  For the investigation of collisions among protoplanetesimal dust aggregates,
we performed microgravity experiments in which the impacts of high-porosity
mm-sized dust aggregates into 2.5 cm-sized high-porosity dust aggregates can be
studied. The dust aggregates consisted of micrometer-sized dust grains and were
produced by random ballistic deposition with porosities between 85% and 93%.
Impact velocities ranged from ~0.1 m/s to ~3 m/s and impact angles were almost
randomly distributed. We also used "molded" target aggregates such that the
radii of the local surface curvatures corresponded to the projectile radii. The
experiments showed that impacts into the highest-porosity targets almost always
led to sticking, whereas for the less porous dust aggregates, the collisions
with intermediate velocities and high impact angles resulted in the bouncing of
the projectile with a mass transfer from the target to the projectile
aggregate. Sticking probabilities for the impacts into the "molded" target
aggregates were considerably decreased. For the impacts into smooth targets, we
measured the depth of intrusion and the crater volume and could derive some
interesting dynamical properties which can help to derive a collision model for
protoplanetesimal dust aggregates. Future models of the aggregate growth in
protoplanetary disks should take into account non-central impacts, impact
compression, the influence of the local radius of curvature on the collisional
outcome and the possible mass transfer between target and projectile
agglomerates in non-sticking collisions.
