Arrokoth (temporary name 2014 MU 69; formerly called Ultima Thule) is a small, icy object located in the Kuiper Belt, which is a region beyond Neptune and Pluto that contains many frozen objects orbiting the Sun. It became the farthest object in the Solar System visited by a spacecraft when NASA’s New Horizons probe passed by it on January 1, 2019. Arrokoth was discovered on June 26, 2014, by astronomer Marc Buie and the New Horizons Search Team. They used the Hubble Space Telescope to find Kuiper Belt objects that New Horizons could explore.
Arrokoth is a contact binary, meaning it is shaped like a "snowman" and made of two connected parts. These parts, named Wenu and Weeyo, are thought to have once been separate pieces of a binary system but gently joined together. Both Wenu and Weeyo have flat shapes with distinct mounds, suggesting they formed from smaller pieces of material about 4.5 billion years ago. Arrokoth’s surface has a red color caused by a type of complex organic material called tholins and shows few craters, which indicates it has remained largely unchanged since its formation. Its nearly circular orbit around the Sun, which is also in a plane similar to other objects in the Solar System, suggests it has not been affected by the gravitational pull of planets.
Name
When Arrokoth was first seen by the Hubble Space Telescope in 2014, it was given the temporary name 1110113Y as part of a search for objects in the Kuiper Belt. It was also called "11" for short. NASA announced in October 2014 that Arrokoth could be a target for the New Horizons spacecraft, and it was called "Potential Target 1" or PT1. In March 2015, the Minor Planet Center officially named it 2014 MU 69 after learning more about its orbit. This name means it was the 1,745th object discovered in the second half of June 2014. In 2017, after more observations, it was given the permanent number 486958.
Before the New Horizons spacecraft flew by Arrokoth on January 1, 2019, NASA asked the public to suggest a nickname. The name "Ultima Thule" was chosen in March 2018. "Thule" refers to a place mentioned in ancient Greek and Roman writings as the farthest known location. The name "Ultima Thule" later became associated with a metaphor for distant, unknown places. After scientists found that Arrokoth has two connected parts, the larger part was called "Ultima" and the smaller part was called "Thule." These names were later changed to "Wenu" and "Weeyo."
The nickname "Ultima Thule" was criticized because "Thule" was used by some in the 19th century to describe a mythical homeland for a group of people. This connection to a group linked to the Nazi Party upset some scientists and historians. In November 2019, the International Astronomical Union (IAU) officially named the object "Arrokoth."
The name "Arrokoth" was chosen by the New Horizons team to honor the Powhatan people, who are indigenous to Virginia and Maryland. The Hubble Space Telescope and the Johns Hopkins University Applied Physics Laboratory, which helped discover Arrokoth, are based in the same region. With permission from the Pamunkey Indian Tribe, part of the Powhatan nation, the name was shared with the IAU. It was officially announced at a ceremony in Washington, D.C., on November 12, 2019. The IAU accepted the name earlier that month, and the decision was published in a scientific report.
The Powhatan language was not widely recorded and became extinct in the late 1700s. In old records, "arrokoth" was described as meaning "sky," but it may have actually meant "cloud."
Shape
Arrokoth is a contact binary made of two hexagonally-shaped lobes connected by a narrow neck or waist. This neck is surrounded by a bright band called Akasa Linea. The two lobes were likely separate objects that joined together during a slow collision. The larger lobe, named Wenu Lobus, is slightly flattened and measures about 20.1 km (12.5 mi) across its longest side. The smaller lobe, Weeyo Lobus, is nearly spherical and measures about 15.0 km (9.3 mi) across its longest side. Shape models from the New Horizons spacecraft show Wenu’s dimensions as approximately 20.1 km × 19.8 km × 13.7 km (12.5 mi × 12.3 mi × 8.5 mi), and Weeyo’s dimensions as approximately 15.0 km × 14.4 km × 13.6 km (9.3 mi × 8.9 mi × 8.5 mi). Wenu’s volume is almost exactly twice that of Weeyo. Together, Arrokoth spans 34.5 km (21.4 mi) along its longest axis and is about 13.8 km (8.6 mi) thick.
The longest sides of both lobes are nearly aligned with each other and with Arrokoth’s rotational axis, which lies between them. This alignment suggests the lobes were locked together before merging, likely because of tidal forces. The alignment supports the idea that each lobe formed from the joining of icy particles.
