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Arrokoth ( minor-planet designation 486958 Arrokoth;
provisional designation Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calcu ...
), formerly nicknamed Ultima Thule, is a trans-Neptunian object located in the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
. Arrokoth became the farthest and most primitive object in the Solar System visited by a spacecraft when the NASA space probe ''
New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...
'' conducted a flyby on 1 January 2019. Arrokoth is a
contact binary In astronomy, a contact binary is a binary star system whose component stars are so close that they touch each other or have merged to share their gaseous envelopes. A binary system whose stars share an envelope may also be called an overcontac ...
long, composed of two planetesimals and 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 A classical Kuiper belt object, also called a cubewano ( "QB1-o"), is a low-eccentricity Kuiper belt object (KBO) that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major axes ...
. Arrokoth was discovered on 26 June 2014 by astronomer Marc Buie and the
New Horizons Search Team ''New Horizons'' is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with ...
using the Hubble Space Telescope as part of a search for a Kuiper belt object for the ''New Horizons'' mission to target in its first extended mission; it was chosen over two other candidates 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 Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calcu ...
, , was assigned by 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 ...
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. ( grc, Θούλη, ) 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. The nickname was criticized by a '' Newsweek'' columnist because of its use by 19th-century racists as the mythical homeland of the Aryan race. 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 Powhatan or Virginia Algonquian was an Eastern Algonquian subgroup of the Algonquian languages. It was formerly spoken by the Powhatan people of tidewater Virginia. Following 1970s linguistic research by Frank Thomas Siebert, Jr., some of the l ...
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 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 ...
on 8 November, and the ''New Horizons'' team's naming citation was published in a
Minor Planet Circular 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 ...
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 Akasa may refer to: Philosophy * Ākāśa, a concept in Indian cosmology * Ākāśa (Jainism), ''space'' in the Jain conception of the cosmos People and persons * Akasa Singh, Indian singer * Akassa tribe (also called Akasa) from Nigeria Place ...
. 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. The 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 the 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 the Wenu to the smaller Weeyo is approximately 1.9:1.0, meaning that Wenu's volume is nearly twice that of the Weeyo. Overall, the volume of Arrokoth is around , though this estimate is largely uncertain due to weak constraints on the thicknesses of the lobi. 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 lobi appeared distinctively spherical. On 8 February 2019, one month after the ''New Horizons'' flyby, Arrokoth was found to be more flattened than initially thought, based on additional images of Arrokoth taken by ''New Horizons'' after its closest approach. The flattened lobus Wenu was described as a "pancake", while Weeyo was described as a "walnut" as it appeared less flattened. By observing how the unseen sections of Arrokoth
occulted An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
background stars, scientists were able to then outline the shapes of both lobi. The cause of Arrokoth's unexpectedly flattened shape is uncertain, with various explanations including sublimation or centrifugal forces. The longest axes of the lobi are nearly aligned with the
rotational axis Rotation around a fixed axis is a special case of rotational motion. The fixed-axis hypothesis excludes the possibility of an axis changing its orientation and cannot describe such phenomena as wobbling or precession. According to Euler's rota ...
, which is situated between them. This near-parallel alignment of the lobi suggests that they were mutually locked to each other, likely due to tidal forces, before merging. The alignment of the lobi supports the idea that the two had individually formed from the coalescence of a cloud of icy particles.


