486958 Arrokoth
486958 Arrokoth is a small, icy Kuiper belt object orbiting the Sun beyond Neptune and Pluto. It became the farthest object in the Solar System visited by a spacecraft when the NASA space probe New Horizons flew past it on 1 January 2019. Arrokoth was discovered on 26 June 2014 by astronomer Marc Buie and the New Horizons Search Team, who had been using the Hubble Space Telescope to find Kuiper belt objects that New Horizons could visit.
Arrokoth is a contact binary—a "snowman"-shaped object composed of two lobes connected by a narrow neck of material. The lobes of Arrokoth, named Wenu and Weeyo, are believed to be former planetesimals that once formed a binary system, but gently merged together. Wenu and Weeyo have flattened shapes consisting of distinct mounds, which indicate that they formed from a collection of smaller planetesimals 4.5 billion years ago. The surface of Arrokoth is tinted red by tholins and shows little cratering, which suggests that it has undergone little change since its formation. The primitive nature of Arrokoth is supported by its nearly circular and low-inclination orbit around the Sun, which suggests that it had not been disturbed by the gravitational influence of the planets.
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. Thule is the northernmost location mentioned in ancient Greek and Roman literature and cartography, while in classical and medieval literature ultima 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 contact binary, the New Horizons team nicknamed the larger lobe "Ultima" and the smaller lobe "Thule". They are now formally named "Wenu" and "Weeyo", respectively.Following the selection of the nickname, it was criticized by a Newsweek columnist because of the use of "Thule" by 19th-century racists as the mythical homeland of the Aryan race. The New York Times, crediting Newsweek, quoted several scientists and historians who expressed being unhappy about the name's connection to the Nazi Party. In November 2019, the International Astronomical Union announced the object's permanent official name, Arrokoth.
Arrokoth
The name Arrokoth was chosen by the New Horizons team to represent the Powhatan people indigenous to the Tidewater region of Virginia and Maryland in the eastern United States. The Hubble Space Telescope and the Johns Hopkins University Applied Physics Laboratory, which were prominently involved in Arrokoth's discovery, were both operated from the Tidewater region of Maryland.With the permission of the elders of the Pamunkey Indian Tribe of the Powhatan nation, 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.
The Powhatan language became extinct in the late 18th century and little was recorded of it. In an old word list, arrokoth is glossed as 'sky', and this was the meaning intended by the New Horizons team, but it would seem that it actually meant 'cloud'.
Shape
Arrokoth is a contact binary consisting of two hexagonally-shaped lobes attached by a narrow neck or waist, which is encircled by a bright band named Akasa Linea. The lobes were likely once two objects that later merged in a slow collision. The larger lobe, formally named Wenu Lobus, is slightly flattened and measures about across its longest axis, while the smaller lobe, Weeyo Lobus, is roughly spherical measures about across its longest axis. Based on shape models of Arrokoth constructed from images taken by the New Horizons spacecraft, the dimensions of Wenu are approximately while the dimensions of Weeyo are approximately. Wenu's volume is nearly exactly twice that of Weeyo. As a whole, Arrokoth is across its longest axis and is about thick.The longest axes of both lobes are nearly aligned with each other and to the rotational axis, which is situated between them. This near-parallel alignment of the lobes suggests that they were mutually locked to each other, likely due to tidal forces, before merging. The alignment of the lobes supports the idea that the two had individually formed from the coalescence of a cloud of icy particles.
Prior to the New Horizons flyby of Arrokoth, stellar occultations by Arrokoth had provided evidence for its bilobate shape. The first detailed image of Arrokoth confirmed its double-lobed appearance and was described as a "snowman" by Alan Stern, as the lobes appeared distinctively spherical. In February 2019, one month after the New Horizons flyby, scientists proposed that Arrokoth was extremely flattened, based on New Horizons images of Arrokoth occulting background stars after its closest approach. However, a 2024 reanalysis of New Horizons imagery found that Arrokoth was not as flattened as previously thought.
Geology
Spectra and surface
Measurements of Arrokoth's absorption spectrum by New Horizons LEISA spectrometer show that Arrokoth's spectrum exhibits a strong red spectral slope extending from red to infrared wavelengths at 1.2–2.5 μm. Spectral measurements from LEISA revealed the presence of methanol and complex organic compounds on the surface of Arrokoth, but no evidence of water ice. One particular absorption band in Arrokoth's spectrum at 1.8 μm indicates that these organic compounds are sulfur-rich. Given the abundance of methanol on Arrokoth's surface, it is predicted that formaldehyde-based compounds resulting from irradiation should also be present, albeit in the form of complex macromolecules. Arrokoth's spectrum shares similarities with that of and the centaur 5145 Pholus, which also display strong red spectral slopes along with signs of methanol present on their surfaces.Preliminary observations by the Hubble Space Telescope in 2016 revealed that Arrokoth has a red coloration, similar to other Kuiper belt objects and centaurs like Pholus. Arrokoth's color is redder than that of Pluto, thus it belongs to the "ultra red" population of cold classical Kuiper belt objects. The red coloration of Arrokoth is caused by the presence of a mix of complex organic compounds called tholins, which are produced from the photolysis of various simple organic and volatile compounds by cosmic rays and ultraviolet solar radiation. The presence of sulfur-rich tholins on Arrokoth's surface implies that volatiles such as methane, ammonia, and hydrogen sulfide were once present on Arrokoth, but were quickly lost due to Arrokoth's small mass. However, less volatile materials such as methanol, acetylene, ethane, and hydrogen cyanide could be retained over a longer period of time, and may likely account for the reddening and production of tholins on Arrokoth. The photoionization of organic compounds and volatiles on Arrokoth was also thought to produce hydrogen gas that would interact with the solar wind, though New Horizons SWAP and PEPSSI instruments did not detect any signature of solar wind interaction around Arrokoth.
From color and spectral measurements of Arrokoth, the surface displays subtle color variation among its surface features. Spectral images of Arrokoth show that the Akasa region and lineation features appear less red compared to the central region of the smaller lobe Weeyo. The larger lobe Wenu also displays redder regions, informally known as "thumbprints" by the New Horizons team. The thumbprint features are located near Wenu's limb. The surface albedo or reflectivity of Arrokoth varies from 5 percent to 12 percent due to various bright features on its surface. Its overall geometric albedo, the quantity of reflected light in visible spectrum, is measured at 21 percent, typical for most Kuiper belt objects. The overall Bond albedo of Arrokoth is measured at 6.3 percent.
Craters
The surface of Arrokoth is lightly cratered and smooth in appearance. Arrokoth's surface has few small craters, implying a paucity of impacts throughout its history. The occurrence of impact events in the Kuiper belt is thought to be uncommon, with a very low impact rate over the course of one billion years. Due to the slower orbital speeds of Kuiper belt objects, the speed of objects impacting Arrokoth is expected to be low, with typical impact speeds around. At such slow impact speeds, large craters on Arrokoth are expected to be rare. With a low frequency of impact events along with the slow speeds of impacts, Arrokoth's surface would remain preserved since its formation. The preserved surface of Arrokoth could possibly give hints to its formation process, as well as signs of accreted material.Numerous small pits on Arrokoth's surface were identified in high resolution images from the New Horizons spacecraft. The size of these pits are measured at about across. The exact cause of these pits is unknown; several explanations for these pits include impact events, the collapse of material, the sublimation of volatile materials, or the venting and escape of volatile gases from the interior of Arrokoth.