View source for 10 Hygiea

{{Short description|Major asteroid}}
{{Use dmy dates|date=October 2019}}
{{Good article}}
{{Infobox planet
 | minorplanet     = yes
 | background      = #D6D6D6
 | name            = 10 Hygiea
 | symbol          = [[File:Hygiea symbol (original, bold).svg|24px|alt=🝹|A zeta-shaped serpent crowned with a star]] (historical astronomical), [[File:Hygiea symbol (bold).svg|24px|alt=⯚|Mercury's caduceus]] (modern astrological)
 | image           = SPHERE image of Hygiea.jpg
 | image_scale      =
 | caption         = Hygiea's southern hemisphere imaged by the [[Very Large Telescope]] in July 2017
 | discovery_ref   = <ref name="jpldata" />
 | discoverer      = [[Annibale de Gasparis]]
 | discovery_site  = [[Astronomical Observatory of Capodimonte]]
 | discovered      = 12 April 1849
 | pronounced      = {{IPAc-en|h|aɪ|ˈ|dʒ|iː|ə}}<ref>{{OED|Hygeia}}</ref>
 | adjectives      = Hygiean {{IPAc-en|h|aɪ|ˈ|dʒ|iː|ə|n}}<ref>{{OED|hygeian}}</ref> 
 | mpc_name        = (10) Hygiea
 | alt_names       = A849 GA{{·}}A900 GA
 | named_after     = [[Hygieia]]
 | mp_category     = [[Asteroid belt|Main belt]] ([[Kirkwood gap|middle]]){{·}}[[Hygiea family]]
 | orbit_ref       = <ref name="jpldata"/>
 | epoch           = 5 May 2025 ([[Julian day|JD]] 2460800.5)
 | uncertainty     = 0
 | observation_arc = <!-- over 170+ years; this asteroid is easily observable, so the observation arc will always increase with time. No need to include this parameter. -->
 | aphelion        = {{convert|3.488|AU|e6km|abbr=unit|lk=on|0}}
 | perihelion      = {{convert|2.799|AU|e6km|abbr=unit|lk=on|0}}
 | time_periastron = 9 February 2028
 | semimajor       = {{convert|3.144|AU|e6km|abbr=unit|lk=on|0}}
 | eccentricity    = 0.1096
 | period          = 5.57 [[Julian year (astronomy)|yr]] (2,036 days)
 | mean_anomaly    = 181.38[[degree (angle)|°]]
 | mean_motion     = {{Deg2DMS|0.17683|sup=ms}} / day
 | inclination     = 3.832° to [[ecliptic]]
 | asc_node        = 283.13°
 | arg_peri        = 312.71°
 | p_orbit_ref     = <ref name="AstDyS-proper"/>
 | p_semimajor     = 3.1418
 | p_eccentricity  = 0.1356
 | p_inclination   = 5.1039°
 | p_mean_motion   = 64.6218
 | perihelion_rate = 128.544
 | node_rate       = −96.9024
 | dimensions      = {{val|450|x|430|x|424|u=km}}<br />(±{{nbsp}}{{val|10|x|10|x|20|u=km}})<ref name="Vernazza2021"/>{{rp|18}}
 | mean_diameter   = {{val|433|8|u=km}} (2021)<ref name="Vernazza2021"/>{{rp|6}}<br />{{val|434|14|u=km}} (2020)<ref name="Vernazza2020"/>{{rp|136}}
 | flattening      = {{val|0.06|0.05}}{{efn|1=Flattening derived from the maximum aspect ratio (c/a): <math>f = 1 - \frac{c}{a}</math>, where (c/a) {{=}} {{val|0.94|0.05}}.<ref name="Vernazza2021"/>{{rp|6}}}}
 | surface_area    = 
 | volume          = 
 | mass            = {{val|8.74|0.69|e=19|u=kg}} (2021)<ref name="Vernazza2021"/>{{rp|6}}<br />{{val|8.32|0.80|e=19|u=kg}} (2020)<ref name="Vernazza2020"/>{{rp|137}}
 | density         = {{val|2.06|0.20|u=g/cm3}} (2021)<ref name="Vernazza2021"/>{{rp|6}}<br />{{val|1.944|0.250|u=g/cm3}} (2020)<ref name="Vernazza2020"/>{{rp|137}}
 | surface_grav    = {{Gr|0.085|217}}{{nbsp}}m/s<sup>2</sup> (average) <!-- The Gr template calculates surface grav. from inputted mass (10^20 kg) and radius (km) -->
 | escape_velocity = {{V2|0.085|217}}{{nbsp}}m/s (average) <!-- The V2 template calculates esc. vel. from inputted mass (10^20 kg) and radius (km) -->
 | sidereal_day    = {{val|13.82559|0.00005|u=h}}<ref name="Vernazza2020"/>{{rp|136}}<ref name="Vernazza2021"/>{{rp|18}}
 | right_asc_north_pole = {{val|319|3|u=°}}<ref name="Vernazza2020"/>{{rp|136}}
 | declination     = {{val|-46|3|u=°}}<ref name="Vernazza2020"/>{{rp|136}}
 | pole_ecliptic_lon = {{val|306|3|u=°}}<ref name="Vernazza2021"/>{{rp|18}}
 | pole_ecliptic_lat = {{val|-29|3|u=°}}<ref name="Vernazza2021"/>{{rp|18}}
 | axial_tilt      = {{val|120|u=°}} to ecliptic<ref name="Vernazza2021"/>{{rp|18}}
 | albedo          = 0.063 (2021)<ref name="Vernazza2021"/>{{rp|6}}<br/>0.072 (2020)<ref name="Vernazza2020"/>{{rp|139}}
 | temp_name1      = [[Kelvin]]
 | min_temp_1      = ≈100<ref name="Johnston1989"/> <!-- page 337. Can't add {{rp|337}} here, otherwise it will widen the infobox too much -->
 | mean_temp_1     = 163<ref name="Johnston1989"/> <!-- page 336. Can't add {{rp|336}} here, otherwise it will widen the infobox too much -->
 | max_temp_1      = {{val|230|5}}<ref name="Barucci2002"/><ref name="Lim2005"/> <!-- page 204 and 396 for Barucci2002 and Lim2005, respectively -->
 | spectral_type   = [[C-type asteroid|C]]<ref name="jpldata"/>
 | magnitude       = 9.0 to 12.0<ref name="MPC-object"/>
 | abs_magnitude   = 5.65<ref name="jpldata"/>
 | angular_size    = 0.321″<!-- Horizons 2078-May-20 --> to 0.133″
}}

'''10 Hygiea''' is a large [[asteroid]] located in the [[Kirkwood gap|outer]] main [[asteroid belt]] between the orbits of [[Mars]] and [[Jupiter]]. It was the tenth known asteroid, discovered on 12 April 1849 by Italian astronomer [[Annibale de Gasparis]] at the [[Astronomical Observatory of Capodimonte]] in [[Naples|Naples, Italy]]. It was named after [[Hygieia]], the [[Greek mythology|Greek goddess]] of health. It is the [[List of exceptional asteroids#Largest by diameter|fourth-largest main-belt asteroid]] by both volume and mass, with a [[mean diameter]] of {{cvt|433|km|mi}} and a mass constituting 3% of the main asteroid belt's total mass.