Geology


Spectra and surface

Measurements of Arrokoth's
absorption spectrum Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating f ...
by ''New Horizons'' LEISA
spectrometer A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...
show that Arrokoth's spectrum exhibits a strong red
spectral slope In astrophysics and planetary science, spectral slope, also called spectral gradient, is a measure of dependence of the reflectance on the wavelength. In digital signal processing, it is a measure of how quickly the spectrum of an audio sound tails ...
extending from red to infrared wavelengths at 1.2–2.5 
μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
. Spectral measurements from LEISA revealed the presence of
methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
,
hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on an ...
, water ice, and complex organic compounds on the surface of Arrokoth. One particular
absorption band According to quantum mechanics, atoms and molecules can only hold certain defined quantities of energy, or exist in specific states. When such quanta of electromagnetic radiation are emitted or absorbed by an atom or molecule, energy of the ...
in Arrokoth's spectrum at 1.8 μm indicates that these organic compounds are
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
-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
macromolecule A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. The ...
s. Arrokoth's spectrum shares similarities with that of and the
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 ...
5145 Pholus 5145 Pholus is an eccentric centaur in the outer Solar System, approximately in diameter, that crosses the orbit of both Saturn and Neptune. It was discovered on 9 January 1992 by American astronomer David Rabinowitz (uncredited) of UA's Space ...
, 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
tholin Tholins (after the Greek (') "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon ...
s, which are produced from the
photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
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 Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
were once present on Arrokoth, but were quickly lost due to Arrokoth's small mass. However, less volatile materials such as methanol,
acetylene Acetylene (systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure ...
, ethane, and
hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on an ...
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 Swap or SWAP may refer to: Finance * Swap (finance), a derivative in which two parties agree to exchange one stream of cash flows against another * Barter Science and technology * Swap (computer programming), exchanging two variables in t ...
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 (neck) 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 (the quantity of reflected light of any wavelength) 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 (from in size to the limits of photographic resolution), implying a paucity of impacts throughout its history. The occurrence of
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
s 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 Sublimation or sublimate may refer to: * ''Sublimation'' (album), by Canvas Solaris, 2004 * Sublimation (phase transition), directly from the solid to the gas phase * Sublimation (psychology), a mature type of defense mechanism * Sublimate of mer ...
of volatile materials, or the venting and escape of volatile gases from the interior of Arrokoth.


