. At most oppositions, Hygiea has a
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
of around +10.2, which is as much as four orders fainter than Vesta, and observation calls for at least a
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
to resolve. However, at a perihelic opposition, Hygiea can reach +9.1 magnitude and may just be resolvable with 10x50
binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes ( binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
, unlike the next two largest asteroids in the asteroid belt,
704 Interamnia 704 Interamnia is a large F-type asteroid. With a mean diameter of around 330 kilometres, it is the fifth-largest asteroid, after Ceres, Vesta, Pallas and Hygiea. Its mean distance from the Sun is 3.067 ( AU). It was discovered on 2 October ...
and
511 Davida Davida (minor planet designation: 511 Davida) is a large C-type asteroid. It is the one of the largest asteroids; approximately tied for 7th place, to within measurement uncertainties, and the 5th or 6th most massive. It was discovered by R. S. ...
, which are always beyond binocular visibility. A total of 17 stellar
occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
s 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 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 An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
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 In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relations ...
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 In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
with Jupiter and Saturn. The computed
Lyapunov time In mathematics, the Lyapunov time is the characteristic timescale on which a dynamical system is chaotic. It is named after the Russian mathematician Aleksandr Lyapunov. It is defined as the inverse of a system's largest Lyapunov exponent. Use ...
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 Panopaea (minor planet designation: 70 Panopaea) is a large main belt asteroid. Its orbit is close to those of the Eunomia asteroid family; however, Panopaea is a dark, primitive carbonaceous C-type asteroid in contrast to the S-type asteroids ...
). Hygiea has a rotation period of about 13.8256 hours, determined from observations with the VLT in 2017 and 2018. Its single-peaked light curve has an amplitude of 0.27 mag, which is largely attributed to
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
variations. , the direction of Hygiea's rotation is not known, 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. Lightcurve analysis indicates that Hygiea's pole points towards either
ecliptic coordinates The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System bod ...
(β, λ) = (30°, 115°) or (30°, 300°) with a 10° uncertainty. This gives an
axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbit ...
of about 60° in both cases.


Hygiea family

Hygiea is the main member of the Hygiean
asteroid family An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions. An a ...
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.


See also

* Former classification of planets


Notes


References


External links


A simulation of the orbit of Hygiea

Stellar occultation of 11 August 2013 (video)
* (displays Elong from Sun and V mag for 2011) * * {{DEFAULTSORT:000010 Hygiea asteroids Hygiea Hygiea Possible dwarf planets C-type asteroids (Tholen) C-type asteroids (SMASS) 18490412 18490412