The most distant world ever visited
Arrokoth
Arrokoth is the most distant and most primitive body ever visited by a spacecraft: a Kuiper Belt object 36 km long, made of two lobes resting against each other like a snowman. New Horizons flew past it on 1 January 2019, 6.6 billion km from Earth, and found an intact fossil of the birth of the Solar System.
- Size
- 36 × 20 × 10 km
- Distance from Sun
- 6,617 M km
- Orbital period
- 294 years
- Rotation
- 15.9 hours
- Discovered
- 2014
- Flyby
- 1 January 2019
Physical characteristics
Arrokoth measures 36 × 20 × 10 km and is a contact binary: two separate bodies that touched and stayed joined. The larger lobe is flattened like a lentil, the smaller one is rounder; the "neck" joining them is the brightest area. The surface is smooth, with very few craters, and deep red in colour, due to organic compounds formed by radiation acting on methanol ice.
The shape itself is the most important discovery. The two lobes show no fractures or deformation where they meet: they came together at a few metres per second, walking pace. It is evidence that planetesimals, the building blocks of planets, did not form through violent collisions but from the gentle collapse of clouds of dust and pebbles.
Orbit and rotation
Arrokoth orbits 6,617 million km from the Sun on average, or 44.2 astronomical units: about 700 million km beyond Pluto's mean distance. It takes 294 years to complete one lap, on an almost circular orbit tilted just 2.5° to the ecliptic.
It belongs to the "cold classicals" of the Kuiper Belt: bodies on quiet orbits that were never moved by the migration of the giant planets and still lie where they formed, 4.5 billion years ago. Out there the Sun is merely the brightest star in the sky and the temperature stays around 230 degrees below zero Celsius. It spins once every 15.9 hours.
Discovery and exploration
It was discovered on 26 June 2014 with the Hubble Space Telescope, during a targeted search: New Horizons needed a second target after Pluto. For years it was known by the designation 2014 MU69 and the provisional nickname Ultima Thule. Its permanent name, chosen in 2019 with the consent of representatives of the Powhatan people, means "sky" in their language.
New Horizons flew past it on 1 January 2019 at a distance of about 3,500 km and a speed of more than 50,000 km/h. The data took nearly two years to reach Earth in full, because from that distance the spacecraft transmits at about a thousand bits per second. It remains the most distant encounter ever made by a spacecraft.
Frequently asked questions
What does Arrokoth mean?
It means "sky" in the language of the Powhatan people, native to the region of Maryland that is home to the centre controlling New Horizons and the institute operating Hubble. The name was proposed by the mission team and approved by the IAU in November 2019; before that the body was informally called Ultima Thule.
Why is Arrokoth shaped like a snowman?
Because it is made of two bodies that originally orbited each other and slowly drew together until they touched, at a few metres per second, without breaking. Similar shapes are found in many comets, such as 67P: a clue that small icy bodies are often born in pairs.
Is Arrokoth an asteroid?
Not strictly speaking. It is a trans-Neptunian object of the Kuiper Belt, made largely of ices, and it orbits well beyond Neptune; asteroids proper are rocky or metallic and mostly lie between Mars and Jupiter. For the IAU it nonetheless belongs, like them, to the category of small Solar System bodies.
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