Surface features

The surfaces of each lobus of Arrokoth display regions of varying brightness along with various
geological feature Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...
s such as troughs and hills. These geological features are thought to have originated from the clumping of smaller planetesimals that come to form the lobi of Arrokoth. The brighter regions of Arrokoth's surface, especially its bright lineation features, are thought to have resulted from the deposition of material that have rolled down from hills on Arrokoth, as surface gravity on Arrokoth is sufficient for this to occur. The smaller lobus, Weeyo, bears a large depression feature named 'Sky' (previously dubbed 'Maryland' after the home state of the ''New Horizons'' team). Assuming Sky has a circular shape, its diameter is , with a depth of . Sky is likely an impact crater that was formed by an object across. Two notably bright streaks of similar size are present within Sky, and may be remnants of avalanches where bright material rolled into the depression. Four subparallel troughs are present near the
terminator Terminator may refer to: Science and technology Genetics * Terminator (genetics), the end of a gene for transcription * Terminator technology, proposed methods for restricting the use of genetically modified plants by causing second generation s ...
of Weeyo, along with two possible kilometer-sized impact craters on the rim of Sky. The surface of Weeyo exhibits bright mottled regions separated by broad, dark regions ('' dm'') which may have undergone scarp retreat, in which they were eroded due to the sublimation of volatiles, exposing
lag deposit A lag deposit is the deposition of material winnowed by physical action. Aeolian processes, fluvial processes, and tidal processes can remove the finer portion of a sedimentary deposit leaving the coarser material behind. Lag deposits are found i ...
s of darker material irradiated by sunlight. Another bright region ('' rm''), located at the equatorial end of Weeyo, exhibits rough terrain along with several topographic features that have been identified as possible pits, craters, or mounds. Weeyo does not display distinct units of rolling topography near Sky, likely as a result of resurfacing caused by the impact event that created the crater. Like on Weeyo, troughs and pit crater chains are also present along the terminator of the larger lobus Wenu. Wenu consists of eight distinctive units or blocks of rolling topography, each similarly sized at around . The units are separated by relatively bright boundary regions. The similar sizes of the units suggests that each was once a small planetesimal, and that they coalesced to form Wenu. The planetesimals are expected to have accreted slowly by astronomical standards (at speeds of several meters per second), though they must have a very low
mechanical strength The field of strength of materials, also called mechanics of materials, typically refers to various methods of calculating the stresses and strains in structural members, such as beams, columns, and shafts. The methods employed to predict the re ...
in order to merge and form compact bodies at these speeds. The central unit ('mh') is encircled by a bright annular feature, Kaan Arcus (initially dubbed "The Road to Nowhere"). From stereographic analysis, the central unit appears to be relatively flat compared to the surrounding units. Stereographic analysis of Arrokoth has also shown that one particular unit located at Wenu's limb ('md') appears to have a higher elevation and tilt than the others. Akasa Linea, the neck region connecting the two lobi, has a brighter and less red appearance than the surfaces of either lobus. The brightness of Akasa Linea is likely due to a composition of a more reflective material than the surfaces of the lobi. One hypothesis suggests the bright material originated in the deposition of small particles that had fallen from the lobi over time. Since Arrokoth's center of gravity lies between the lobi, small particles are likely to roll down the steep slopes toward the center between each lobus. Another proposal suggests the bright material is produced by the deposition of ammonia ice. Ammonia vapor present on the surface of Arrokoth would solidify around Akasa Linea, where gases cannot escape due to the concave shape of the neck. The brightness of Akasa is thought to be maintained by high seasonal axial tilt as Arrokoth orbits around the Sun. Over the course of its orbit, Akasa Linea is shadowed when the lobi are coplanar to the direction of the Sun, at which times the neck region receives no sunlight, cooling and trapping volatiles in the region. In May 2020, the IAU's Working Group for Planetary System Nomenclature (WGPSN) formally established a naming theme for all features of Arrokoth, which are to be named after words for "sky" in the languages of the world, past and present. In 2021, the first few names were approved, including Sky Crater on the small lobe, later named Weeyo Lobus. In 2022, Kaʼan Arcus was approved for the circular arc on Wenu Lobus.


Internal structure

Topography variations at the limb of Arrokoth suggest that its interior is likely composed of mechanically strong material consisting of mostly amorphous water ice and rocky material. Trace amounts of methane and other volatile gases in the form of vapors may also be present in Arrokoth's interior, trapped in water ice. Under the assumption that Arrokoth has a low comet-like density of around , its internal structure is expected to be porous, as volatile gases trapped in Arrokoth's interior are thought to escape from the interior to the surface. Assuming that Arrokoth may have an internal heat source caused by the
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
of
radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transfer ...
s, the trapped volatile gases inside Arrokoth would migrate outward and escape from the surface, similarly to the scenario of outgassing of comets. The escaped gases may subsequently freeze and deposit on Arrokoth's surface, and could possibly account for the presence of ices and tholins on its surface.


