  We present OSIRIS/NAC observations of decimetre-sized, likely ice-containing
aggregates ejected from a confined region on the surface of comet
67P/Churyumov-Gerasimenko. The images were obtained in January 2016 when the
comet was at 2 AU from the Sun out-bound from perihelion. We measure the
acceleration of individual aggregates through a two-hour image series.
Approximately 50% of the aggregates are accelerated away from the nucleus, and
50% towards it, and likewise towards either horizontal direction. The
accelerations are up to one order of magnitude stronger than local gravity, and
are most simply explained by the combined effect of gas drag accelerating all
aggregates upwards, and the recoil force from asymmetric outgassing, either
from rotating aggregates with randomly oriented spin axes and sufficient
thermal inertia to shift the temperature maximum away from an aggregate's
subsolar region, or from aggregates with variable ice content. At least 10% of
the aggregates will escape the gravity field of the nucleus and feed the
comet's debris trail, while others may fall back to the surface and contribute
to the deposits covering parts of the northern hemisphere. The rocket force
plays a crucial role in pushing these aggregates back towards the surface. Our
observations show the future back fall material in the process of ejection, and
provide the first direct measurement of the acceleration of aggregates in the
innermost coma (<2km) of a comet, where gas drag is still significant.
