Satellite formation around the largest asteroids

A subset of large asteroids (Diameter larger than 100km) are found to be in multi-asteroid systems: asteroids with one or more satellites. The formation of these large multi-asteroid systems is generally thought to occur through impacts into the parent body [e.g., Durda et al. 2004]. The specifics of the impact process that could achieve this has been confounded by a lack of satellites found amongst the largest remnants of very large asteroid families. These asteroids are known to have suffered catastrophic impacts, yet rarely have satellites. Meanwhile, the asteroids that do have satellites are almost always rotating rapidly and are elongated.

Here, through combined modeling of impacts with smoothed particle hydrodynamics, reaccumulation with discrete element methods and long term satellite stability with N-body modeling, we find a direct pathway to satellite formation in sub-catastrophic impacts. Targets with rapid pre-impact rotation, or that suffer highly oblique impacts, can be distorted into an elongated shape immediately after the impact event. The distorted shape of the rotating body provides a means to launch some debris onto initial trajectories distinct from simple ballistic trajecotires that would otherwise be doomed to re-impact. The debris most commonly launched into stable orbits around the parent body is sourced from its longest-axis and relatively shallow depths (10-20km below the surface of a 50-km-radius object). Thus, the specific energy of a collision does not control satellite formation, as sub-catastrophic impacts are sufficiently energetic to generate ejecta that dynamically evolve to stable orbits in relatively short timescales. Catastrophic or super-catastrophic collisions are not guaranteed to result in the formation of satellites around the parent body. These characteristics of disruption-level asteroid collisions explains the observed lack of a correlation between the presence of satellites around large main belt asteroids and the presence of a family.

The figure shows a modeled impact and reaccumulation after the handoff to pkdgrav at times. The particles are colored by their instantaneous orbit relative to the largest remnant in the system, where the shades of red show particles with an orbit pericenter smaller than the primary radius and blue have pericenter larger than the largest remnant radius. A cluster of blue particles is found at later times amidst the tail of debris and are the source of debris that reach temporary orbits that are not immediately re-impacting the primary. This case was for a 13km impactor hitting a 100km target that was initially rotating with a 10hr at 5km/s at and angle 60 degrees. The largest remnant has a period of 6.75hrs and a short over long axis ratio of 0.69.