Orbit and classification

Arrokoth orbits the Sun at an average distance of , taking 297.7 years to complete a full orbit around the Sun. Having a low
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 ...
of 0.042, Arrokoth follows a nearly circular orbit around the Sun, only slightly varying in distance from 42.7 AU at perihelion to 46.4 AU at aphelion. Because Arrokoth has a low orbital eccentricity, it does not approach close enough to
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 ...
for its orbit to become perturbed. (Arrokoth's minimum orbital intersection distance from Neptune is 12.75 AU.) Arrokoth's orbit appears to be stable over the long term; simulations by the
Deep Ecliptic Survey The Deep Ecliptic Survey (DES) is a project to find Kuiper belt objects (KBOs), using the facilities of the National Optical Astronomy Observatory (NOAO). The principal investigator is Robert L. Millis. Since 1998 through the end of 2003, the sur ...
show that its orbit will not significantly change over the next 10 million years. At the time of the ''New Horizons'' flyby in January 2019, Arrokoth's distance from the Sun was . At this distance, light from the Sun takes over six hours to reach Arrokoth. Arrokoth has last passed aphelion around 1906 and is currently approaching the Sun at a rate of approximately 0.13 AU per year, or about . Arrokoth will approach perihelion by 2055. Having an observation arc of 851 days, Arrokoth's orbit is fairly well-determined, with an
uncertainty parameter The uncertainty parameter ''U'' is introduced by the Minor Planet Center (MPC) to quantify the uncertainty of a perturbed orbital solution for a minor planet. The parameter is a logarithmic scale from 0 to 9 that measures the anticipated longit ...
of 2 according to the Minor Planet Center. Hubble Space Telescope observations in May and July 2015 as well as in July and October 2016 have greatly reduced the uncertainties in Arrokoth's orbit, which prompted the Minor Planet Center to assign its permanent minor planet number. In contrast to the orbit calculated by the Minor Planet Center, Arrokoth's observation arc in the JPL Small-Body Database does not include these additional observations and purports the orbit to be highly uncertain, with an uncertainty parameter of 5. Arrokoth is generally classified as a distant minor planet or trans-Neptunian object by the Minor Planet Center as it orbits in the outer Solar System beyond Neptune. Having a non-resonant orbit within the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
region 39.5–48 AU from the Sun, Arrokoth is formally classified as a classical Kuiper belt object, or cubewano. Arrokoth's orbit is inclined to the ecliptic plane by 2.45 degrees, relatively low compared to other classical Kuiper belt objects such as . Since Arrokoth has a low orbital inclination and eccentricity, it is part of the dynamically cold population of classical Kuiper belt objects, which are unlikely to have undergone significant perturbations by Neptune during its outward migration in the past. The cold classical population of Kuiper belt objects are thought to be remnant planetesimals left over from the accretion of material during the
formation of the Solar System The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a ...
.


Rotation and temperature

Results from photometric Hubble Space Telescope observations show that the brightness of Arrokoth varies by around 0.3  magnitudes as it rotates. Though the rotation period and light curve amplitude of Arrokoth could not be determined from Hubble observations, the subtle brightness variations suggested that Arrokoth's rotational axis is either pointed toward the Earth or is being viewed at an equator-on configuration with a nearly spherical shape, with a constrained ''a''/''b'' best-fit aspect ratio around 1.0–1.15. Upon the ''New Horizons'' spacecraft's approach to Arrokoth, no rotational light curve amplitude was detected by the spacecraft despite Arrokoth's irregular shape. To explain the lack of its rotational light curve, scientists surmised that Arrokoth is rotating on its side, with its rotational axis pointing nearly directly at the approaching ''New Horizons'' spacecraft. Subsequent images of Arrokoth from ''New Horizons'' upon approach confirmed that its rotation is tilted, with its south pole facing towards the Sun. The rotational axis of Arrokoth is tilted 99 
degree Degree may refer to: As a unit of measurement * Degree (angle), a unit of angle measurement ** Degree of geographical latitude ** Degree of geographical longitude * Degree symbol (°), a notation used in science, engineering, and mathematics ...
s to its orbit. Based on occultation and ''New Horizons'' imaging data, Arrokoth's rotation period is determined to be 15.938 hours. Due to the high axial tilt of its rotation, the
solar irradiance Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/ ...
of the northern and southern hemispheres of Arrokoth varies greatly over the course of its orbit around the Sun. As it orbits around the Sun, one polar region of Arrokoth faces the Sun continuously while the other faces away. The solar irradiance of Arrokoth varies by 17 percent due to the low eccentricity of its orbit. The average temperature of Arrokoth is estimated to be around , with a maximum of around on the illuminated
subsolar point The subsolar point on a planet is the point at which its sun is perceived to be directly overhead (at the zenith); that is, where the sun's rays strike the planet exactly perpendicular to its surface. It can also mean the point closest to the sun ...
of Arrokoth. Radiometric measurements from the ''New Horizons'' REX instrument indicate that the mean surface temperature of Arrokoth's unilluminated face is about , higher than the modeled range of . The higher temperature of Arrokoth's unilluminated face as measured by REX implies that thermal radiation is emitted from Arrokoth's subsurface, which was predicted to be intrinsically warmer than the exterior surface.