Hygiea has a nearly spherical shape, with two known [[impact crater|craters]] about {{cvt|100 and 180|km|mi}} in diameter. Because of its shape and large size, some researchers consider Hygiea a [[List of possible dwarf planets|possible dwarf planet]]. Hygiea has a dark, [[carbon|carbonaceous]] surface consisting of [[hydrate|hydrated]] and [[ammonia|ammoniated]] [[silicate mineral]]s, with [[carbonate]]s and water ice. Hygiea's subsurface likely contains a large fraction of water ice. These characteristics make Hygiea very similar to the main-belt [[dwarf planet]] [[Ceres (dwarf planet)|Ceres]], which suggests the two objects have similar origins and evolutionary histories.<ref name="Rivkin2025"/>{{rp|2}}

Hygiea is the [[parent body]] of the [[Hygiea family]], an [[asteroid family]] comprising over 7,000 known asteroids that share similar orbital and compositional characteristics with Hygiea.<ref name="eso1918"/> The Hygiea family is believed to have formed by a giant impact on Hygiea about 2 to 3 billion years ago.<ref name="Carruba2014"/> This impact is thought to have shattered Hygiea, which led to its reaccumulation as a nearly spherical body.
<!-- The leads in most featured articles contain about 250 to 400 words. See [[MOS:LEADLENGTH]] for details on the appropriate length of an introductory lead. -->

== History ==
=== Discovery ===
[[File:Annibale De Gasparis.gif|thumb|left|Annibale de Gasparis discovered Hygiea on 12 April 1849.]]

Hygiea was discovered on the evening of 12 April 1849 by Italian astronomer [[Annibale de Gasparis]] at the [[Astronomical Observatory of Capodimonte]] in [[Naples|Naples, Italy]].<ref name="Annals"/>{{rp|XV}}<ref name="Gargano2023"/>{{rp|10}} On that night he was using the observatory's Reichenbach [[Equatorial mount|equatorial telescope]]<ref name="Palma2019"/>{{efn|The Reichenbach equatorial telescope was the smaller of the Capodimonte Observatory's two [[Equatorial mount|equatorial telescopes]], with an [[Objective (optics)|objective]] [[lens]] diameter of {{cvt|8.3|cm|in}} and [[focal length]] of {{cvt|120|cm|in}}.<ref name="Gargano2023"/>{{rp|9}} The Reichenbach equatorial telescope was located in the northern dome of the observatory.<ref name="Gargano2023"/>{{rp|10}}}} to observe a section of the sky in the 12th [[hour]] of [[right ascension]], as part of his larger project of [[star catalogue|cataloguing stars]] along the [[ecliptic]] up to the 14th [[apparent magnitude]].<ref name="Annals"/>{{rp|XV–XVI}}<ref name="Gargano2023"/>{{rp|10}} While comparing his observations to the [[Prussian Academy of Sciences|Berlin Academy's]] [[star chart]],<ref name="Gargano2023"/>{{rp|10}}{{efn|The Berlin Academy star chart, which was published in years between 1830 and 1859, was compiled by a collaboration of astronomers who were each allotted a section of the sky spanning one hour of right ascension.<ref name="Jones2002"/>{{rp|367}} The stars in the 12th (XXII) hour of right ascension were charted by [[Carl August von Steinheil]],<ref name="Jones2002"/>{{rp|368}} whose map aided de Gasparis's discovery of Hygiea.<ref name="Schumacher"/><ref name="Hind"/>{{rp|126}}<ref name="Annals"/>{{rp|XVI}}}} de Gasparis noticed a starlike object between magnitude 9 and 10 which was not recorded before.<ref name="Hind"/>{{rp|126}}<ref name="Gargano2023"/>{{rp|10}}{{efn|Hygiea was located at the 12th hour of [[right ascension]] in the sky at the time of its discovery.<ref name="discovery-location"/>}} Although poor weather prevented further observations for a few days, de Gasparis reobserved the object on 14 and 17 April and confirmed it had moved between these dates.<ref name="Hind"/>{{rp|126}}<ref name="Schumacher"/> Judging by how much the object had moved, de Gasparis concluded it must be another new [[planet]]{{efn|Asteroids were referred to as "[[planet]]s" at the time of Hygiea's discovery. These would eventually be recategorized as "[[minor planet]]s".<ref name="Ashworth2022"/>}} between the orbit of [[Mars]] and [[Jupiter]].<ref name="Hind"/>{{rp|126}} De Gasparis announced his discovery to Erasmo Fabri Scarpellini,<ref name="Leuschner"/>{{rp|28}} secretary of the ''Correspondenza Scientifica'' bulletin at [[Rome|Rome, Italy]],<ref name="Hind"/>{{rp|126}} who in turn passed on the news to [[Heinrich Christian Schumacher]], who published it in the ''[[Astronomische Nachrichten]]'' journal on 11 May 1849.<ref name="Schumacher"/><ref name="Ashworth2022"/> Hygiea was the first asteroid discovered by de Gasparis and the tenth asteroid discovered in history.<ref name="Ashworth2022"/><ref name="Gould1852"/>{{rp|80}} He would later discover another six asteroids between 1850 and 1853 and two more during the 1860s.<ref name="Ashworth2022"/>

=== Name ===
De Gasparis invited [[Ernesto Capocci|Ernesto Capocci Belmonte]], his friend and director of the Capodimonte Observatory, to assign a name to the asteroid as gratitude for his help and advice.<ref name="Annals"/>{{rp|XVI}}<ref name="Gargano2023"/>{{rp|10}} Capocci suggested the name ''Igea''—the Italian spelling of [[Hygieia]], the [[Greek mythology|Greek goddess]] of health and daughter of [[Asclepius]].<ref name="Palma2019"/><ref name="Gargano2023"/>{{rp|10}} De Gasparis added the adjective ''Borbonica'' to Capocci's suggested name, to honor [[Ferdinand II of the Two Sicilies|King Ferdinand II]] of the [[House of Bourbon|Bourbons of Naples]] who ruled the [[Kingdom of the Two Sicilies]] and supported the work of the Capodimonte Observatory.<ref name="Badolati2007"/><ref name="Annals"/>{{rp|XVI}} Together, de Gasparis and Capocci proposed the name ''Igea Borbonica'' ("Bourbon Hygieia") in a letter forwarded to Ferdinand II on 8 May 1849.<ref name="Capocci-letter"/><ref name="Gargano2023"/>{{rp|10}} De Gasparis's commemoration of the Bourbons may have helped him and Capocci circumvent punishment for their participation in the [[Revolutions of 1848 in the Italian states|liberal movements of 1848]].<ref name="Treccani"/>

