Arrokoth

Arrokoth
source
Name	Arrokoth
Caption	Color image from the New Horizons spacecraft
Discoverer	Marc W. Buie using the Hubble Space Telescope
Discovery date	26 June 2014
Designations	(486958) Arrokoth, nicknamed Ultima Thule
Mp category	Trans-Neptunian object, Cold classical Kuiper belt object
Epoch	31 May 2020 (JD 2459000.5)
Semimajor	44.581 AU
Eccentricity	0.044
Inclination	2.451°
Mean anomaly	304.7°
Arg peri	177.0°
Asc node	159.0°
Dimensions	36 × 20 × 10 km
Rotation period	15.92 h
Spectral type	Ultra-red (very red)
Albedo	0.21 (geometric), 0.062 (Bond)
Abs magnitude	10.9

Arrokoth. It is a contact binary Kuiper belt object located in the outer Solar System and is the most distant and most primitive object ever visited by a spacecraft. Composed of two distinct lobes that gently merged in a slow-speed collision, it provides a pristine, frozen record of the early stages of planetary formation. The data returned by the New Horizons mission has revolutionized our understanding of how planetesimals, the building blocks of planets, assembled over four billion years ago.

Discovery and naming

The object was discovered on 26 June 2014 by astronomer Marc W. Buie using the Hubble Space Telescope as part of a dedicated search for potential post-Pluto flyby targets for the New Horizons spacecraft. Initially designated (486958) 2014 MU₆₉, it was later given the provisional nickname "Ultima Thule" by the mission team, a term from medieval cartography for a distant place beyond the known world. In November 2019, the International Astronomical Union approved the official name Arrokoth, a word meaning "sky" in the language of the Powhatan people, chosen to honor the indigenous cultures of the Chesapeake Bay region where the discovery observations were conducted at the Space Telescope Science Institute.

Physical characteristics

Arrokoth presents a distinctive "snowman" shape, consisting of two flattened, pancake-like lobes in contact. The larger lobe, nicknamed "Ultima," measures approximately 22 by 20 by 7 kilometers, while the smaller "Thule" lobe is about 14 by 14 by 10 kilometers. Its surface is uniformly ultra-red, one of the reddest objects in the Kuiper belt, indicating a surface rich in complex organic molecules known as tholins. The object has a slow rotation period of just under 16 hours and shows a remarkably smooth surface with few identifiable impact craters, suggesting it has remained largely unaltered since its formation.

Formation and composition

Scientific analysis of data from New Horizons strongly supports a model of gentle, hierarchical accretion for its formation. The two lobes likely formed separately as independent bodies in the same region of the primordial Solar System before merging at a speed of just a few meters per second. Spectroscopic measurements from the spacecraft's Ralph instrument confirmed the presence of methanol, water ice, and complex organic molecules on its surface. The lack of significant heating or geological activity has preserved its primordial composition, making it a direct relic from the era of protoplanetary disk evolution.

Exploration by New Horizons

The New Horizons spacecraft conducted its historic flyby on 1 January 2019, passing within 3,500 kilometers at a relative speed of over 14 kilometers per second. The encounter was managed by the Johns Hopkins Applied Physics Laboratory and the principal investigator, Alan Stern. Key instruments like the Long Range Reconnaissance Imager (LORRI) and the Alice ultraviolet spectrometer collected gigabytes of data, which were transmitted back to Earth over a period of many months due to the immense distance and low data rates from the Deep Space Network.

Significance and implications

The study of this object has had profound implications for planetary science, providing the first clear look at an undisturbed planetesimal. Its bi-lobed structure and smooth surface challenge older models of violent, hierarchical accretion in the Kuiper belt, instead supporting a local, gentle collapse of a cloud of solid particles. Findings from the mission have been published in high-impact journals like *Science*, significantly informing theories about the formation of other binary asteroid systems and the early architectural processes that built Pluto, Charon, and other dwarf planets.