Granular materials, ensembles of huge quantities of macroscopic particles not subject to thermal motion fluctuations, surround us every day. They are present for example in the form of sugar, salt, cereals and other food, sand and construction materials, even in the form of pedestrian or car traffic.

Granular gases are ensembles of low particle number density that interact only by occasional collisions. They are found in sand storms or snow avalanches, but also in cosmic dust, nebulae and planetesimals as the precursors of larger, solid celestial bodies. Because of the dissipative nature of the particle interactions, their behaviour is fundamentally different from that of molecular gases. A stationary state requires permanent energy supply, otherwise the granular gas ‘cools’ to a state where kinetic energy continuously decays. Interesting scientific questions concern the distribution of kinetic energies on the individual degrees of freedom, the cooling rates, i. e. the decay rate of the kinetic energy in the translational and rotational degrees of freedom, and the homogeneity of the system during cooling (clustering).

Numerical and analytical studies of such ensembles have been reported for decades, while experiments were largely limited to simplified 2D ‘toy’ systems. Real 3D experiments require microgravity. We report the results of investigations performed on suborbital rocket flights, on parabolic airplane flights and in the ZARM Drop Tower, Bremen. The equipartition theorem as well as the granular cooling law was tested and results are compared to theoretical models from literature.