  We present a model of cometary dust capable of simulating the dynamics within
the first few tens of km of the comet surface. Recent measurements by the GIADA
and COSIMA instruments on Rosetta show that the nucleus emits fluffy dust
particles with porosities above 50% and sizes up to at least mm (Schulz et al.
2015, Rotundi et al. 2015, Fulle et al. 2015). Retrieval of the physical
properties of these particles requires a model of the effective forces
governing their dynamics. Here, we present a model capable of simulating
realistic, large and porous particles using hierarchical aggregates, which
shows previous extrapolations to be inadequate. The main strengths of our
approach are that we can simulate very large (mm-scale) non-spherical
agglomerates and can accurately determine their 1) effective cross-section and
ratio of cross-section to mass, 2) gas drag coefficient, and 3) light
scattering properties. In practical terms, we find that a more detailed
treatment of the dust structure results in 3-5 times higher velocities for
large dust particles in the inner coma than previously estimated using
spherical particles of the same mass. We apply our model to the dynamics of
dust in the vicinity of the nucleus of comet 67P and successfully reproduce the
dust speeds reported early on when the comet was roughly 3.5 AU from the Sun.
At this stage, we employ a simple spherical comet nucleus, we model activity as
constant velocity gas expansion from a uniformly active surface, and use Mie
scattering. We discuss pathways to improve on these simplifications in the
future.