The name ''Igea Borbonica'' was not mentioned in the first communications to the European scientific community.<ref name="Gargano2023"/>{{rp|10}} When English astronomer [[John Herschel]] received the news of Hygiea's discovery from his [[Naples|Neapolitan]] colleagues,<ref name="Gargano2023"/>{{rp|10}} he suggested that [[Parthenope (siren)|Parthenope]] would be an apt name for de Gasparis's discovery, as it comes from the siren who founded Naples in Greek mythology.<ref name="Ashworth2022"/><ref name="Hind"/>{{rp|127}} In a letter written to English mathematician [[Augustus De Morgan]] in April 1849, Herschel wrote:

{{Blockquote
|text=No name has yet been mentioned. What do you think of Parthenope (being a Neapolitan?) I should think it will occur as a matter of course to Gasparis if he has any classical reading.
|author=John Herschel
|source=in a letter to Augustus De Morgan (April 1849)<ref name="Palma2019"/><ref name="Gargano2023"/>{{rp|10–11}}
}}

Herschel corresponded with other European scientists and astronomers to hear their thoughts on the name Parthenope for de Gasparis's discovery, although by the time he reached out to de Gasparis, Hygiea had already been named one month earlier.<ref name="Palma2019"/> When Schumacher, editor of ''Astronomische Nachrichten'', learned of the name proposed by de Gasparis and Capocci, he complained in a letter addressed to Herschel on 26 June 1849 that it had the Italian spelling instead of the Latin form, as conventionally done for all the other planets.<ref name="Palma2019"/> Regardless, de Gasparis recognized Herschel's suggestion and expressed desire to realize Herschel's wish.<ref name="Palma2019"/><ref name="Hind"/>{{rp|167}} On 11 May 1850, de Gasparis discovered the asteroid [[11 Parthenope]] and acknowledged Herschel for the name in his announcement.<ref name="Ashworth2022"/>

There are multiple variations on the Latin spelling of Hygiea, including ''Hygièa'', ''Hygia'', and ''Hygea''.<ref name="Palma2019"/><ref name="Latin"/> The modern English form ''Hygiea'' is a less common variant of the name of the goddess, which in Greek is Ὑγίεια (''Hygieia'')<ref name="Greek-Hygieia"/> or Ὑγεῖα (''Hygeia'').<ref name="Greek-Hygeia"/> The name was often spelled ''Hygeia'' (and occasionally ''Hygea''<ref name="Ferguson1850"/><ref name="Gould1852"/>{{rp|80}}) by astronomers during the early 1850s, though by the 1860s, the spelling ''Hygiea'' became commonplace.<ref name="ADS-search"/> According to the English astronomer [[John Russell Hind]], the adjective ''Borbonica'' had been dropped by 1852.<ref name="Hind"/>{{rp|126}}<ref name="Murdin"/>{{rp|68-70}}

=== Symbol and designation ===
As with the other previously discovered asteroids, Hygiea was given an [[astronomical symbols|astronomical symbol]] as a way of representing it. The symbol for Hygiea was proposed by de Gasparis in a letter addressed to Hind on 4 November 1850, in which he wrote, "The symbol of Hygeia {{sic}} is a serpent (like a Greek ''[[zeta|ζ]]'') crowned with a star."<ref name="deGasparis1850"/> Visually, de Gasparis's symbol for Hygiea is depicted as: [[File:Hygiea symbol (original, fixed width).svg|24px|The traditional planetary symbol for 10 Hygiea|alt=A zeta-shaped serpent crowned with a star]] (U+1F779 🝹 in [[Unicode]] 17.0).<ref name="astronomy-unicode"/>{{rp|7}}<ref name="unicodepipeline"/> The serpent, particularly when [[bowl of Hygieia|drinking from a bowl]], is a traditional symbol of the goddess Hygieia (cf. U+1F54F 🕏).<ref name="DrugTopics"/>

However, perhaps due to the late announcement of Hygiea's intended symbol, it was not used in following astronomical [[almanac]]s such as the ''[[Berliner Astronomisches Jahrbuch]]'' in 1850 and the ''[[American Ephemeris and Nautical Almanac|Nautical Almanac and Astronomical Ephemeris]]'' in 1852, marking the first time an asteroid was listed without its symbol in these almanacs.<ref name="astronomy-unicode"/>{{rp|7}} American astronomer [[Benjamin Apthorp Gould]] interpreted Hygiea's symbol as a [[rod of Asclepius]], depicting it as a serpent coiled around a staff in a January 1852 publication of the ''[[Astronomical Journal]]'': [[File:Rod of Asclepius (fixed width).svg|24px|rod of Asclepius]] (U+2695 ⚕).<ref name="Gould1852"/>{{rp|80}}<ref name="astronomy-unicode"/>{{rp|7}}

Both symbols of Hygiea are now largely obsolete as the number of asteroids discovered had grown too much for each to have unique symbols.<ref name="minorplanets"/><ref name="Gould1852"/>{{rp|80}} In 1851 [[Johann Franz Encke]] proposed a different identification system, suggesting using a number corresponding to the order of discovery enclosed in a small circle<ref name="minorplanets"/><ref name="astronomy-unicode"/>{{rp|5}}—for Hygiea it would be ⑩.<ref name="Gould1852"/>{{rp|80}} Astronomers began adopting Encke's circled number scheme in scientific publications, though as the number of asteroids discovered grew, astronomers eventually switched to enclosing the number in [[parenthesis|parentheses]], which became the modern [[minor-planet designation]] scheme.<ref name="minorplanets"/><ref name="astronomy-unicode"/>{{rp|5}} In the case of Hygiea, its modern minor planet designation would be ''(10) Hygiea''<ref name="MPC-object"/> or ''10 Hygiea''.<ref name="jpldata"/>

