Aggregation of dust through sticking collisions is the first step of planet formation .
Basic physical properties of the evolving dust aggregates strongly depend on the porosity of the aggregates , e.g .
mechanical strength , thermal conductivity , gas-grain coupling time .
Also the outcome of further collisions depends on the porosity of the colliding aggregates .
In laboratory experiments we study the growth of large aggregates of \sim 3 mm to 3 cm through continuous impacts of small dust agglomerates of 100 \mu m size , consisting of \mu m grains at different impact velocities .
The experiments show that agglomerates grow by direct sticking as well as gravitational reaccretion .
The latter can be regarded as suitable analog to reaccretion of fragments by gas drag in protoplanetary disks .
Experiments were carried out in the velocity range between 1.5 m/s and 7 m/s .
With increasing impact velocities the volume filling factor of the resulting agglomerates increases from \phi = 0.2 for 1.5 m/s to \phi = 0.32 for 7 m/s .
These values are independent of the target size .
Extrapolation of the measured velocity dependence of the volume filling factor implies that higher collision velocities will not lead to more compact aggregates .
Therefore , \phi = 0.32 marks a degree of compaction suitable to describe structures forming at v > 6 m / s .
At small collision velocities below 1 m/s highly porous structures with \phi \approx 0.10 will form .
For intermediate collision velocities porosities vary .
Depending on the disk model and resulting relative velocities , objects in protoplanetary disks up to dm-size might evolve from highly porous ( \phi \approx 0.10 ) to compact ( \phi = 0.32 ) with a more complex intermediate size range of varying porosity .