Arrokoth

On 1 January 2019, at 05:33 UTC, the New Horizons spacecraft flew past a small object 43.4 times as far from the Sun as Earth. Three and a half years after its famous flyby of Pluto, it was now passing within 3 538 kilometres of something nobody had seen as more than a dot. The pictures it sent back showed a shape like a flattened snowman.

Two worlds that touched

Arrokoth is about 35 kilometres long and made of two lobes: a larger one about 20 kilometres across and a smaller one about 15. They are joined at a narrow neck with almost no sign of damage. The two must have formed close together in the early Solar System, circled each other, and gradually drawn together until they touched at walking pace.

That gentle meeting is the important part. It shows that, at least in this part of the Solar System, small worlds formed by soft collapse of clouds of pebbles rather than by violent collisions. Arrokoth has spent the last 4.5 billion years in the cold, quiet Kuiper belt, so it is one of the best-preserved relics of how planets began.

Red and organic

Arrokoth is very red, redder even than Pluto. The colour comes from tholins, organic compounds produced when radiation acts on simple ices. New Horizons found methanol and complex organics on its surface, but no sign of water ice.

A name, and a nickname

The New Horizons team found the object with the Hubble Space Telescope in 2014. Before the flyby it was given a nickname, Ultima Thule, a medieval term for a land beyond the known world. The choice was criticised because “Thule” had also been used by 19th-century racists and by the Nazi movement. In November 2019 it received its official name, Arrokoth, a word from the Powhatan language for “sky”, chosen with the agreement of Powhatan elders. New Horizons left it behind and continues outward through the Kuiper belt.