Before the New Horizons flyby, star occultations showed Arrokoth had a bilobate shape. The first detailed image confirmed its two-lobed appearance and was described as resembling a "snowman" by Alan Stern, as the lobes looked distinctively round. In February 2019, scientists thought Arrokoth was extremely flattened based on images of it blocking background stars. However, a 2024 reanalysis of New Horizons images found Arrokoth was not as flattened as previously believed.
Geology
Measurements of Arrokoth's absorption spectrum by New Horizons' LEISA spectrometer show that Arrokoth's spectrum has a strong red slope from red to infrared wavelengths at 1.2–2.5 μm. LEISA found methanol and complex organic compounds on Arrokoth's surface but no water ice. A specific absorption band at 1.8 μm suggests these organic compounds contain sulfur. Because methanol is abundant, scientists think formaldehyde-based compounds, formed by irradiation, may also be present as complex macromolecules. Arrokoth's spectrum is similar to 2002 VE 95 and centaur 5145 Pholus, which also show strong red slopes and methanol on their surfaces.
Hubble Space Telescope observations in 2016 showed Arrokoth has a red color, like other Kuiper belt objects and centaurs such as Pholus. Arrokoth is redder than Pluto, placing it in the "ultra red" group of cold classical Kuiper belt objects. Its red color comes from complex organic compounds called tholins, formed when cosmic rays and ultraviolet light break down simple compounds. Sulfur-rich tholins suggest methane, ammonia, and hydrogen sulfide were once present but were lost due to Arrokoth's small size. Less volatile materials like methanol, acetylene, ethane, and hydrogen cyanide may have remained, contributing to tholins. Photoionization of organic compounds may have produced hydrogen gas that interacts with the solar wind, but New Horizons' SWAP and PEPSSI instruments found no evidence of this interaction.
Color and spectral data show Arrokoth's surface has subtle color differences. The Akasa (neck) region and lineation features are less red than the central part of the smaller lobe, Weeyo. The larger lobe, Wenu, has redder areas called "thumbprints" near its edge. Arrokoth's surface reflectivity, or albedo, ranges from 5% to 12%, with an overall geometric albedo of 21% and Bond albedo of 6.3%, typical for Kuiper belt objects.
Arrokoth's surface is lightly cratered and smooth, with few small craters (1 km or smaller) suggesting few impacts over its history. Impact events in the Kuiper belt are rare, and impacts on Arrokoth likely occurred at slow speeds (about 300 m/s), making large craters uncommon. This suggests Arrokoth's surface has remained largely unchanged since its formation, possibly preserving clues about its origin and material.
High-resolution images from New Horizons show small pits about 700 m wide on Arrokoth's surface. The cause of these pits is unknown, but possibilities include impacts, material collapse, sublimation of volatiles, or gas escaping from the interior.
Each lobe of Arrokoth has varying brightness and geological features like troughs and hills. These features may have formed when smaller planetesimals clumped together to create the lobes. Brighter areas, such as lineation features, may result from material rolling down hills due to Arrokoth's weak gravity.
The smaller lobe, Weeyo, has a large depression called "Sky" (6.7 km wide, 0.51 km deep), likely an impact crater from a 700 m object. Two bright streaks inside Sky may be avalanche remnants. Four parallel troughs and two possible craters are near Sky's rim. Weeyo also has bright mottled regions and dark areas that may have eroded, exposing darker material. A bright region at Weeyo's equator has rough terrain with pits, craters, or mounds. Sky's area lacks rolling topography, possibly due to resurfacing from the impact.
The larger lobe, Wenu, has troughs and pit crater chains along its edge. Wenu consists of eight units, each about 5 km wide, separated by bright boundaries. These units likely formed from small planetesimals that merged slowly at low speeds. The central unit, 'mh,' is surrounded by a bright ring called Kaʼan Arcus and appears flatter than surrounding units. Another unit, 'md,' at Wenu's edge has higher elevation and tilt.
The neck region, Akasa Linea, is brighter and less red than the lobes. This brightness may come from reflective material deposited from the lobes or ammonia ice that accumulates in the concave neck. The brightness of Akasa is thought to be maintained by seasonal changes.
