486958 Arrokoth

Lore & Background

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. When first observed, it was designated 1110113Y and nicknamed '11'. Its existence as a potential target was announced by NASA in October 2014, and it was unofficially designated 'Potential Target 1', or PT1. Its official provisional designation, 2014 MU69, was assigned by the Minor Planet Center in March 2015. Before the flyby, NASA invited public suggestions for a nickname; 'Ultima Thule' was selected on 13 March 2018. Following criticism over the nickname's connection to 19th-century racists and the Nazi Party, the International Astronomical Union announced the permanent official name Arrokoth in November 2019. The name was chosen by the New Horizons team to represent the Powhatan people indigenous to the Tidewater region of Virginia and Maryland, with permission of the elders of the Pamunkey Indian Tribe. In an old word list, 'arrokoth' is glossed as 'sky', but it actually meant 'cloud'.

Reader's Guide

Arrokoth is a contact binary consisting of two hexagonally-shaped lobes, named Wenu and Weeyo, attached by a narrow neck called Akasa Linea. The lobes are believed to be former planetesimals that once formed a binary system and gently merged. Wenu measures about 20.1 km across its longest axis, while Weeyo measures about 15.0 km; Wenu's volume is nearly exactly twice that of Weeyo. The surface is tinted red by tholins and shows little cratering, suggesting little change since its formation 4.5 billion years ago. Its nearly circular and low-inclination orbit suggests it has never been disturbed by planetary gravitational influence. Spectral measurements revealed methanol and complex organic compounds, but no water ice. The presence of sulfur-rich tholins implies volatiles such as methane, ammonia, and hydrogen sulfide were once present but lost. The primitive nature of Arrokoth makes it a key object for understanding the early Solar System.

Did You Know?

• Arrokoth is a contact binary with lobes named Wenu and Weeyo.
• The name 'Arrokoth' comes from the Powhatan language and means 'cloud'.

Discovery and the Historic Flyby

Arrokoth was spotted on 26 June 2014 by astronomer Marc Buie working with the New Horizons Search Team, who were scanning the outer Solar System with the Hubble Space Telescope to identify a suitable target for the probe's extended mission. NASA publicly confirmed the object as a potential flyby candidate in October 2014, and the Minor Planet Center assigned it the provisional designation 2014 MU69 in March 2015 once enough orbital data had accumulated. The permanent number 486958 followed on 12 March 2017. On 1 January 2019, the New Horizons spacecraft executed a flyby that made Arrokoth the most distant body in the Solar System ever visited by a human-made object. The encounter provided the first close-up images of a Kuiper belt object and confirmed the double-lobed structure that stellar occultation data had hinted at before the mission.

The Snowman Shape and a Gentle Merger

Arrokoth presents as a contact binary, a snowman-shaped body made of two lobes joined by a narrow waist called Akasa Linea. The larger lobe, Wenu Lobus, spans roughly 20.1 km along its longest axis and is slightly flattened, while the smaller, Weeyo Lobus, is closer to spherical at about 15.0 km across. Wenu's volume is nearly exactly double that of Weeyo. The two lobes' longest axes are almost parallel to each other and to the body's rotational axis, a geometry that points to tidal locking before the pair merged in an extremely slow collision. Their flattened, mound-studded surfaces indicate each lobe had already coalesced from a cloud of smaller icy planetesimals roughly 4.5 billion years ago. Before the flyby, stellar occultations had already suggested a bilobate silhouette, but New Horizons imagery confirmed the distinct double-lobed form. A 2019 analysis proposed extreme flattening, yet a 2024 reanalysis of the same images concluded Arrokoth was not as squashed as initially thought.

From Ultima Thule to Arrokoth

The object's naming history is as layered as its geology. Initially tracked as 1110113Y and informally called "11," it became "Potential Target 1" before receiving its provisional designation. In 2018, NASA opened a public poll, and the crowd-favorite nickname Ultima Thule was selected on 13 March 2018, evoking the classical idea of the farthest known land. The choice drew criticism from a Newsweek columnist and, as reported by the New York Times, from scientists and historians uncomfortable with the term's appropriation by 19th-century racial theorists and the Nazi Party. After the flyby revealed the contact-binary structure, the team informally split the nickname, calling the larger lobe "Ultima" and the smaller "Thule." In November 2019, the IAU accepted the permanent name Arrokoth, a Powhatan word proposed with the blessing of Pamunkey tribal elders. The New Horizons team unveiled it at a NASA Headquarters ceremony on 12 November 2019, honoring the Powhatan people of the Tidewater region where Hubble and the Applied Physics Laboratory are based.

A Pristine Relic of the Early Solar System

Arrokoth's surface carries a reddish hue produced by tholins, complex organic molecules created when ultraviolet light and cosmic rays interact with simple ices. Remarkably, the surface shows very little cratering, a sign that the body has experienced almost no geological or impact modification since it first took shape. Its orbit reinforces this picture of preservation: nearly circular and tilted only slightly relative to the ecliptic, Arrokoth's path has apparently never been perturbed by the gravitational pull of the giant planets. Together, the pristine surface and the undisturbed orbit suggest Arrokoth has remained essentially unchanged for 4.5 billion years, preserving a snapshot of conditions in the outer protoplanetary disk. The mounds visible on both lobes are interpreted as remnants of smaller planetesimals that accreted into the larger bodies before their gentle merger, making Arrokoth a direct window into the earliest stages of planetary formation.