Arrokoth is a small object, measuring 22 miles long by 12 miles wide and 6 miles thick, which orbits in the Kuiper belt beyond the orbit of Neptune. It was discovered while the New Horizons space craft was already in flight. New Horizons was redirected to pass within ~2200 miles of Arrokoth to photograph and measure it.
Arrokoth appears to be two small globular objects that collided at a slow speed and eventually fused together. It is the most distant celestial object yet visited by humans.