Fundamental Physics: Physics of plasmas and solid/liquid dust particles

Astrophysics, Planetology
Granular systems

51st ESA Parabolic Flight Campaign

3 November 2009

B. Rozitis ⁽¹⁾, S. Green ⁽¹⁾, T. Lophem de ⁽¹⁾, P. Michel ⁽²⁾, N. Murdoch ⁽³⁾

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The experiment will investigate how a steady state (constant) flow is achieved in a granular material in microgravity conditions. A granular flow will be started by applying rotational shear forces to a granular material consisting of glass beads. Individual glass beads will be tracked by high speed cameras so that their positions and velocities can be determined. By monitoring the glass bead velocities, the time required to initiate a steady state flow can be
determined. Once a steady state flow is achieved the glass beads can be tracked for the length of microgravity time available during a single parabola. This will tell us how a steady state flow in microgravity differs to a steady state flow on Earth. Another investigation will determine what effect reversing the direction of shear has on the steady state flow already initiated in microgravity. Again this will be compared with ground based results. These three investigations (time to start a steady state flow, monitoring the flow, and effect of shear reversal on the flow) will be repeated with glass beads of different sizes, and with different shear rates to determine what effects these properties have. The aim of these experimental results is to help understand the nature of asteroid surfaces, and to design a suitable asteroid sampling mechanism. Due to their small size and very low mass, asteroids have
microgravity conditions at their surface. Despite having a microgravity surface, asteroids can still have granular material residing on their surface.

The experiment will be conducted using a Taylor-Couette shear cell modified for microgravity conditions. In a Taylor-Couette shear cell there are two concentric cylinders; a larger outer cylinder, and a smaller inner cylinder that fits inside the larger one. The outer cylinder is fixed in place (unable to move or rotate) and its inside surface is rough with a layer of glued on glass beads. The inner cylinder’s outer surface is also rough with a layer of glued on beads but it is free to rotate. The bottom floor between the two cylinders is smooth and fixed in place. The gap between the two cylinders is filled with granular material consisting of glass beads to which the rotating inner cylinder applies shear stresses to. This gap between the cylinders is filled with spherical glass beads of diameter d, such that the gap has a width of ~50d. Glass beads of the same diameter are glued to both cylinder surfaces as described previously. The inner cylinder is driven by a variable speed motor, and in a typical experiment the inner cylinder is rotated at a speed of 4 to 16 mHz. The motions of the glass beads are then tracked with high speed cameras.

A better understanding of granular material dynamics under different conditions would lead to a wide range of applications.
The results will have implications for surface geology on small bodies in the solar system. Applying the experimental results to asteroids indicates that the dynamics of granular materials on their surfaces could also depend on the direction of shear that they have undergone. For instance impact phenomena, tidal forces from planetary encounters, and spin up (due to YORP) could apply shear forces to the surface. Due to possible previous shear stresses their surface materials may not necessarily behave as one might expect. The results will also have implications for interpreting the angle of repose of observed surface slopes on an asteroid.
We can apply the results to space missions to help with efficiently designing an anchoring mechanism, a sub-surface mechanism or any device aimed at interacting with the surface of regolith covered bodies. In addition to the obvious benefits this understanding will bring to asteroid space mission design, there are also important implications for space missions designed to interact with the surface of other solar system bodies such as the Moon or Mars.
With the Moon and Mars having larger gravity and for the case of Mars an atmosphere, means that the conditions found at the surface of both these bodies differ from those on the surface of an asteroid. However, they also differ greatly from the conditions found on Earth.
If we can develop our understanding of granular materials in such a way as to eventually describe a multitude of gravity environments our results could be invaluable in numerous space missions.

Please find the final report as attachment to this experiment record.

Natacha Callens (e-mail: natacha.callens@esa.int)