The TAGSAM Experiment simulated with a Contact Dynamics Code

The NASA OSIRIS-REx mission had as one of its main objectives whilst visiting asteroid Bennu to capture and bring back to Earth a sample of the regolith that covers said asteroid. In order to carry this out, Lockheed-Martin Aerospace developed what was called the Touch-and-Go Sample Acquisition Mechanism, TAGSAM. In addition to the real-life development, two scientific teams used state-of-the-art Discrete Element Method (DEM) codes to simulate the event. These codes used Soft-sphere DEM implementations, which means that their particles are allowed to overlap as the contact forces are calculated as a function of it. Additionally, all particles were simulated as perfect spheres and rolling friction was implemented in order to mimic the behaviour of non-spherical particles.

Images of the surface of asteroid Bennu revealed a very rough, porous surface which gave in upon contact with the TAGSAM head. In order to improve our understanding of the event, we recreate the sampling experiment with a Non-Smooth Contact Dynamics (CD) code that allows us to simulate non-spherical particles. The Contact Dynamics method is based on two main ingredients: non-smoothed contact and non-smooth dynamics. This means: 1. an implicit time integration scheme TO describe the dynamic evolution of the particles and TO manage contacts and shocks at the same time;and, 2. a contact resolution algorithm ensuring the respect of constraints related to the choice of non-smooth interaction laws. To use the CD method within a DEM philosophy, both previous ingredients are combined in a contact detection algorithm to deal with potential interactions between particles.

As in this method forces are calculated implicitly through momentum transfer in collisions, different contact types can be easily implemented. This makes the simulation of non-spherical particles much simpler than in other DEM methods.

Details about the method, the simulation of the TAGSAM experiment and how it relates to the TAGSAM measurements will be discussed during the conference.