944 Hidalgo is a

centaur A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as bein ...

and unusual object on an eccentric,

comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or Coma (cometary), coma, and sometimes also a Comet ta ...

ary-like orbit between the

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 ...

and the

outer Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

, approximately in diameter. Discovered by German astronomer

Walter Baade Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was a German astronomer who worked in the United States from 1931 to 1959. Biography The son of a teacher, Baade finished school in 1912. He then studied maths, physics and astro ...

in 1920, it is the first member of the dynamical class of

s ever to be discovered. The dark D-type object has a

rotation period The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to its sidereal rotation period, i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the ...

of 10.1 hours and likely an elongated shape. It was named after Mexican revolutionary Miguel Hidalgo y Costilla.

Discovery and naming

Hidalgo was discovered by German astronomer

on 31 October 1920 at

Bergedorf Observatory Hamburg Observatory (german: Hamburger Sternwarte) is an astronomical observatory located in the Bergedorf borough of the city of Hamburg in northern Germany. It is owned and operated by the University of Hamburg, Germany since 1968, although it ...

in Hamburg, Germany. It is named for Miguel Hidalgo y Costilla (1753–1811), who was responsible for declaring Mexico's independence in 1810 and the ensuing

Mexican War of Independence The Mexican War of Independence ( es, Guerra de Independencia de México, links=no, 16 September 1810 – 27 September 1821) was an armed conflict and political process resulting in Mexico's independence from Spain. It was not a single, co ...

. German astronomers who were in

Mexico Mexico ( Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guate ...

to observe a total

eclipse An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ce ...

on 10 September 1923 had an audience with

President President most commonly refers to: *President (corporate title) * President (education), a leader of a college or university *President (government title) President may also refer to: Automobiles * Nissan President, a 1966–2010 Japanese f ...

Álvaro Obregón Álvaro Obregón Salido (; 17 February 1880 – 17 July 1928) better known as Álvaro Obregón was a Sonoran-born general in the Mexican Revolution. A pragmatic centrist, natural soldier, and able politician, he became the 46th President of Me ...

. During this meeting, they asked his permission to name the asteroid after Hidalgo ().

Orbit and classification

Hidalgo orbits the Sun at a distance of 1.9–9.5 AU once every 13 years and 9 months (5,024 days;

semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the lo ...

of 5.74 AU). Its orbit has a high eccentricity of 0.66 and an

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

of 43 ° with respect to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agai ...

. The body's

observation arc In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly use ...

begins with its official discovery observation on 31 October 1920. With a semi-major axis between that of

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandt ...

(5.2 AU) and

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 time ...

(30.1 AU), Hidalgo is a member of the dynamically unstable population of

s, located between the classical

asteroid An asteroid is a minor planet of the Solar System#Inner solar system, 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 o ...

s and the

trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typicall ...

s. The

Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Function ...

classifies it both as

main-belt asteroid 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, cal ...

and unusual object due to an

orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values b ...

higher than 0.5. Hidalgo has traditionally been considered an asteroid because centaurs were not recognized as a distinct class until the discovery of

2060 Chiron 2060 Chiron is a small Solar System body in the outer Solar System, orbiting the Sun between Saturn and Uranus. Discovered in 1977 by Charles Kowal, it was the first-identified member of a new class of objects now known as centaurs—bodies ...

in 1977. Alternatively,

cis-Neptunian object A cis-Neptunian object is, literally, any astronomical body found within the orbit of Neptune. However, the term is typically used for those distant minor planets other than trans-Neptunian objects: that is, all sub-planetary bodies orbiting the ...

and distant object are more generic terms that also includes objects further out the Solar System. Hidalgo orbit takes it to the inner edge of the asteroid belt and as far out as to the orbit of

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; ...

(9.0–10.1 AU), a characteristic normally associated with Saturn's family of

s. Some astronomers therefore suspect that it was once a

. It is a Jupiter- and Saturn-crosser. Strictly speaking, Hidalgo is a Saturn-grazer rather than a crosser as its aphelion does not clear Saturn's. The object's severe orbital inclination is suspected to be the result of a close encounter with Jupiter. Even as recently as 1922, Hidalgo passed within 0.89 AU of Jupiter. Its orbit has a Jupiter

minimum orbit intersection distance Minimum orbit intersection distance (MOID) is a measure used in astronomy to assess potential close approaches and collision risks between astronomical objects. It is defined as the distance between the closest points of the osculating orbits of ...

of only . It is unlikely there are any large satellites orbiting Hidalgo larger than about 15 kilometers across. Observations from 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, ...

in 2003 ruled out any secondary objects brighter than V=19.5 further than 200 kilometers from the asteroid. However, smaller moons have not been ruled out.

Physical characteristics

In the

Tholen Tholen () is a 25,000 people Municipalities of the Netherlands, municipality in the southwest of the Netherlands. The municipality of Tholen takes its name from the town of Tholen, which is the largest population center in the municipality. The m ...

and

Bus–DeMeo classification An asteroid spectral type is assigned to asteroids based on their emission spectrum, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition. For small bodies that are not internally differentiat ...

, Hidalgo is a dark, carbonaceous

D-type asteroid D-type asteroids have a very low albedo and a featureless reddish spectrum. It has been suggested that they have a composition of organic-rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors. D-type asteroids ...

. According to the surveys carried out by the Japanese Akari satellite and the

NEOWISE Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 20 ...

mission of NASA's

Wide-field Infrared Survey Explorer Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 2 ...

, Hidalgo measures 52.45 and 61.4 kilometers in diameter and its surface has a low

albedo Albedo (; ) is the measure of the diffuse reflection of 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 that refl ...

of 0.042 and 0.028, respectively. The prominent

JPL Small-Body Database The JPL Small-Body Database (SBDB) is an astronomy database about small Solar System bodies. It is maintained by Jet Propulsion Laboratory (JPL) and NASA and provides data for all known asteroids and several comets, including orbital parameters an ...

currently gives a diameter of 38 kilometers taken from the publication ''Hazards due to Comets and Asteroids'' (

Tom Gehrels Anton M.J. "Tom" Gehrels (February 21, 1925 – July 11, 2011) was a Dutch–American astronomer, Professor of Planetary Sciences, and Astronomer at the University of Arizona, Tucson. Biography Youth and education Gehrels was born at Haa ...

, 1994). The ''Collaborative Asteroid Lightcurve Link'' assumes a standard albedo for a

carbonaceous 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 ...

of 0.057 and derives a diameter of 44.6 km based on an

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it we ...

of 10.48, while Johnston's archive adopts Akari's diameter of 52 km with an albedo of 0.042. In the late 1990s, a network of astronomers worldwide gathered

lightcurve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...

data that was ultimately used to derive the spin states and shape models of 10 new asteroids, including Hidalgo. The authors describe the shape model as having 'very large flat areas and a "rectangular" pole-on silhouette, which are strong indications of a highly nonconvex shape'. Some of the light curves show sharp minima, which indicates the object shape may have two lobes. Lightcurve data has also been recorded by observers at the Antelope Hills Observatory in Colorado, United States. When

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest k ...

was discovered Hidalgo was the furthest known minor planet from the Sun.

Discovery and naming

Orbit and classification

Physical characteristics

Notes

References

External links