  The New Horizons spacecraft's encounter with the cold classical Kuiper belt
object (486958) Arrokoth (formerly 2014 MU69) revealed a contact-binary
planetesimal. We investigate how it formed, finding it is the product of a
gentle, low-speed merger in the early Solar System. Its two lenticular lobes
suggest low-velocity accumulation of numerous smaller planetesimals within a
gravitationally collapsing, solid particle cloud. The geometric alignment of
the lobes indicates the lobes were a co-orbiting binary that experienced
angular momentum loss and subsequent merger, possibly due to dynamical friction
and collisions within the cloud or later gas drag. Arrokoth's contact-binary
shape was preserved by the benign dynamical and collisional environment of the
cold classical Kuiper belt, and so informs the accretion processes that
operated in the early Solar System.