The [[Minor Planet Center]] uses the [[Provisional_designation_in_astronomy#Minor_planets|minor planet provisional designations]] ''A849 GA'' and ''A900 GA'' for Hygiea,<ref name="MPC-object"/> though these are only retrospective extensions of the new-style provisional designation scheme established in 1925.<ref name="ProvDes"/> While the provisional designation A849 GA refers to the discovery date of Hygiea, the designation A900 GA refers to the date when Hygiea was serendipitously observed and catalogued (as ''Arequipa 38''<ref name="Strobel1963"/>{{rp|13}}) by the [[Harvard College Observatory]]'s photographic [[sky survey]] at [[Arequipa|Arequipa, Peru]] in 1900.<ref name="Ebell1908"/><ref name="MPC-object"/>{{efn|name=newstyle|In the [[Provisional_designation_in_astronomy#New-style_provisional_designation|new-style provisional designation scheme]] for pre-1925 minor planet discoveries, the designation begins with the letter A followed by three digits corresponding to the year of discovery ("A849" for 1849, and "A900" for 1900) and ends with two letters, with the first indicating the discovery date's half-month ("G" for the first half of April) and the second indicating the order of discovery in that half-month ("A" meaning first minor planet discovered in that half-month).<ref name="ProvDes"/>}}

Hygiea has seen some minor [[astrology|astrological]] use, though its symbol was confused once again, with Asclepsius's rod replaced by [[Mercury (mythology)|Mercury]]'s [[caduceus]]: [[File:Hygiea symbol (fixed width).svg|24px|Astrological symbol for Hygiea]], though in a more elaborate form (U+2BDA ⯚) than the caduceus symbol of the planet Mercury.<ref name="astronomy-unicode"/>{{rp|6}}<ref name="astrology-unicode"/>{{rp|11}} The caduceus has long been mistaken for the rod of Asclepius (see [[caduceus as a symbol of medicine]]).

=== Classification ===
[[File:Iau dozen.jpg|thumb|upright=1.5|This graphic used in the [[IAU definition of planet#First draft proposal|IAU 2006 draft proposal]] listed Hygiea as a potential planet.<ref name="IAU-Q&A"/>{{rp|17}}]]

While the first four asteroids discovered—[[Ceres (dwarf planet)|Ceres]] (1801), [[2 Pallas|Pallas]] (1802), [[3 Juno|Juno]] (1804), and [[4 Vesta|Vesta]] (1807)—were considered planets during the first half of the 19th century (with some astronomers continuing to consider them as such until the end of the century), the discovery of Hygiea occurred when the classification of asteroids was already evolving.<ref name="minorplanets"/> Hygiea was referred to as a ''planet'' in the following years after its discovery,<ref name="Palma2019"/> though by 1852 astronomers have also called Hygiea a ''[[minor planet]]''<ref name="Hind"/>{{rp|127}} or an ''asteroid''.<ref name="Gould1852"/>{{rp|80}} These terms were introduced prior to Hygiea's discovery and became more used with the discovery of more asteroids beginning in the 1850s.<ref name="minorplanets"/> From the second half of the 19th century, asteroids began to be treated in a predominantly collective way, grouped according to orbital or spectral characteristics, with less attention to the individual object.<ref name="Peebles"/>

In 2006, the [[International Astronomical Union]]'s (IAU) committee for [[IAU definition of planet#First draft proposal|drafting a new definition of planet]] had considered classifying Hygiea among the planets or the newly-coined category of [[dwarf planet]]s, if it were found that the asteroid is massive enough to be shaped predominantly by [[hydrostatic equilibrium]].<ref name="Weintraub"/>{{rp|238}}<ref name="IAU-Q&A"/>{{rp|17}}<ref name="IAU-report"/>{{rp|50}} This proposal for the definition of planet has since been revised and adopted to decree that a planet must orbit the Sun, have sufficient mass to attain a spherical shape by hydrostatic equilibrium, and [[Clearing the neighbourhood|clear its neighbourhood]]; objects that satisfy all but the last criterion are considered dwarf planets.<ref name="SyfyWire-2019"/><ref name="Vernazza2020"/>{{rp|137–138}} Since 2019, telescope images have resolved Hygiea's nearly spherical shape, suggesting that it may have reached hydrostatic equilibrium.<ref name="eso1918"/> Because Hygiea already orbits the Sun and is located in the asteroid belt where it has not cleared its orbital neighborhood, Hygiea's nearly spherical shape could qualify it as a dwarf planet, according to Pierre Vernazza and collaborators who studied Hygiea with telescope imaging.<ref name="SyfyWire-2019"/><ref name="Vernazza2020"/>{{rp|138–139}}<ref name="eso1918"/> If so, Hygiea could be the smallest dwarf planet known.<ref name="eso1918"/><ref name="ArsTechnica-2019"/> However, the IAU has not yet officially classified Hygiea as a dwarf planet.<ref name="Rivkin2025"/>{{rp|1}}

== Orbit ==
[[File:10 Hygiea orbit.jpg|upright=1.3|thumb|alt=A diagram of Hygiea's orbit|Diagram of Hygiea's orbit (white), with the inner planets' and Jupiter's orbits labelled]]

Hygiea [[orbit]]s the Sun at an average distance ([[semi-major axis]]) of 3.14{{nbsp}}[[astronomical unit]]s (AU; {{convert|470|e6km|e6mi|abbr=unit|disp=or|sigfig=2}}),<ref name="jpldata"/> which places it in the [[Kirkwood gap|outer portion]] of the [[asteroid belt|main asteroid belt]] between [[Mars]] and [[Jupiter]].<ref name="DeMeo2015"/>{{rp|18}} Hygiea has an [[orbital period]] of 5.57 [[years|Earth years]] and follows an [[elliptical orbit]] where its distance from the Sun ranges between {{convert|2.79|AU|e6km e6mi|abbr=unit|sigfig=2}} at [[perihelion]] to {{convert|3.49|AU|e6km e6mi|abbr=unit|sigfig=2}} at [[aphelion]].<ref name="jpldata"/> Hygiea has a relatively low<ref name="Mothe-Diniz2001"/>{{rp|122}}<ref name="Carruba2013b"/>{{rp|3569}} [[orbital inclination]] of 3.8° with respect to the [[ecliptic]] plane.<ref name="jpldata"/> In its current orbit, Hygiea never comes closer than {{convert|1.24|AU|e6km e6mi|abbr=unit}} from Mars and {{convert|1.54|AU|e6km e6mi|abbr=unit}} from Jupiter.<ref name="MPC-object"/>

Hygiea is in a [[orbital resonance#N-body mean motion resonance|three-body mean motion orbital resonance]] with Jupiter and [[Saturn]].<ref name="Nesvorny1998"/>{{rp|3032}} In this resonance, the [[mean motion]]s or orbital frequencies of Jupiter, Saturn, and Hygiea follow the relation <math>8n_{\rm Jupiter} - 4n_{\rm Saturn} - 3n_{\rm Hygiea} = 0</math>, where a [[linear combination]] of their mean motions (<math>n</math>) with [[integer]] coefficients equals zero.<ref name="Nesvorny1998"/>{{rp|3032}}<ref name="Sidlichovsky1999"/>{{rp|78}} Simulations show that this three-body resonance leads to a [[chaos theory|chaotic]] evolution in Hygiea's orbit, with a short<ref name="Nesvorny1998"/>{{rp|3029}} [[Lyapunov time]] between 14,000<ref name="AstDyS-proper"/> and 16,000 years.<ref name="Sidlichovsky1999"/>{{rp|78}} This means that changes in Hygiea's orbit become exponentially less predictable over this time scale.<ref name="Nesvorny1998"/>{{rp|3029}}

