  Collisional evolution is a key process in planetesimal formation and
decimetre bodies play a key role in the different models. However, the outcome
of collisions between two dusty decimetre bodies has never been studied
experimentally. Therefore, we carried out microgravity collision experiments in
the Bremen drop tower. The agglomerates consist of quartz with irregularly
shaped micrometre-sized grains and the mean volume filling factor is 0.437
$\pm$ 0.004. The aggregates are cylindrical with 12 cm in height and 12 cm in
diameter and typcial masses are 1.5 kg. These are the largest and most massive
dust aggregates studied in collisions to date. We observed rebound and
fragmentation but no sticking in the velocity range between 0.8 and 25.7 cm
s$^{-1}$. The critical fragmentation velocity for split up of an aggregate is
16.2 $\pm$ 0.4 cm s$^{-1}$. At lower velocities the aggregates bounce of each
other. In this velocity range the coefficient of restitution decreases with
increasing collision velocity from 0.8 to 0.3. While the aggregates are very
weak the critical specific kinetic energy for fragmentation $Q_{\mu =1}$ is a
factor 6 larger than expected. Collisions of large bodies in protoplanetary
discs are supposed to be much faster and the generation of smaller fragments is
likely. In planetary rings collision velocities are of the order of a few cm
s$^{-1}$ and are thereby in the same range investigated in these experiments.
The coefficient of restitution of dust agglomerates and regolith covered ice
particles, which are common in planetary rings, are similar.
