Context. The COmetary Secondary Ion Mass Analyzer (COSIMA) on board
Rosetta is dedicated to the collection and compositional analysis of the dust particles in
the coma of 67P/Churyumov-Gerasimenko (67P).

Aims. Investigation of the physical properties of the dust particles
collected along the comet trajectory around the Sun starting at a heliocentric distance of
3.5 AU.

Methods. The flux, size distribution, and morphology of the dust
particles collected in the vicinity of the nucleus of comet 67P were measured with a daily
to weekly time resolution.

Results. The particles collected by COSIMA can be classified according
to their morphology into two main types: compact particles and porous aggregates. In
low-resolution images, the porous material appears similar to the chondritic-porous
interplanetary dust particles collected in Earth’s stratosphere in terms of texture. We
show that this porous material represents 75% in volume and 50% in number of the large
dust particles collected by COSIMA. Compact particles have typical sizes from a few tens
of microns to a few hundreds of microns, while porous aggregates can be as large as a
millimeter. The particles are not collected as a continuous flow but appear in bursts.
This could be due to limited time resolution and/or fragmentation either in the collection
funnel or few meters away from the spacecraft. The average collection rate of dust
particles as a function of nucleo-centric distance shows that, at high phase angle, the
dust flux follows a 1/d²_comet law, excluding fragmentation of the dust particles
along their journey to the spacecraft. At low phase angle, the dust flux is much more
dispersed compared to the 1/d²_comet law but cannot be explained by fragmentation of the
particles along their trajectory since their velocity, indirectly deduced from the COSIMA
data, does not support such a phenomenon. The cumulative size distribution of particles
larger than 150 μm follows a power law close to r^{− 0.8 ± 0.1},
confirming measurements made by another Rosetta dust instrument Grain Impact Analyser and
Dust Accumulator (GIADA). The cumulative size distribution of particles between 30
μm and 150
μm has a
power index of −1.9 ± 0.3.
The excess of dust in the 10–100 μm  range in comparison to the 100 μm–1 mm range together with
no evidence for fragmentation in the inner coma, implies that these particles could have
been released or fragmented at the nucleus right after lift-off of larger particles. Below
30 μm,
particles exhibit a flat size distribution. We interprete this knee in the size
distribution at small sizes as the consequence of strong binding forces between the
sub-constitutents. For aggregates smaller than 30 μm, forces stronger than
Van-der-Waals forces would be needed to break them apart.