Orbit and classification
Arrokoth orbits the Sun at an average distance of 44.2 astronomical units (AU), which is about 6.61 billion kilometers or 4.11 billion miles. It takes 294 years to complete one full orbit around the Sun. Arrokoth has a low orbital eccentricity of 0.04, meaning its orbit is nearly circular. Its distance from the Sun varies slightly, from 42.6 AU at its closest point (perihelion) to 45.9 AU at its farthest point (aphelion). Because its orbit is nearly circular, Arrokoth does not come close enough to Neptune for Neptune’s gravity to significantly change its path. The closest Arrokoth ever comes to Neptune is 12.7 AU. Studies show that Arrokoth’s orbit remains stable for at least 10 million years. During the New Horizons flyby in January 2019, Arrokoth was 43.28 AU from the Sun, or about 6.47 billion kilometers and 4.02 billion miles. At this distance, sunlight takes more than six hours to reach Arrokoth.
Arrokoth is classified as a distant minor planet or trans-Neptunian object because it orbits the Sun beyond Neptune. It is located in the Kuiper belt region, between 39.5 and 48 AU from the Sun, and is classified as a classical Kuiper belt object, or cubewano. Arrokoth’s orbit is tilted 2.45 degrees from the ecliptic plane, which is a relatively small angle compared to other classical Kuiper belt objects. Because it has a low orbital tilt and a nearly circular orbit, Arrokoth belongs to the dynamically cold population of classical Kuiper belt objects. These objects are thought to have remained largely undisturbed by Neptune’s gravity during the early history of the Solar System. Scientists believe this group of objects may be leftover pieces from the formation of the Solar System.
Rotation and temperature
Photometric observations from the Hubble Space Telescope show that Arrokoth's brightness changes by about 0.3 magnitudes as it spins. While Hubble data could not determine Arrokoth's rotation period or the full range of brightness changes, the small variations suggest its rotation axis is either pointing directly toward Earth or viewed from the side, with a shape close to a perfect sphere and an estimated width-to-length ratio of 1.0 to 1.15.
As the New Horizons spacecraft approached Arrokoth, no brightness changes were detected, even though Arrokoth has an irregular shape. Scientists explained this by suggesting Arrokoth's rotation axis is nearly aligned with the direction of New Horizons. Images from New Horizons confirmed that Arrokoth's rotation is tilted, with its south pole facing the Sun. Arrokoth's rotation axis is tilted 99 degrees relative to its orbit. Using data from occultation events and New Horizons images, scientists calculated Arrokoth's rotation period as 15.938 hours.
Because of Arrokoth's extreme tilt, the amount of sunlight reaching its northern and southern hemispheres changes greatly as it orbits the Sun. At certain points in its orbit, one pole faces the Sun continuously, while the other is in darkness. The amount of sunlight Arrokoth receives varies by 17% due to the nearly circular shape of its orbit. Arrokoth's average temperature is estimated to be about 42 K (−231.2 °C; −384.1 °F), with the sunlit area reaching up to 60 K. Measurements from the New Horizons REX instrument show that the temperature on the unlit side of Arrokoth is about 29 ± 5 K, higher than predicted models of 12–14 K. This suggests that heat from beneath Arrokoth's surface is being released, as subsurface materials are warmer than the outer surface.
Mass and density
The mass and density of Arrokoth are not known. A clear estimate of its mass and density cannot be made because the two lobes of Arrokoth are touching each other, not orbiting around a shared center. While a natural satellite orbiting Arrokoth could help calculate its mass, no such satellites have been discovered. If the two lobes are held together by their own gravity, with the pull of gravity between them stronger than forces that might pull them apart, Arrokoth is thought to have a very low density, similar to comets. Its minimum estimated density is 0.29 grams per cubic centimeter. To keep the neck-shaped area between the lobes from being crushed, Arrokoth’s density must be less than 1 gram per cubic centimeter. If the density were higher, the gravity between the lobes would cause the object to collapse into a round shape.
Formation
Arrokoth is believed to have formed when two separate parent objects joined together. These parent objects were created over time from a rotating cloud of small, icy bodies that existed since the Solar System formed 4.6 billion years ago. Arrokoth likely formed in a cold, dark region of the early Kuiper belt, where the Sun’s light was blocked by dust. In the early Kuiper belt, icy particles slowed down because of drag from gas and dust, and then came together to form larger pieces.
Because Arrokoth has not been hit by large objects since its formation, its history remains well-preserved. The two parts of Arrokoth, called lobes, appear different, suggesting they grew separately while orbiting each other. Both parent objects seem to have come from the same material, as they share similar brightness, color, and composition. The larger lobe, Wenu, shows signs of being made from smaller pieces that joined together before merging with the smaller lobe. Wenu may have been formed from about eight smaller pieces, each about 5 kilometers (3 miles) wide.