Other asteroids occasionally pass close to Hygiea, sometimes close enough that their trajectories become visibly [[Perturbation (astronomy)|perturbed]] or deflected by Hygiea's gravity.<ref name="Scholl1987"/>{{rp|311}}<ref name="Mammana1997"/>{{rp|29}} This allows astronomers to measure Hygiea's mass.<ref name="Scholl1987"/>{{rp|311}} A search conducted in 2002 identified 40 known asteroids that have passed within {{convert|0.05|AU|e6km e6mi|abbr=unit}} of Hygiea between 1970 and 2000.<ref name="Galad2002"/>{{rp|1118}} Over a time span of 30 million years, Hygiea's gravitational perturbations can cause the [[proper orbital elements|proper semi-major axes]] of close-passing asteroids to change by {{cvt|0.000908|AU|km mi|sigfig=3}}.<ref name="Carruba2013a"/>{{rp|5}}

== Hygiea family ==
[[File:Hygiea_family.png|thumb|upright=1.5|Location and structure of the Hygiea family]]
{{main|Hygiea family}}
{{expand section|date=March 2025}}

Hygiea is the [[parent body]] of the [[Hygiea family]], an [[asteroid family]] comprising over 7,000 known asteroids that share similar orbital and compositional characteristics with Hygiea.<ref name="eso1918"/> The Hygiea family is the most populous asteroid family in the outer main belt,<ref name="Carruba2013b"/> and is believed to have formed by a giant impact on Hygiea that ejected at least 1.7% of the asteroid's original mass<ref name="Vernazza2020"/> about 2 to 3 billion years ago.<ref name="Carruba2014"/> The impactor that formed the Hygiea family was probably between {{cvt|75 and 150|km|mi}} in diameter.<ref name="Vernazza2020"/> Hygiea contains almost all the mass (over 98%) of the family.<ref name="Vernazza2020"/>

== Rotation ==
{{multiple image
 | align = right
 | total_width = 350
 | image1 = 10 Hygiea lightcurve TRAPPIST 2018.png
 | caption1 = Rotational light curve of Hygiea measured by the [[TRAPPIST]] telescope in 2018
 | image2 = Hygiea shape model rotation.gif
 | caption2 = Simulation of Hygiea's retrograde (clockwise) rotation
}}

The [[rotation period]] of Hygiea is about 13.83{{nbsp}}hours (13 hours and 49.5 minutes), with an uncertainty of 0.2 seconds.<ref name="Vernazza2020"/>{{rp|136}}<ref name="Vernazza2021"/>{{rp|18}} This rotation period was determined via direct imaging and analysis of Hygiea's [[light curve]], or changes in brightness over time.<ref name="Ferrais2021"/><ref name="Vernazza2020"/>{{rp|139}} From Earth, Hygiea's brightness periodically fluctuates by up to 15% from its average value<ref name="Vernazza2020-supp"/>{{rp|3}} as [[albedo feature]]s on Hygiea's surface rotate in and out of view.<ref name="Ferrais2021"/> The north pole of Hygiea's rotation axis points toward the ecliptic south, in the direction of [[ecliptic coordinate system|ecliptic longitude]] {{val|306|3|u=°}} and ecliptic latitude {{val|-29|3|u=°}}.<ref name="Vernazza2021"/>{{rp|18}} This means Hygiea's rotation axis is highly tilted with an [[obliquity]] of 120° with respect to the ecliptic.<ref name="Vernazza2021"/>{{rp|18}} Since Hygiea's obliquity is greater than 90°, it has a [[retrograde motion|retrograde]] rotation, meaning it rotates backwards with respect to the direction of its orbit around the Sun.<ref name="Michalowski1991"/>{{rp|54}}

Before it was directly imaged by telescopes in 2017–2018, Hygiea was thought to have a rotation period twice as long as the currently accepted value.<ref name="Ferrais2021"/><ref name="Vernazza2020"/>{{rp|139}} The first measurements of Hygiea's rotation period beginning in the 1950s reported 18 hours<ref name="Groeneveld1954"/>{{rp|538–539}}<ref name="Michalowski1991"/>{{rp|53}} while studies from 1991 and onward reported 27.6 hours.<ref name="Vernazza2020"/>{{rp|139}} These past observations did not collect enough datapoints to produce reliable light curves, which led to these inaccurate rotation period measurements.<ref name="Ferrais2021"/><ref name="Vernazza2020"/>{{rp|139}}

== Size and mass ==
[[File:The Four Largest Asteroids.jpg|thumb|upright=1.5|Relative sizes of the four largest asteroids. Hygiea is furthest right.]]

Hygiea is the [[List of exceptional asteroids#Largest by diameter|fourth-largest main-belt asteroid]] by both volume and mass, with a volume-equivalent [[mean diameter]] of {{cvt|433 ± 8|km|mi|0}} and a mass of {{val|8.74|0.69|e=19|u=kg}}.{{efn|These diameter and mass values come from Vernazza et al. (2021).<ref name="Vernazza2021"/>{{rp|6}} In an earlier paper from 2020, the same authors measured a diameter of {{val|434|14|u=km}} and a mass of {{val|8.32|0.80|e=19|u=kg}}.<ref name="Vernazza2020"/>{{rp|136}}}} Compared to the largest asteroid Ceres, Hygiea is less than half its diameter and is roughly 10% as massive.<ref name="Rivkin2025"/>{{rp|15}} Hygiea constitutes 3% of the total mass of the entire main belt<ref name="DeMeo2013"/>{{rp|736}} and is the largest asteroid in the outer main belt.<ref name="DeMeo2015"/>{{rp|18}} The four largest asteroids (Ceres, Pallas, Vesta, and Hygiea) together make up a little more than half of the entire main belt's mass.<ref name="DeMeo2013"/>{{rp|738}} These four largest asteroids have sometimes been collectively referred to as the "Big Four" by astronomers,<ref name="Rivkin2011"/><ref name="Galad2002"/> although historically this term meant the first four asteroids discovered (Ceres, Pallas, Juno, and Vesta).<ref name="Cunningham"/>{{rp|26}} Hygiea was not recognized as the fourth-largest asteroid until 1974, when [[David Morrison (astrophysicist)|David Morrison]] measured its diameter and [[albedo]] for the first time by using its [[infrared]] [[thermal radiation|thermal emission]].<ref name="Morrison1974"/>{{rp|211}}

