Characterization of the Agilkia region through discrete-element simulation of Philae's rebound

Context. The cold gas system and anchoring harpoons that were designed to hold Philae down after it landed on the Agilkia region of comet 67P/Churyumov-Gerasimenko (67P) failed. This caused the lander to move on a complex trajectory that comprised multiple impacts and rebounds. The motion of Philae was mainly dominated by the gravitational environment and the physical properties of the surface on Agilkia. This allows us to determine the physical properties of the surface layer by high-fidelity discrete-element simulations of Philae's rebounds. Aims. We explore the surface physical properties of the Agilkia region on comet 67P by minimizing the difference in Philae's rebound status between observational data and simulations based on the assumption of a granular-boulder hybrid surface material. Methods. We first developed an efficient gravity model to accurately approximate the high-resolution polyhedral shape of the comet. This allowed us to run many simulations for the landing trajectory. We developed a complete dynamical model of the motion of Philae, including a mechanical model of the lander and the hybrid surface model. This focused in particular on the interaction of discrete elements in Philae and the granular regolith layer with a boulder added on top of it. We used mixed discrete optimization to determine the input physical variables on Agilkia to fit the rebound observational data (Philae's rebound velocity). Results. The discrete-element simulation constrained by Philae's rebound velocity implies that Philae first impacted on a boulder and scratched it with the landing gear. After this, its three soles interacted with the granular regolith, which consists of particles with a mean diameter of 0.014 +/- 0.004 cm. The thickness of the region interaction is estimated to be 0.272 +/- 0.062 m with a corresponding density of 1443 +/- 231 kg m(-3). The Young modulus for each particle is estimated to be 10(8) Pa. Based on a porosity of 0.75, the friction of the surface of particles is derived to be moderate, with a friction coefficient of about 0.6.