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 being ...

and unusual object on an eccentric, cometary-like orbit between the asteroid belt and the outer Solar System, approximately in diameter. Discovered by German astronomer Walter Baade 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 Don (honorific), Don Miguel Gregorio Antonio Ignacio Hidalgo y Costilla y Gallaga Mandarte Villaseñor (8 May 1753 – 30 July 1811), more commonly known as Miguel Hidalgo y Costilla or Miguel Hidalgo (), was a Catholic priest, leader ...

Discovery and naming

Hidalgo was discovered by German astronomer Walter Baade on 31 October 1920 at Bergedorf Observatory in Hamburg, Germany. It is named for

(1753–1811), who was responsible for declaring Mexico's independence in 1810 and the ensuing Mexican War of Independence. German astronomers who were in Mexico 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 Álvaro Obregón. 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 long ...

of 5.74 AU). Its orbit has a high eccentricity of 0.66 and an inclination of 43 ° with respect to the ecliptic. The body's observation arc begins with its official discovery observation on 31 October 1920. With a semi-major axis between that of Jupiter (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 times ...

(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 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. ...

s and the trans-Neptunian objects. 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 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 betwee ...

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 orb ...

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; h ...

(9.0–10.1 AU), a characteristic normally associated with Saturn's family of comets. Some astronomers therefore suspect that it was once a comet. 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 of only . It is unlikely there are any large satellites orbiting Hidalgo larger than about 15 kilometers across. Observations from the Very Large Telescope 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 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. According to the surveys carried out by the Japanese Akari satellite and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, Hidalgo measures 52.45 and 61.4 kilometers in diameter and its surface has a low albedo of 0.042 and 0.028, respectively. The prominent JPL Small-Body Database 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 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 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 was discovered Hidalgo was the furthest known minor planet from the Sun.

Discovery and naming

Orbit and classification

Physical characteristics

Notes

References

External links