The best estimate for Hygiea's diameter was measured via direct imaging using the [[Very Large Telescope]] (VLT) in [[Chile]], whose [[Astronomical seeing|atmospheric turbulence]]-correcting [[adaptive optics]] could resolve the asteroid in high [[angular resolution]].<ref name="eso1918"/><ref name="Vernazza2021"/>{{rp|1}} Hygiea's mass was measured by observing how its gravity deflects the paths of other asteroids that pass near it.<ref name="Vernazza2020-supp"/>{{rp|18–19}} This requires an extensive number of observations with accurate positional measurements of the deflected asteroid before and after its close approach.<ref name="Goffin2014"/>{{rp|1}}<ref name="Scholl1987"/>{{rp|311}} The measurement of Hygiea's mass through this method was first attempted in 1986, with the analysis of a past close approach by the asteroid [[829 Academia]] (which passed {{convert|0.006|AU|e3km e3mi|abbr=unit|disp=or}} from Hygiea on 19 May 1927<ref name="Michalak2001"/>{{rp|705}}).<ref name="Scholl1987"/>{{rp|311–313}} Hygiea's mass has since been refined with additional asteroid close approaches and more accurate observations.<ref name="Michalak2001"/>{{rp|705}}<ref name="Vernazza2020-supp"/>{{rp|18–19}}

== Geology ==
=== Shape, craters, and topography ===
[[File:10 Hygiea VLT shape model annotated.png|thumb|upright=1.5|The nearly spherical shape model of Hygiea as seen from three mutually orthogonal views]]
[[File:Hygiea VLT Serpens Calix craters.png|thumb|upright=1.2|Hygiea's two large craters "Serpens" and "Calix" identified in VLT images from 2017. A bright spot is also present near Calix.]]

Hygiea is a nearly spherical or [[ellipsoid]]al object<ref name="SyfyWire-2019"/><ref name="Vernazza2020"/>{{rp|137}} whose diametric dimensions are {{cvt|450 x 430 x 424|km|mi}}, with an uncertainty of ±{{cvt|10|km|mi}} for the equatorial axes and ±{{cvt|20|km|mi}} for the polar axis.<ref name="Vernazza2020"/>{{rp|136}} It is the second-most spherical main-belt asteroid after Ceres.<ref name="Vernazza2021"/>{{rp|5}} The nearly spherical shape of Hygiea suggests it may be in [[hydrostatic equilibrium]], having attained its shape by its own gravity.<ref name="Vernazza2020"/>{{rp|138}} Specifically, Hygiea's polar [[flattening]] and [[specific angular momentum]] closely matches that of a [[Maclaurin spheroid]], which is the shape assumed by a rotating self-gravitating fluid in hydrostatic equilibrium.<ref name="Vernazza2020"/>{{rp|137}}<ref name="Vernazza2021"/>{{rp|7}}

In contrast to the larger asteroid Vesta, Hygiea lacks large [[impact crater]]s or [[Multi-ringed basin|basins]] that would otherwise deform its shape from a sphere.<ref name="Vernazza2020"/>{{rp|137}} Only two craters on Hygiea have been confidently identified in VLT imaging, with their respective diameters being {{cvt|180 ± 15|km|mi}} and {{cvt|97 ± 10|km|mi}}.<ref name="Vernazza2020"/>{{rp|139}}<ref name="Vernazza2020-supp"/>{{rp|11}} Both craters appear to have [[Complex crater#Central-peak craters|central peaks]].<ref name="Vernazza2020-supp"/>{{rp|11}} Vernazza and collaborators who studied these images have informally named the larger crater "Serpens" and smaller crater "Calix", after the Latin words for 'snake' and 'cup', respectively.<ref name="SyfyWire-2019"/><ref name="Vernazza2020-supp"/>{{rp|11}} These names, which are not approved by the IAU, were chosen because their [[eponym]]s commonly appear in symbols of Hygiea.<ref name="Vernazza2020-supp"/>{{rp|11}}

Besides Serpens and Calix, VLT images of Hygiea show no obvious or bowl-shaped craters larger than {{cvt|30|km|mi}} in diameter.<ref name="Vernazza2020"/>{{rp|139}} Ceres similarly lacks large bowl-shaped craters and is instead dominated by flat-floored [[complex crater]]s, which are difficult to see from Earth—this may be the case for Hygiea.<ref name="Vernazza2020"/>{{rp|139}}<ref name="Vernazza2021"/>{{rp|9}} An abundance of flat-floored craters on Hygiea could indicate a water ice-rich subsurface, which would allow for the relaxation of its surface topography.<ref name="Vernazza2020"/>{{rp|139}}

Although Hygiea lacks major impact basins, the existence of the Hygiea family points to a major [[impact event|collisional event]] in its past.<ref name="eso1918"/> Rather than leaving a giant impact basin like on Vesta, the impact is thought to have completely shattered Hygiea, which erased all of its pre-existing surface features.<ref name="SyfyWire-2019"/><ref name="Vernazza2020"/>{{rp|137}} Simulations of the giant impact show that most of Hygiea's debris reaccumulated into an oscillating fluid-like body, which was able to relax into a sphere by its own gravity.<ref name="ArsTechnica-2019"/><ref name="Vernazza2020"/>{{rp|137–138}} The present-day ellipsoidal shape of Hygiea suggests it solidified around four hours after the impact.<ref name="Vernazza2020"/>{{rp|137}}<ref name="Vernazza2020-supp"/>{{rp|8}} Other large asteroids that are parents of asteroid families, such as [[8 Flora]] and [[31 Euphrosyne]], have also been found to show roughly spherical shapes and are thus believed to have undergone similar collisional disruption and reaccumulation.<ref name="Vernazza2021"/>{{rp|5}}

=== Surface ===
==== Albedo, temperature, and regolith ====
[[File:Hygiea VLT 2017-2018 map.png|thumb|upright=1.5|Albedo and shadow map of Hygiea's southern hemisphere in an [[equirectangular projection]], constructed from VLT images.<ref name="Vernazza2020"/>{{rp|139}}<ref name="Vernazza2020-supp"/>{{rp|10}} A bright spot is present at longitude {{val|290|u=°}} and latitude {{val|-30|u=°}}.<ref name="Vernazza2020"/>{{rp|139}}]]

