486958 Arrokoth

486958 Arrokoth is the most distant object a spacecraft has ever visited. When NASA’s New Horizons probe swept past on 1 January 2019, more than a billion kilometres beyond Pluto, its cameras returned this enhanced-color view of a small, reddish world frozen since the dawn of the Solar System.

Arrokoth is a contact binary: two separate lobes, the larger Wenu and the smaller Weeyo, that drifted together so gently they merged without shattering. The whole body spans about 34.5 kilometres along its longest axis, spins once every 16 hours, and is extremely dark and red — its surface stained by tholins, organic compounds built up over billions of years of irradiation.

As a cold classical Kuiper belt object, Arrokoth has orbited far from the Sun on a nearly circular path since it formed, never warmed or disturbed. That makes it a pristine sample of planetary building blocks, and its softly joined lobes are direct evidence that the earliest planetesimals grew by slow, gentle accumulation rather than violent collisions.