Patrick Michel - UCA, OCA, CNRS - Keywords: granular material dynamics, asteroid, space mission, discrete element method, continuum approach

Patrick Michel - UCA, OCA, CNRS - Keywords: granular material dynamics, asteroid, space mission, discrete element method, continuum approach

Contribution title: Granular material dynamics and space missions to celestial bodies: a transdisciplinary approach

Collaborators at OCA are members of science teams of 3 space missions developed by 3 main space agencies (JAXA, NASA, ESA). These missions are planned rendezvous with distinct near-Earth asteroids (small rocky bodies passing close to the Earth) within the next few years. This involvement will offer great visibility to UCA given the great public interest for asteroid-related topics. The Japanese Hayabusa2 mission will visit the primitive near-Earth asteroid Ryugu of 900 meters in diameter in 2018-2019, and will return a sample to Earth in 2020. The NASA OSIRIS-REx mission will visit the primitive near-Earth asteroid Bennu of 450 meters in diameter in 2018-2020, and will return a sample to Earth in 2023. Both missions will be at their respective asteroid during the same years, which promises amazing new images and a large media coverage. They will allow us to make a big step in our understanding of the early phases of our Solar System, in particular the role of asteroids in planet formation and emergence of life on Earth. Finally, the AIDA mission in collaboration between ESA and NASA consists of two separate spacecraft that will be launched to a binary asteroid system (an Earth-Moon system at small scale), the near-Earth asteroid Didymos, to test our ability to deflect an asteroid from its original trajectory by using an artificial projectile. AIDA will be the first test ever to deflect an asteroid, which will allow us to have a secured plan to protect the Earth from asteroid impacts.

The project funded by Academy 2 and 3 will contribute to different important activities related to these space missions, in which members of OCA are co-Investigators (Hayabusa2, OSIRIS-REx) or Science lead (AIDA). In particular, the surface of small asteroids are usually covered with a layer made of granular material, called regolith. The properties of this regolith is not known by telescope observations from the ground. Moreover, its behavior in the low-gravity conditions of a small asteroid is poorly understood and a subject of intense research. Our project joins the independent expertise of a group from OCA and another group from Ecole des Mines ParisTech to study the behavior and motion of granular materials in conditions adapted to those existing at the surface of asteroids, comets, planets and their satellites. This study relies on the development of numerical simulations and their validations by comparison with experiments performed both in low-gravity using a drop tower and under Earth gravity. This allows us to address various important issues related to the motion of regolith on asteroid surfaces, to model the interaction of tools that aims to interact with an asteroid surface (to land and perform in-situ measurements or collect a sample) and to be best prepared to interpret the space mission data. We thus contribute to space mission activities (design, image interpretation) but also more generally improve our knowledge of small body’s and planetary surface evolutions and histories based on our understanding of the complex dynamics of regolith on their surface.

Our highly transdisciplinary project will allow UCA to contribute greatly to on-going space missions and future ones, adding to the visibility of our institutions. A few results achieved since the beginning of this project will be presented.