Mass and density

The mass and density of Arrokoth is unknown. A definitive mass and density estimate cannot be given as the lobi are in contact rather than orbiting each other. Although a possible natural satellite orbiting Arrokoth could help determine its mass, no such satellites were found. Under the assumption that both lobi are bound by self-gravity, with the mutual gravity of the two overcoming centrifugal forces that would otherwise separate them, Arrokoth is estimated to have a very low density similar to that of comets, with an estimated minimum density of . In order to maintain the shape of the neck, the density of Arrokoth must be less than the maximum possible density of , otherwise the neck would be excessively compressed by the mutual gravity of the lobi such that the entire object would gravitationally collapse into a spheroid.


Formation

Arrokoth is thought to have formed from two separate progenitor objects that formed over time from a rotating cloud of small, icy bodies since the
formation of the Solar System The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a ...
4.6 billion years ago. Arrokoth had likely formed in a colder environment within a dense, opaque region of the early Kuiper belt where the Sun appeared heavily obscured by dust. Icy particles within the early Kuiper belt experienced
streaming instability In planetary science a streaming instability is a hypothetical mechanism for the formation of planetesimals in which the drag felt by solid particles orbiting in a gas disk leads to their spontaneous concentration into clumps which can gravitational ...
, in which they slowed down due to
drag Drag or The Drag may refer to: Places * Drag, Norway, a village in Tysfjord municipality, Nordland, Norway * ''Drág'', the Hungarian name for Dragu Commune in Sălaj County, Romania * Drag (Austin, Texas), the portion of Guadalupe Street adj ...
against the surrounding gas and dust, and gravitationally coalesced into clumps of larger particles. Because there have been few to no disruptive impacts on Arrokoth since it formed, the details of its formation have been preserved. From the differing present appearances of the lobi, each is thought to have accreted separately while in orbit around each other. Both progenitor objects are believed to have formed from a single source of material as they appear to be homogeneous in albedo, color, and composition. The presence of rolling topography units on the larger object indicates that it had likely formed from the coalescence of smaller planetesimal units prior to merging with the smaller object. The larger lobus Wenu appears to be an aggregate of 8 or so smaller components, each approximately across.


Flattening and merging

It is unclear how Arrokoth has attained its present flattened shape, though two leading hypotheses have been postulated to explain the mechanisms leading to its flattened shape during the formation of the Solar System. The ''New Horizons'' team hypothesizes that the two progenitor objects formed with initially rapid rotations, causing their shapes to become flattened due to centrifugal forces. Over time, the rotation rates of the progenitor objects gradually slowed down as they experienced impacts by small objects and transferred their angular momentum to other orbiting debris left over from their formation. Eventually, loss of momentum, caused by impacts and momentum shifting to other bodies in the cloud, caused the pair to slowly spiral closer until they touched—where over time the joints fused together, forming its present bilobate shape. In an alternative hypothesis formulated by researchers of the Chinese Academy of Sciences and the Max Planck Institute in 2020, the flattening of Arrokoth may have resulted from the process of sublimation-driven mass loss over a timescale of several million years after the merging of the lobi. At the time of formation, Arrokoth's composition had a higher volatile concentration from the accretion of condensed volatiles within the dense and opaque Kuiper belt. After the surrounding dust and nebula subsided, solar radiation was no longer obstructed, allowing for photon-induced sublimation to occur in the Kuiper belt. Due to Arrokoth's high rotational obliquity, one polar region faces the Sun continuously for half of its orbital period, resulting in extensive heating and consequent sublimation and loss of frozen volatiles at Arrokoth's poles. Regardless of the uncertainty surrounding the mechanisms for the flattening of Arrokoth, the subsequent merging of the bodies ancestral to the lobi appeared to be gentle. The present appearance of Arrokoth does not indicate deformation or compression fractures, suggesting that the two progenitor objects had merged very slowly at a speed of —comparable to the average walking speed of a person. The progenitor objects must have also merged obliquely at angles greater than 75 degrees in order to account for the present shape of Arrokoth's thin neck while keeping the lobi intact. By the time the two progenitor objects merged, both of them had already been tidally locked in synchronous rotation. The long-term frequency of
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
s occurring on Arrokoth was low due to the slower speeds of objects in the Kuiper belt. Over a period of 4.5 billion years, photon-induced sputtering of water ice on Arrokoth's surface would minimally reduce its size by . With the lack of frequent cratering events and perturbations of its orbit, the shape and appearance of Arrokoth would remain virtually pristine since the conjoining of two separate objects that formed its bilobate shape.