The surface of Hygiea is dark,<ref name="Sykes2006"/>{{rp|1}} with an average [[visible light|visual]] [[geometric albedo]] estimated between 6.3% and 7.2%.<ref name="Vernazza2021"/>{{rp|6}}<ref name="Vernazza2020"/>{{rp|139}} The apparent brightness or [[reflectance]] varies across Hygiea's surface due to variations in [[albedo]] and shadows cast by topographic features.<ref name="Vernazza2020"/>{{rp|139}} VLT images of Hygiea show several bright spots on its surface, with the brightest one located on Hygiea's southern hemisphere at longitude {{val|290|u=°}} and latitude {{val|-30|u=°}}.<ref name="Vernazza2020"/>{{rp|139}} This bright spot is 10% brighter than the average reflectance of Hygiea's surface.<ref name="Vernazza2020"/>{{rp|139}} A large dark region is present along Hygiea's equator at longitude {{val|60|u=°}} and latitude {{val|0|u=°}}, though it is most likely a shadowed region.<ref name="Vernazza2020"/>{{rp|139}} The albedo and reflectance variation across Hygiea's surface resembles those of Ceres.<ref name="Vernazza2020"/>{{rp|139}}

The temperature on Hygiea's surface depends on the asteroid's rotation, distance from the Sun,<ref name="Johnston1989"/>{{rp|336}} and the properties of its surface [[regolith]].<ref name="Lebofsky1985"/>{{rp|194}} While the surface of Hygiea have been observed to reach temperatures as high as {{cvt|230|K|C F}} at the [[subsolar point]],<ref name="Barucci2002"/>{{rp|204}}<ref name="Lim2005"/>{{rp|394}} the surface cools down it rotates away from the Sun, which averages out the temperature.<ref name="Lebofsky1985"/>{{rp|196}} Between Hygiea's perihelion and aphelion distances, the average temperature can range from {{cvt|150 to 180|K|C F}}.<ref name="Rivkin2014"/>{{rp|434}} At Hygiea's semi-major axis, the average temperature would be {{cvt|163|K|C F}}.<ref name="Johnston1989"/>{{rp|336}}

Hygiea has been described as a "fairly dusty object",<ref name="Lebofsky1985"/>{{rp|199}}<ref name="Mothe-Diniz2001"/>{{rp|120}} with a regolith layer consisting mostly of fine dust rather than coarse rock.<ref name="Johnston1989"/>{{rp|335}} This fine regolith has a low [[thermal inertia]], meaning it radiates away heat more easily and leads to cooler temperatures on Hygiea's surface.<ref name="Lebofsky1985"/>{{rp|194–195}} The regolith of Hygiea may be dustier (and thus finer<ref name="Colwell2007"/>{{rp|4}}) than [[lunar regolith|that of Earth's Moon]], which could imply a more mature regolith<ref name="Lebofsky1985"/>{{rp|199}} that has experienced more [[space weathering]].<ref name="Colwell2007"/>{{rp|4}} The regolith layer of Hygiea is at least {{cvt|8|cm|in}} deep.<ref name="Johnston1989"/>{{rp|335}}<ref name="Barucci2002"/>{{rp|203}} These properties of Hygiea's regolith were inferred from the asteroid's lower-than-expected temperature and [[thermal radiation|thermal emission]] in infrared, [[Submillimetre astronomy|submillimetre]], and [[microwave]] [[wavelength]]s.<ref name="Lebofsky1985"/>{{rp|195}}<ref name="Johnston1989"/>{{rp|335}}

==== Composition ====
[[File:Ceres-Hygiea visible-NIR spectrum comparison.png|thumb|upright=2|The similar spectra of Ceres (red) and Hygiea (blue) in visible to near-infrared [[wavelength]]s indicate they have similar compositions. Both spectra show absorption signatures of hydrated and ammoniated phyllosilicates.]]

Hygiea is a [[carbon|carbonaceous]] asteroid whose surface is largely composed of [[hydrate|hydrated]] and [[ammonia|ammoniated]] [[silicate mineral]]s ([[Silicate_mineral#Phyllosilicates|phyllosilicates]]) and [[carbonate]]s,<ref name="Rivkin2025"/>{{rp|1, 4}} with hints of water ice.<ref name="DePra2020"/>{{rp|9}} This composition is inferred from [[spectroscopy|spectroscopic]] observations of Hygiea,<ref name="Rivkin2025"/> whose visible and [[near-infrared]] [[Electromagnetic spectrum|spectrum]] characterize it as a [[C-type asteroid]].<ref name="Barucci2002"/>{{rp|203}} More broadly, it makes Hygiea a member of the [[C-type_asteroid#C-group_classifications|C spectral complex]],<ref name="Vernazza2017"/>{{rp|1}} which is common in the outer main belt where Hygiea resides.<ref name="DeMeo2015"/>{{rp|18}}

The spectrum and composition of Hygiea's surface is very similar to those of [[carbonaceous chondrite]] [[meteorite]]s and Ceres,<ref name="Rivkin2025"/>{{rp|1}} which has led some astronomers to call Hygiea a "primitive" object<ref name="Barucci2002"/>{{rp|209}} and a "virtual [[Geology_of_Ceres#Surface_composition|spectral twin of Ceres]]".<ref name="Rivkin2025"/>{{rp|2}} Hygiea's resemblance to carbonaceous chondrites (particularly [[CM chondrite]]s<ref name="Rivkin2025"/>{{rp|1}}) suggests it shares a similar history of having undergone little thermal alteration and some aqueous alteration of its original minerals, hence its likely primitive nature.<ref name="Barucci2002"/>{{rp|209}} Slight differences between the near-infrared spectra of Hygiea and Ceres indicate they have slightly different concentrations of hydrated and ammoniated minerals.<ref name="Vernazza2017"/>{{rp|9}}<ref name="Rivkin2025"/>{{rp|12}} High resolution near-infrared spectroscopy by the [[James Webb Space Telescope]] has found that Hygiea's phyllosilicates are [[magnesium]]-rich<ref name="Rivkin2025"/>{{rp|5, 15}} and appear to match ammoniated [[saponite]].<ref name="Rivkin2025"/>{{rp|12}}

