Hyper-Velocity Impacts on Rubble Pile Asteroids

Most asteroids in the size range of approximately 100m to 100km are rubble piles, aggregates of rocky material held together mainly by gravitational forces, and only weak cohesion. They contain high macroporosities, indicating a large amount of void space in their interiors. How these voids are distributed is not yet known, as in-situ measurements are still outstanding.

In this work, a model to create rubble pile asteroid simulants for use in SPH impact simulations is presented. Rubble pile asteroids are modelled as gravitational re-aggregating remnant fragments of a catastrophically disrupted parent body, which are represented by spherical pebbles. It is shown that this approach allows to explicitly follow the internal restructuring of rubble pile asteroids during impact events, while preserving the expected properties of the bulk asteroid as known from observations and experiments. The bulk behaviour of asteroid simulants, as characterized by the stability against disruption and fragment size distribution, follows the expected behaviour and is not sensitive to the exact distribution of voids in the interior structure, but rather to the void fraction as the amount of consolidated void space in between the constituent fragment pebbles. No exact a priory knowledge of the fragment size distribution inside the body is therefore needed to use this model in impact simulations.

Modelling the behaviour of the large-scale rubble pile constituents during impact events is used as a tool to infer the internal structure of asteroids by linking surface features like hills or pits to the creation of sub-catastrophic craters. In this work, the small rubble pile asteroid (2867) S?teins is analysed. The flyby of the Rosetta spacecraft at S?teins has revealed several interesting features: the large crater Diamond close to the southern pole, a hill like feature almost opposite to the crater, and a catena of crater pits extending radially from the rim of the crater.

A possible link between these two structures and the cratering event is investigated in a series of impact simulations varying the interior of a plausible shape of S?teins prior to the event that formed crater Diamond. A connection between the cratering event and the hill is shown to be highly unlikely. Therefore, the hill is most likely a remnant of the formation of S?teins. Its size therefore helps to infer the initial size distribution of fragments forming the asteroid.

The formation of a fracture radially from the crater can be observed for rubble pile simulants with highly collimated voids. This fracture could plausibly form the catena of pits observed on S?teins. This can therefore serve as a link between observable surface features and S?teins internal structure. The interior of S?teins is most likely an aggregate of fragments that themselves are only lightly fractured, and large void spaces might be found inside the asteroid. As S?teins seems to be a good example of a YORPoid, an asteroid that has been evolved to a top-like shape by radiative forces due to the YORP effect, this gives first insights in the distribution of voids in the interior of this class of rubble pile asteroids.