Constraining the final merger of contact binary (486958) Arrokoth with soft-sphere discrete element simulations

The New Horizons mission has returned stunning images of the bilobate Kuiper belt object (486958) Arrokoth. It is a contact binary, formed from two intact and relatively undisturbed predecessor objects joined by a narrow contact region. We use a version of pkdgrav, an N-body code that allows for soft-sphere collisions between particles, to model a variety of possible merger scenarios with the aim of constraining how Arrokoth may have evolved from two Kuiper belt objects into its current contact binary configuration. We find that the impact must have been quite slow (≲ 5 m/s) and grazing (impact angles ≳ 75^∘) in order to leave intact lobes after the merger, in the case that both progenitor objects were rubble piles. A gentle contact between two bodies in a close synchronous orbit seems most plausible.