  The first macroscopic bodies in protoplanetary disks are dust aggregates. We
report on a number of experimental studies with dust aggregates formed from
micron-size quartz grains. We confirm in laboratory collision experiments an
earlier finding that producing macroscopic bodies by the random impact of
sub-mm aggregates results in a well-defined upper-filling factor of 0.31 \pm
0.01. Compared to earlier experiments, we increase the projectile mass by about
a factor of 100. The collision experiments also show that a highly porous
dust-aggregate can retain its highly porous core if collisions get more
energetic and a denser shell forms on top of the porous core. We measure the
mechanical properties of cm-sized dust samples of different filling factors
between 0.34 and 0.50. The tensile strength measured by a Brazilian test,
varies between 1 kPa and 6 kPa. The sound speed is determined by a runtime
measurement to range between 80 m/s and 140 m/s while Young's modulus is
derived from the sound speed and varies between 7MPa and 25MPa. The samples
were also subjected to quasi-static omni- and uni-directional compression
todetermine their compression strengths and flow functions. Applied to planet
formation, our experiments provide basic data for future simulations, explain
the specific collisional outcomes observed in earlier experiments, and in
general support a scenario where collisional growth of planetesimals is
possible.