Scientists are still unsure how Arrokoth became its current flattened shape. One idea is that the parent objects spun quickly at first, causing them to flatten from the force of their rotation. Over time, their spin slowed as they were hit by small objects, which transferred their motion to other debris. Eventually, the parent objects moved closer together and merged slowly, forming Arrokoth’s two-lobed shape.
Another idea, proposed by researchers in 2020, suggests that Arrokoth’s flattened shape may have happened after its lobes joined. At first, Arrokoth had a high amount of volatile materials, such as frozen gases. After the surrounding dust cleared, sunlight reached Arrokoth, causing ice to turn directly into gas (a process called sublimation). Because Arrokoth spins at an angle, one of its poles faced the Sun for half its orbit, leading to ice loss at that pole.
Although the exact reason for Arrokoth’s flattened shape is unclear, its lobes merged gently. Arrokoth shows no signs of being crushed or broken, which suggests the parent objects joined slowly, at about 2 meters per second (6.6 feet per second)—similar to a person walking. The parent objects likely merged at a steep angle (more than 75 degrees) to create Arrokoth’s thin neck without breaking the lobes. By the time they merged, both objects had already locked in place, always facing the same direction as they spun.
The Kuiper belt has few objects moving quickly, so Arrokoth has been hit by other objects very rarely. Over 4.5 billion years, sunlight has only slightly worn away about 1 centimeter (0.39 inches) of Arrokoth’s surface. Because of the lack of impacts and changes to its orbit, Arrokoth’s shape and appearance have remained almost unchanged since its two parts joined to form its current two-lobed shape.
Observation
Arrokoth was discovered on June 26, 2014, using the Hubble Space Telescope during an early search to find a suitable Kuiper belt object for the New Horizons spacecraft to fly by. Scientists on the New Horizons team were looking for an object in the Kuiper belt that the spacecraft could study after Pluto. Their next target had to be reachable with the spacecraft’s leftover fuel. Researchers used large telescopes on Earth to search for possible targets starting in 2011. However, none of the objects found were reachable by New Horizons, and most Kuiper belt objects were too far away and too faint to see through Earth’s atmosphere. To find these faint objects, the New Horizons team used the Hubble Space Telescope to search for targets on June 16, 2014.
Hubble first took pictures of Arrokoth on June 26, 2014, 10 days after the search began. Astronomer Marc Buie, part of the New Horizons team, found Arrokoth while reviewing Hubble images. He shared his discovery with the search team for further study. Arrokoth was the second object found during the search, after 2014 MT 69. Three more candidates were later found with Hubble, but follow-up observations ruled them out. Of the five possible targets found with Hubble, Arrokoth was chosen as the best option because it required the least fuel for the spacecraft’s flyby compared to the second best option, 2014 PN 70. On August 28, 2015, NASA officially selected Arrokoth as the flyby target for New Horizons.
Arrokoth is too small and far away for scientists to see its shape from Earth. To learn more, scientists used an event called a stellar occultation, where Arrokoth passes in front of a star from Earth’s view. Since this event is only visible from certain places on Earth, the New Horizons team used data from Hubble and the European Space Agency’s Gaia observatory to predict when and where Arrokoth would block the star’s light. They found that three occultations would occur on June 3, July 10, and July 17, 2017. Scientists traveled to parts of the world to observe these events and study Arrokoth’s shape.
In June and July 2017, Arrokoth blocked the light of three stars. A team led by Marc Buie, called “KBO Chasers,” observed these events from South America, Africa, and the Pacific Ocean. On June 3, 2017, scientists in Argentina and South Africa tried to see Arrokoth’s shadow but did not detect it. At first, this made scientists think Arrokoth might be smaller or brighter than expected. Later, Hubble observations showed that the telescopes had been placed in the wrong locations, and the earlier conclusions were incorrect.
On July 10, 2017, the SOFIA airborne telescope was positioned near the predicted path of the second occultation over the Pacific Ocean. Scientists used this event to look for dangerous material, like rings or dust, near Arrokoth that might threaten the New Horizons spacecraft during its 2019 flyby. Data collection was successful. At first, scientists thought the central shadow was missed, but later analysis showed that SOFIA had observed a very short dip in the star’s light. This data helped scientists understand the amount of dust near Arrokoth. Results from this search were shared at a science meeting in October 2017.
