  Over the past years the processes involved in the growth of planetesimals
have extensively been studied in the laboratory. Based on these experiments, a
dust-aggregate collision model was developed upon which computer simulations
were based to evaluate how big protoplanetary dust aggregates can grow and to
analyze which kinds of collisions are relevant in the solar nebula and are
worth further studies in the laboratory. The sticking threshold velocity of
millimeter-sized dust aggregates is such a critical value that had so far only
theoretically been derived, as the relevant velocities could not be reached in
the laboratory. We developed a microgravity experiment that allows us for the
first time to study free collisions of mm-sized dust aggregates down to
velocities of ~0.1 cm/s to assess this part of the protoplanetary dust
evolution model. Here, we present the results of 125 free collisions between
dust aggregates of 0.5 to 2 mm diameter. Seven collisions with velocities
between 0.2 and 3 cm/s led to sticking, suggesting a transition from perfect
sticking to perfect bouncing with a certain sticking probability instead of a
sharp velocity threshold. We developed a model to explain the physical
processes involved in dust-aggregate sticking, derived dynamic material
properties of the dust aggregates from the results of the collisions, and
deduced the velocity below which aggregates always stick. For millimeter-sized
porous dust aggregates this velocity is 8e-5 m/s.
