  We carry out three-dimensional Smoothed Particle Hydrodynamics simulations of
spherical homogeneous SiO2 dust aggregates to investigate how the mass and the
porosity of the aggregates affects their ability to survive an impact at
various different collision velocities (between 1 - 27.5m/s). We explore how
the threshold velocities for fragmentation vary with these parameters.
Crucially, we find that the porosity plays a part of utmost importance in
determining the outcome of collisions. In particular, we find that aggregates
with filling factors >37% are significantly weakened and that the velocity
regime in which the aggregates grow is reduced or even non-existent (instead,
the aggregates either rebound off each other or break apart). At filling
factors less than ~37% we find that more porous objects are weaker but not as
weak as highly compact objects with filling factors >37%. In addition we find
that (for a given aggregate density) collisions between very different mass
objects have higher threshold velocities than those between very similar mass
objects. We find that fragmentation velocities are higher than the typical
values of 1m/s and that growth can even occur for velocities as high as
27.5m/s. Therefore, while the growth of aggregates is more likely if collisions
between different sized objects occurs or if the aggregates are porous with
filling factor <37%, it may also be hindered if the aggregates become too
compact.