On July 17, 2017, the Hubble Space Telescope checked for debris around Arrokoth, limiting the possibility of rings or debris within 75,000 km (47,000 mi) of Arrokoth. For the third occultation, scientists set up 24 mobile telescopes along the predicted path of Arrokoth’s shadow in southern Argentina. These telescopes were spaced about 4 km (2.5 mi) apart. Using Hubble’s precise data, scientists successfully observed Arrokoth’s shadow from at least five telescopes. This, combined with SOFIA’s findings, helped scientists understand the amount of debris near Arrokoth.
Results from the July 17 occultation suggested Arrokoth might have an irregular shape or be a close binary object. Based on the time the shadow lasted, scientists found Arrokoth had two “lobes” with diameters of about 20 km (12 mi) and 18 km (11 mi). Early data also suggested Arrokoth might have a moonlet orbiting 200–300 km (120–190 mi) away. Later, scientists realized a software error had shifted the target’s position. After correcting this, the short dip observed on July 10 was confirmed to be from Arrokoth itself.
By combining data about Arrokoth’s light patterns, color, and occultation results, scientists created images showing what Arrokoth might look like before the spacecraft flew by.
In 2018, two more occultations were predicted: one on July 16 and one on August 4. These were less useful than the 2017 events. Scientists did not observe the July 16 event, which occurred over the South Atlantic and Indian Ocean. For the August 4 event, teams of about 50 researchers traveled to Senegal and Colombia. This event received media attention in Senegal, where scientists shared knowledge with the public. Despite bad weather, scientists successfully observed the event. On September 6, 2018, NASA confirmed that at least one observer saw the star’s light dip, providing new information about Arrokoth’s size and shape.
Hubble observations on August 4, 2018, supported the occultation campaign. Although Hubble could not be placed directly in the path of the event, it studied the region up to 1,600 km (990 mi) from Arrokoth. No changes in the star’s brightness were seen, ruling out thick rings or debris within 1,600 km (990 mi) of Arrokoth. Results from the 2017 and 2018 occultation events were shared at a science meeting in October 2018.
Exploration
In July 2015, the New Horizons spacecraft completed its flyby of Pluto. In October and November 2015, it made four course changes to adjust its path toward Arrokoth. Arrokoth is the first object to be targeted for a flyby that was discovered after the spacecraft was launched. It is also the farthest object in the Solar System ever visited by a spacecraft. Traveling at a speed of 51,500 km/h (32,000 mph), New Horizons passed Arrokoth at a distance of 3,538 km (2,198 mi). This distance is equivalent to a few minutes of travel at the spacecraft’s speed and one-third the distance of its closest encounter with Pluto. The closest approach occurred on 1 January 2019 at 05:33 UTC (Spacecraft Event Time – SCET), when the spacecraft was 43.4 AU from the Sun, in the direction of the constellation Sagittarius. At this distance, radio signals between Earth and New Horizons took 6 hours to travel one way.
New Horizons’ closest approach to Arrokoth was toward the celestial north of the object. Because Arrokoth’s rotation is tilted, with its south pole facing the Sun, its northern hemisphere was mostly in darkness during the flyby.
The science goals of the flyby include studying Arrokoth’s geology, surface features, and composition, such as searching for ammonia, carbon monoxide, methane, and water ice. Scientists also looked for orbiting moonlets, a coma (a cloud of gas and dust), or rings around Arrokoth. Images with details as small as 30 meters (98 feet) to 70 meters (230 feet) were expected. Observations from the Hubble Space Telescope showed no faint, small moons orbiting Arrokoth beyond 2,000 km (1,200 mi). Arrokoth has no detectable atmosphere, large rings, or satellites larger than 1.6 km (1 mi) in diameter. However, scientists continue searching for smaller moons, which may help explain how Arrokoth formed from two separate objects.
New Horizons first detected Arrokoth on 16 August 2018, from a distance of 172 million km (107 million mi). At that time, Arrokoth had a brightness of magnitude 20, in the direction of Sagittarius. By mid-November, it was expected to reach magnitude 18, and by mid-December, magnitude 15. From the spacecraft’s perspective, Arrokoth became bright enough to see with the naked eye (magnitude 6) just 3–4 hours before closest approach. If obstacles were found, the spacecraft could have changed its path to a safer distance, but no moons, rings, or hazards were detected. High-resolution images were taken on 1 January 2019. Lower-resolution images arrived the next day. The data collected during the flyby was expected to take 20 months to transmit back to Earth, ending in September 2020.