Asteroids and other small bodies of the Solar System are of high general and scientific interest in many aspects. Space missions for sample return are under development or have been proposed recently to space agencies (Hayabusa II, Osiris-REX, MarcoPolo-R). Threat to our planet by Near-Earth Objects (NEO) is also considered at the international level with dedicated research and possible space mission for mitigation. Even ideas of industrial exploitation have merged during the last years. Future space mission projects will undoubtedly target some asteroids or other SBSS. Last, the origin, formation, and evolution of our Solar System (and other planetary system) can be better understood when analysing the constitution and physical properties of small bodies through the Solar System. In all theses cases knowledge of the structure and behaviour of the celestial body is crucial; in this respect binary asteroids as those targets of space missions are of particular interest, since one can derive the mass and other parameters of their internal structure. A large proportion of such bodies is moreover believed to consist of gravitational aggregates (rubble piles) with no, or low, internal cohesion, with varying macro-porosity, with varying surface properties (from smooth regolith covered terrain, to very rough collection of boulders), and with varying topography (cratering, depressions, ridges). Such bodies can sustain some plastic deformation in contrast to the classical visco-elastic models, and are better approached by granular systems theories, which have been a subject of intense research during the last 25 years.

Our aim is to get an enhanced understanding of the physics that governs gravitational aggregates (which can also be extended to planetary rings). We believe that the key problems of mechanics and nonlinear dynamics of self-gravitating celestial bodies can best be approached by granular media modelling. Granular systems are complex as a result of non-linear and dissipative interactions among a large number of solid particles. Their discrete nature leads to gas-like, liquid-like and solid-like behaviours that may occur simultaneously in different parts of a granular system. Moreover, as a result of dissipative contact interactions and steric exclusions, the rheological behaviour of a granular material is both pressure-dependent and density-dependent and involves complex phenomena such as jamming and size segregation. Most such phenomena have not yet been investigated in the context of self-gravitating granular systems. But using concepts and investigation tools developed for granular materials is obviously an enormous added value by providing new insights about the shapes of the asteroids, the appearance of their surface, their internal structure, their formation scenarios, their post-collisional evolution and their relaxation over long time scales and external perturbations. In this context, numerical methods (Contact Dynamics, Soft and Hard-Sphere Discrete Element Methods, Finite Element Methods, …) may be applied as a powerful tool to simulate asteroids for a broad range of parameters.

Scientific rationale

Planetary science and space exploration of Solar System Bodies are constantly progressing with outbreaking results on many research fronts. The recent discovery of water vapour outgassing from the dwarf planet Ceres, as observed by HERSCHEL, is one of such impressive results. On the other hand, the Rosetta rendez-vous with comet 67P will be one of the greatest events in planetology in 2014. In this respect, the next step is to perform sample return mission from an asteroid (Hayabusa II, Osiris-Rex) as well as from Mars, or to visit binary and multiple asteroids.

In many of these studies the physical characterisation of the surfaces and interiors of the planetary bodies to be visited requires insight into their formation and evolution. In spite of this need, the interior of asteroids, comets and small satellites remains largely unknown. Fundamental parameters such as density, mass, macro-porosity, ice or volatiles content to mention some, are calculated making educated, but strong assumptions. This leads to a great uncertainty about the evolution of these bodies, uncertainty that is of paramount importance when it comes to hazardous NEOs as this could lead to erroneous mitigation strategies.

An additional complication is that a large fraction of small bodies is supposed to have a “rubble-pile” structure, i.e. they are gravitational aggregates with no, or low internal cohesion which governs their maximum spin rate, deformation, and disruption mode among others. The surfaces of these small bodies also show a large variety of ‘geological’ terrain with different surface roughness/smoothness and thermal inertia.

In general, the complexity of granular media appears as these systems can behave in different ways depending on the nature of the contacts between their solid components (“grains”), whose most important property is that they are highly dissipative. The study at hand then refers to the mechanics and dynamics of self-gravitating and nonlinear systems, and states of equilibrium.

In granular systems, segregation, pattern formation, flow and percolation among many other phenomena, are directly related to the microscopic properties of the grains. Their shape, density, mass, surface friction, strength, grain-grain cohesive forces dictate the behaviour of the aggregate. This being the case, researchers in the field of granular physics have developed numerical methods and theoretical frameworks that have motivated new simulation codes and laboratory experiments. The resulting scaling laws have further been compared with theory and simulation.

Keeping all this in mind, it seems fitting, and timely, to use the methods and techniques developed for granular systems on Earth and apply them to the study of small NEOs as well as larger main-belt asteroids. With one extra complexity, these are not only granular systems, but self-gravitating aggregates. This interdisciplinary research effort should provide the, now indispensable clear understanding of gravitational aggregates that will help us understand small planetary bodies formation and evolution. Various approaches exist and different modeling approaches have already been implemented; we now need to validate the models for typical asteroids, and benchmark the various approaches to identify their advantages and/or limitations. At the same time, this will better enable the planned sample return missions and will help the scientific community devise others.

This reasoning led us to organize in 2011 and 2013, within the European Planetary Science Congress (and the one joint with the AAS Division of Planetary Science) dedicated sessi
ons on binary asteroids and small bodies as granular systems. Additionally, two schools were sponsored in France and in the USA to provide the much needed cross-pollination between Planetary and Granular Matter scientist.
The work that is proposed here is the continuation of our efforts to consolidate strong interdisciplinary research among leading researchers in both fields.