Observation


Discovery

Arrokoth was discovered on 26 June 2014 using the Hubble Space Telescope during a preliminary survey to find a suitable Kuiper belt object for the ''New Horizons'' spacecraft to fly by. Scientists of the ''New Horizons'' team were searching for an object in the Kuiper belt that the spacecraft could study after Pluto, and their next target had to be reachable on ''New Horizons'' remaining fuel. Using large ground-based telescopes on Earth, researchers began looking in 2011 for candidate objects and searched multiple times per year for several years. However, none of the objects found were reachable by the ''New Horizons'' spacecraft and most Kuiper belt objects that may be suitable were just too distant and faint to be seen through Earth's atmosphere. In order to find these fainter Kuiper belt objects, the ''New Horizons'' team initiated a search for suitable targets with the Hubble Space Telescope on 16 June 2014. Arrokoth was first imaged by Hubble on 26 June 2014, 10 days after the ''New Horizons'' team began their search for potential targets. While digitally processing images from Hubble, Arrokoth was identified by astronomer Marc Buie, member of the ''New Horizons'' team. Buie reported his finding to the search team for subsequent analysis and confirmation. Arrokoth was the second object found during the search, after . Three more candidate targets were later discovered with Hubble, though follow-up
astrometric Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. Histor ...
observations eventually ruled them out. Of the five potential targets found with Hubble, Arrokoth was deemed to be the most feasible target for the spacecraft as the flyby trajectory required the least amount of fuel compared to that for , the second most feasible target for ''New Horizons''. On 28 August 2015, Arrokoth was officially selected by NASA as a flyby target for the ''New Horizons'' spacecraft. Arrokoth is too small and distant for its shape to be observed directly from Earth, but scientists were able to take advantage of an astronomical event called a stellar occultation, in which the object passes in front of a star from the vantage point of Earth. Since the occultation event is only visible from certain parts of the Earth, the ''New Horizons'' team combined data from Hubble and the
European Space Agency , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (1205 ...
's ''
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'' space observatory to figure out exactly when and where on Earth's surface Arrokoth would cast a shadow. They determined that occultations would occur on 3 June, 10 July, and 17 July in 2017, and set off for places around the world where they could see Arrokoth cover up a different star on each of these dates. Based on this string of three occultations, scientists were able to trace out the object's shape.