Ground-based observations have shown that the near-infrared spectrum of Hygiea varies over time, which implies there is compositional variation across Hygiea's surface.<ref name="DePra2020"/>{{rp|9}}<ref name="Busarev2016"/>{{rp|18}} A 2011 study by Vladimir Busarev observed Hygiea's spectral type changing between C, [[B-type asteroid|B]], and [[F-type asteroid|F]] over the course of its rotation,<ref name="Busarev2011"/>{{rp|47–48}} which he interpreted as an indication of local dehydration of Hygiea's surface material,<ref name="Busarev2011"/>{{rp|50}}<ref name="Busarev2016"/>{{rp|18}} possibly as a result of heating by impacts.<ref name="Hsieh2018"/>{{rp|15}} A 2019 study by Andrew Rivkin and colleagues reported changes in the {{val|3.1|u=um}} [[absorption band|absorption feature]] in Hygiea's near-infrared spectrum over a time scale of years.<ref name="Rivkin2019"/>{{rp|1401}} While this absorption feature is often attributed to ammoniated minerals,<ref name="Rivkin2019"/>{{rp|1394, 1403}} its variations could also correspond to either hydrated minerals or potentially water ice frost.<ref name="Rivkin2019"/>{{rp|1394, 1401}}<ref name="DePra2020"/>{{rp|9}} Rivkin and colleagues speculated that the variations could be caused by exposed subsurface material on some parts of Hygiea's surface,<ref name="Rivkin2019"/>{{rp|1405}} although they do not appear to be correlated with one particular surface location.<ref name="Rivkin2019"/>{{rp|1403}}

=== Density and interior ===
Hygiea's [[bulk density]] has been estimated as either {{val|1.944|0.250|u=g/cm3}} or {{val|2.06|0.20|u=g/cm3}}, based on measurements of its diameter and mass from 2020 and 2021, respectively.<ref name="Vernazza2020"/>{{rp|136–137}}<ref name="Vernazza2021"/>{{rp|6}} Hygiea's bulk density is similar to that of Ceres ({{val|2.16|u=g/cm3}}),<ref name="Vernazza2020"/>{{rp|137}} which together with Hygiea's large size suggests it could have a [[Planetary differentiation|differentiated]] interior.<ref name="DePra2020"/>{{rp|7, 9}}<ref name="Hanus2017"/>{{rp|10}} Hygiea likely has a water ice-rich subsurface, as hinted by Hygiea's bulk density, spectroscopic detections of water ice, and its lack of large, bowl-shaped craters.<ref name="Vernazza2020"/>{{rp|139}}<ref name="Hanus2020"/>{{rp|7}} Assuming the higher density estimate of {{val|2.06|0.20|u=g/cm3}}, Hygiea would be among the denser members of the largest C-type asteroids (diameters larger than {{cvt|300|km|mi|disp=or}}), whose densities range between {{val|1.5|to|2.16|u=g/cm3}} and may have negligible interior [[macropore|macroporosities]], according to Vernazza and colleagues.<ref name="Vernazza2021"/>{{rp|12}} They further speculated that the higher densities of Hygiea and other large C-type asteroids may be due to a more [[Lithification|lithified]] or compacted interior, as a result of significant aqueous alteration.<ref name="Vernazza2021"/>{{rp|12}}

== Observation and exploration ==
=== Observation ===
[[File:10 Hygiea 2MASS.jpg|thumb|Image of 10 Hygiea taken by the [[2MASS]] survey]]
{{expand section|date=March 2025}}
Despite its size, Hygiea appears very dim when observed from [[Earth]]. This is due to its dark surface and its position in the [[Asteroid belt#Orbits|outer main belt]]. For this reason, six smaller asteroids were observed before [[Annibale de Gasparis]] discovered Hygiea on 12 April 1849. 

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 [[Magnitude (astronomy)|magnitude]]s.<ref name=brightestasteroids/> At most oppositions, Hygiea has a [[apparent magnitude|magnitude]] of around +10.2,<ref name=brightestasteroids/> which is as much as four orders fainter than Vesta, and observation calls for at least a {{convert|4|in|mm|adj=on}} [[telescope]] to resolve.<ref name=telescope/> 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.<ref name="inthesky"/>

A total of 17 [[star|stellar]] [[occultation]]s by Hygiea have been tracked by Earth-based astronomers,<ref name=Watanabe/><ref name=Hilton2008/> 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.<ref name=Hanus2017/> The [[Hubble Space Telescope]] has resolved the asteroid and ruled out the presence of any orbiting companions larger than about {{convert|16|km|mi}} in diameter.<ref name="Storrs1999"/>

=== Exploration ===
Hygiea has not been explored by any [[space probe]]. In 2006, Mark V. Sykes and others from the [[Planetary Science Institute]] proposed the ''Exploring the Very Earliest Epoch'' (EVE) mission to [[NASA]], as part of the [[Discovery Program]]. The proposal, which did not get approved, suggested launching a copy of the ''[[Dawn Mission|Dawn]]'' probe to Hygiea in October 2011 and arriving to the asteroid via [[space rendezvous|rendezvous]] in 2021.<ref name="Sykes2006"/>{{rp|1–2}} In 2013,<ref name="Ulivi"/>{{rp|431}} Pierre Vernazza and Philippe Lamy proposed the [[European_Space_Agency_Science_Programme#Medium_class|medium-class mission]] ''INSIDER'' for the [[European Space Agency]]'s [[European Space Agency Science Programme|Space Programme]].<ref name="Vernazza2014"/> The proposal, which also did not get approved, suggested rendezvousing and orbiting around two or three large main-belt asteroids before releasing one or two [[lander (spacecraft)|landers]]. Hygiea and [[24 Themis]] were among the potential targets of the ''INSIDER'' mission concept.<ref name="Vernazza2014"/>  The exploration of primitive main-belt asteroids like Hygiea could provide clues to the processes that led to the [[Formation and evolution of the solar system|formation of the solar system]].<ref name="Vernazza2014"/><ref name="Sykes2006"/>{{rp|1}}

== See also ==
* [[704 Interamnia]] – the fifth-[[List of exceptional asteroids#Largest by diameter|largest main-belt asteroid]], whose shape is close to hydrostatic equilibrium<ref name="Hanus2020"/>
* [[65 Cybele]] – one of the largest main-belt asteroids, whose shape is also close to hydrostatic equilibrium
* [[List of former planets]]
* [[List of gravitationally rounded objects of the Solar System]]

== Notes ==
{{notelist|group=lower-alpha}}

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}} <!-- end of reflist -->

== External links ==
{{Commons category}}
* [https://www.youtube.com/watch?v=C-6s1LBaTsE Stellar occultation by Hygiea on 11 August 2013 (video)]
* [https://astro.troja.mff.cuni.cz/projects/damit/asteroid_models/view/4392 (10) Hygiea shape model and observation details], Astronomical Institute of the Charles University
* {{AstDys|10}}
* {{JPL small body}}

{{Minor planets navigator |9 Metis |number=10 |11 Parthenope}}
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{{DEFAULTSORT:000010}}
[[Category:Hygiea asteroids]]
[[Category:Discoveries by Annibale de Gasparis|Hygiea]]
[[Category:Named minor planets|Hygiea]]
[[Category:Possible dwarf planets]]
[[Category:C-type asteroids (Tholen)]]
[[Category:C-type asteroids (SMASS)]]
[[Category:1840s in science|18490412]]
[[Category:Astronomical objects discovered in 1849|18490412]]