486958 Arrokoth (provisional designation ; formerly nicknamed Ultima Thule) is a trans-Neptunian object located in the Kuiper belt. Arrokoth became the farthest and most primitive object in the Solar System visited by a spacecraft when the NASA space probe New Horizons conducted a flyby on 1 January 2019. Arrokoth is a contact binary long, composed of two planetesimals across, that are joined along their major axes. With an orbital period of about 298 years and a low orbital inclination and eccentricity, Arrokoth is classified as a cold classical Kuiper belt object.
Arrokoth was discovered on 26 June 2014 by astronomer Marc Buie and the New Horizons Search Team using the Hubble Space Telescope as part of a search for a Kuiper-belt object for New Horizons to target in its first extended mission; it was chosen over two other candidates, and , to become the primary target of the mission.
Name
When Arrokoth was first observed by the Hubble Space Telescope in 2014, it was designated in the context of the telescope's search for Kuiper belt objects, and was nicknamed "11" for short. Its existence as a potential target of the New Horizons probe was announced by NASA in October 2014 and it was unofficially designated as "Potential Target 1", or . Its official provisional designation, , was assigned by the Minor Planet Center in March 2015, after sufficient orbital information had been gathered. The provisional designation indicates that Arrokoth was the 1745th minor planet to be assigned a provisional designation during the second half of June 2014. After further observations refining its orbit, it was given the permanent minor planet number 486958 on 12 March 2017.
Ultima Thule
Before the flyby on 1 January 2019, NASA invited suggestions from the public on a nickname to be used for the object. One of the choices, Ultima Thule, was selected on 13 March 2018. (, ) is the northernmost location mentioned in ancient Greek and Roman literature and cartography, while in classical and medieval literature (Latin for 'farthermost Thule') acquired a metaphorical meaning of any distant place located beyond the "borders of the known world". Once it was determined that the body was a bilobate contact binary, the New Horizons team nicknamed the larger lobus "Ultima" and the smaller lobus "Thule". They are now formally named "Wenu" and "Weeyo", respectively.
In November 2019, the International Astronomical Union (IAU) announced the object's permanent official name, Arrokoth.
Arrokoth
Arrokoth was named for a word in the Powhatan language of the Tidewater region of Virginia and Maryland in the eastern United States. The Powhatan language became extinct in the late 18th century and little was recorded of it. In an old word list, is glossed as 'sky', but it would seem that it actually meant 'cloud'.
The name Arrokoth was chosen by the New Horizons team to represent the Powhatan people indigenous to the Tidewater region. The Hubble Space Telescope and the Johns Hopkins University Applied Physics Laboratory were both operated in Maryland and were prominently involved in Arrokoth's discovery. With the permission of the elders of the Pamunkey Indian Tribe, the name Arrokoth was proposed to the IAU and formally announced by the New Horizons team in a ceremony held at the NASA Headquarters in the District of Columbia on 12 November 2019. Prior to the ceremony, the name was accepted by the IAU's Minor Planet Center on 8 November, and the New Horizons team's naming citation was published in a Minor Planet Circular on 12 November.
Shape
Arrokoth is a contact binary consisting of two lobes (lobi) attached by a narrow neck or waist, which is encircled by a bright band named Akasa Linea. The lobi were likely once two objects that later merged in a slow collision. The larger lobus, Wenu, is measured at about across its longest axis while the smaller lobus, Weeyo, is measured at across its longest axis. Wenu is lenticular in shape, being highly flattened and moderately elongated. Based on shape models of Arrokoth constructed from images taken by the New Horizons spacecraft, the dimensions of Wenu are approximately . In contrast, Weeyo is less flattened, with dimensions of . As a whole, Arrokoth is across its longest axis and is about thick, with the centers of the lobi separated from each other by .
Given the volume equivalent lobus diameters of and , the volume ratio of Wenu to the smaller Weeyo is approximately 1.9:1.0, meaning that Wenu's volume is nearly twice that of Weeyo. Overall, the volume of Arrokoth is around , though this estimate is largely uncertain due to weak constraints on the thicknesses of the lobi.