2017 occultations

In June and July 2017, Arrokoth
occulted An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
three background stars. The team behind ''New Horizons'' formed a specialized "KBO Chasers" team led by Marc Buie to observe these stellar occultations from South America, Africa, and the Pacific Ocean. On 3 June 2017, two teams of NASA scientists tried to detect the shadow of Arrokoth from Argentina and South Africa. When they found that none of their telescopes had observed the object's shadow, it was initially speculated that Arrokoth might be neither as large nor as dark as previously expected, and that it might be highly reflective or even a swarm. Additional data taken with the Hubble Space Telescope in June and July 2017 revealed that the telescopes had been placed in the wrong location, and that these estimations were incorrect. On 10 July 2017, the airborne telescope SOFIA was successfully placed close to the predicted centerline for the second occultation while flying over the Pacific Ocean from Christchurch, New Zealand. The main purpose of those observations was the search for hazardous material like rings or dust near Arrokoth that could threaten the ''New Horizons'' spacecraft during its flyby in 2019. Data collection was successful. A preliminary analysis suggested that the central shadow was missed; only in January 2018 was it realized that SOFIA had indeed observed a very brief dip from the central shadow. The data collected by SOFIA will also be valuable to put constraints on dust near Arrokoth. Detailed results of the search for hazardous material were presented on the 49th Meeting of the AAS Division for Planetary Sciences, on 20 October 2017. On 17 July 2017, the Hubble Space Telescope was used to check for debris around Arrokoth, setting constraints on rings and debris within the Hill sphere of Arrokoth at distances of up to from the main body. For the third and final occultation, team members set up another ground-based "fence line" of 24 mobile telescopes along the predicted ground track of the occultation shadow in southern Argentina (
Chubut Chubut may refer to: * Chubut Province, Argentina * Chubut River in the Chubut Province * Chubut steamer duck The Chubut steamer duck or white-headed flightless steamer duck (''Tachyeres leucocephalus'') is a flightless duck endemic to Argenti ...
and Santa Cruz provinces) to better constrain the size of Arrokoth. The average spacing between these telescopes was around . Using the latest observations from Hubble, the position of Arrokoth was known with much better precision than for the 3 June occultation, and this time the shadow of Arrokoth was successfully observed by at least five of the mobile telescopes. Combined with the SOFIA observations, this put constraints on possible debris near Arrokoth. Results from the occultation on 17 July showed that Arrokoth could have had a very oblong, irregular shape or be a close or contact binary. According to the duration of the observed chords, Arrokoth was shown to have two "lobes", with diameters of approximately and , respectively. A preliminary analysis of all collected data suggested that Arrokoth was accompanied by an orbiting moonlet about away from the primary. It was later realized, however, that an error with the data processing software resulted in a shift in the apparent location of the target. After accounting for the bug, the short dip observed on 10 July was considered to be a detection of the primary body. By combining data about its light curve, spectra (e.g. color), and stellar occultation data, illustrations could rely on known data to create a concept of what it might look like prior to spacecraft flyby.


2018 occultations

There were two potentially useful Arrokoth occultations predicted for 2018: one on 16 July and one on 4 August. Neither of these were as good as the three 2017 events. No attempts were made to observe the 16 July 2018 occultation, which took place over the South Atlantic and the Indian Ocean. For the 4 August 2018 event, two teams, consisting of about 50 researchers in total, went to locations in Senegal and Colombia. The event gathered media attention in Senegal, where it was used as an opportunity for
science outreach Science outreach, also called Education and Public Outreach (EPO or E/PO) or simply public outreach, is an umbrella term for a variety of activities by research institutes, universities, and institutions such as science museums, aimed at promoting ...
. Despite some stations being affected by bad weather, the event was successfully observed, as reported by the ''New Horizons'' team. Initially, it was unclear whether a chord on the target had been recorded. On 6 September 2018, NASA confirmed that the star had indeed been seen to dip by at least one observer, providing important information about the size and shape of Arrokoth. Hubble observations were carried out on 4 August 2018, to support the occultation campaign. Hubble could not be placed in the narrow path of the occultation, but due to the favourable location of Hubble at the time of the event, the space telescope was able to probe the region down to from Arrokoth. This is much closer than the region that could be observed during the 17 July 2017 occultation. No brightness changes of the target star have been seen by Hubble, ruling out any optically thick rings or debris down to from Arrokoth. Results of the 2017 and 2018 occultation campaigns were presented at the 50th meeting of the
American Astronomical Society The American Astronomical Society (AAS, sometimes spoken as "double-A-S") is an American society of professional astronomers and other interested individuals, headquartered in Washington, DC. The primary objective of the AAS is to promote the adv ...
Division for Planetary Sciences on 26 October 2018.


