2011 Interdisciplinary Summer School: Granular Flows

Laboratory Impacts

Patrick Michel, Nice Observatory in France

All solid celestial bodies’ surfaces are covered with craters, and the shapes of the smallest bodies suggest that they are the fragments of large bodies that were disrupted by a collision. Thus, the understanding of the impact process is fundamental to determine the role of collisions during the formation and evolution of our Solar System. Moreover, the understanding of the response of a surface, as a function of its mechanical properties and composition, to an impact is also crucial to determine the relationship between the size of a projectile and the size of the resulting craters for a given impact energy, the amount and fate of produced ejecta, as well as to interpret the crater morphologies and degradations observed on celestial bodies’ surfaces. This knowledge has also great implications in the design of efficient deflection techniques of a potentially threatening object: the amount of momentum transfer from the impact of a kinetic impactor highly depends on the amount and velocities of ejecta that are produced, which in turn highly depends on the response of the surface of the object to the impact.

Impact experiments in laboratory have been developed to understand the impact physics at small scales over a wide range of target properties. The impact velocities can range between a few hundreds of meters per second to a few kilometers per seconds, depending on the techniques used to perform the impact. Targets are either represented by finite solids (e.g. spherical, cubic blocks) and made of different kinds of materials (e.g. basalt, pumice) or by semi-finite surfaces made of solid or granular materials. The outcomes of the impacts are typically imaged with high-speed cameras, allowing a determination of the size and velocities of the ejecta. Both catastrophic impacts (leading to the disruption of the target) and cratering ones are investigated. These experiments not only allow a great improvement of the understanding of the impact process at small scale, but also allow the validation of the numerical models that can then be used to investigate this process at scales unreachable by experiments. This lecture will present some laboratory experiments made on different kinds of targets and their confrontation with numerical models and theoretical scaling laws. For completeness and for comparisons, impacts on both solid and granular media will be presented, showing the effect of the surface characteristics on the outcome. Confrontation with numerical models will also be presented.