The soft-sphere discrete element method implemented by N-body code pkdgrav is used to simulate the merging process of two planetesimals that formed the precursor body of Arrokoth3,37. The primary and secondary bodies in the upper panels (oblate and spherical shape, respectively) are of the same scale as the lobes of SHAPE-SYN. The two lobes are constructed with 40,000 and 25,775 spherical particles with a power-law distribution of radii ranging from 309 m to 927 m using a -3 index, and an inter-particle cohesion of 27500 Pa is assumed1. A set of material parameters are selected to obtain a friction angle of 40° and the bulk density3 is set to be 0.47 g/cm−3. The resulting contact binary shown in the lower panels is obtained with impact speed v = 5 m/s and impact angle α = 45° (measured with respect to the vertical at the impact point), which is close to the SHAPE-SYN. The upper panels show the two original objects from -y axis and +z axis viewpoints, in which blue and red particles belong to the spherical and oblate body, respectively. The bottom panel shows the resulting contact binary for the same viewing geometry, and the wireframe in yellow outlines the SHAPE-SYN we build from the inverse evolution result. This simulation result shows a good agreement and indicates that our idealized bilobed body is consistent with physics of slow collisions, and, as already noted, it is taken as our starting shape in our evolution model.