Exploration

Having completed its flyby of Pluto in July 2015, the ''New Horizons'' spacecraft made four course changes in October and November 2015 to place itself on a trajectory towards Arrokoth. It is the first object to be targeted for a flyby that was discovered after the visiting spacecraft was launched, and is the farthest object in the Solar System ever to be visited by a spacecraft. Moving at a speed of ''New Horizons'' passed by Arrokoth at a distance of , equivalent to a few minutes of travel at the craft's speed, and one third of the distance of the spacecraft's closest encounter with Pluto. Closest approach occurred on 1 January 2019, at 05:33 UTC (
Spacecraft Event Time Spacecraft Event Time (SCET) is the spacecraft-local time for events that happen at the spacecraft. SCET is used for command programs that control the timing of spacecraft operations and to identify when specific events occur on the spacecraft rel ...
– SCET) at which point it was from the Sun in the direction of the constellation Sagittarius. At this distance, the one-way transit time for radio signals between Earth and ''New Horizons'' was 6 hours. The science objectives of the flyby include characterizing the geology and morphology of Arrokoth, and mapping the surface composition (searching for ammonia, carbon monoxide, methane, and water ice). Surveys of the surrounding environment to detect possible orbiting moonlets, a coma, or rings, were conducted. Images with resolutions showing details of to are expected. From Hubble observations, faint, small satellites orbiting Arrokoth at distances greater than have been excluded to a depth of >29th  magnitude. The object has no detectable atmosphere, and no large rings or satellites larger than in diameter. Nonetheless, a search for a related moon (or moons) continues, which may help better explain the formation of Arrokoth from two individual orbiting objects. ''New Horizons'' made its first detection of Arrokoth on 16 August 2018, from a distance of . At that time, Arrokoth was visible at magnitude 20, in the direction of the constellation Sagittarius. Arrokoth was expected to be magnitude 18 by mid-November, and magnitude 15 by mid-December. It reached naked eye brightness (magnitude 6) from the spacecraft's point of view just 3–4 hours before closest approach. If obstacles were detected, the spacecraft had the option of diverting to a more distant rendezvous, though no moons, rings or other hazards were seen. High-resolution images from ''New Horizons'' were taken on 1 January. The first images of mediocre resolution arrived the next day. The downlink of data collected from the flyby was expected to last 20 months, through September 2020.


Gallery


See also

*
List of minor planets and comets visited by spacecraft The following tables list all minor planets and comets that have been visited by robotic spacecraft. List of minor planets visited by spacecraft A total of 17 minor planets (asteroids, dwarf planets, and Kuiper belt objects) have been visit ...
*
132524 APL 132524 APL, provisional designation , is a small background asteroid in the intermediate asteroid belt. It was discovered by Lincoln Near-Earth Asteroid Research in May 2002, and imaged by the ''New Horizons'' space probe on its flyby in June 200 ...
– Another minor planet visited by ''New Horizons'' * List of trans-Neptunian objects


Notes


References


External links


Arrokoth (): Most Distant Object Explored Up Close
at NASA Solar System Exploration
About Arrokoth ()

LORRI Images from the Arrokoth Flyby
* * {{DEFAULTSORT:486958 2019 in spaceflight Articles containing video clips 20140626 Cold classical Kuiper belt objects Contact binary (small Solar System body)
Arrokoth Arrokoth (minor-planet designation 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 ...
Discoveries by the New Horizons KBO Search Discoveries using the Hubble Space Telescope Minor planets visited by spacecraft
Arrokoth Arrokoth (minor-planet designation 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 ...
New Horizons Objects observed by stellar occultation