  Binary asteroid formation is a highly complex process, which has been
highlighted with recent observations of satellites with unexpected shapes, such
as the oblate Dimorphos by the NASA DART mission and the contact binary Selam
by NASA's Lucy mission. There is no clear consensus on which dynamical
mechanisms determine the final shape of these objects. In turn, we explore a
formation pathway where spin-up and rotational failure of a rubble pile
asteroid lead to mass-shedding and a wide circumasteroidal debris disk in which
the satellite forms. Using a combination of smooth-particle hydrodynamical and
N-body simulations, we study the dynamical evolution in detail. We find that a
debris disk containing matter corresponding to a few percent of the primary
asteroid mass extending beyond the fluid Roche limit can consistently form both
oblate and bilobate satellites via a series of tidal encounters with the
primary body and mergers with other gravitational aggregates. Principally,
satellites end up prolate (elongated) and on synchronous orbits, accreting
mainly in a radial direction while tides from the primary asteroid keep the
shape intact. However, close encounters and mergers can break the orbital
state, leading to orbital migration and deformation. Satellite-satellite
impacts occurring in this regime have lower impact velocities than
merger-driven moon formation in e.g. planetary rings, leading to soft impacts
between differently sized, non-spherical bodies. The resulting post-merger
shape of the satellite is highly dependent on the impact geometry. Only moons
having experienced a prior mild or catastrophic tidal disruption during a close
encounter with the primary asteroid can become oblate spheroids, which is
consistent with the predominantly prolate observed population of binary
asteroid satellites.
