  Dust is an important constituent in cometary comae; its analysis is one of
the major objectives of ESA's Rosetta mission to comet
67P/Churyumov-Gerasimenko (C-G). Several instruments aboard Rosetta are
dedicated to studying various aspects of dust in the cometary coma, all of
which require a certain level of exposure to dust to achieve their goals. At
the same time, impacts of dust particles can constitute a hazard to the
spacecraft. To conciliate the demands of dust collection instruments and
spacecraft safety, it is desirable to assess the dust environment in the coma
even before the arrival of Rosetta. We describe the present status of modelling
the dust coma of 67P/C-G and predict the speed and flux of dust in the coma,
the dust fluence on a spacecraft along sample trajectories, and the radiation
environment in the coma. The model will need to be refined when more details of
the coma are revealed by observations. An overview of astronomical observations
of 67P/C-G is given and model parameters are derived from these data where
possible. For quantities not yet measured for 67P/C-G, we use values obtained
for other comets. One of the most important and most controversial parameters
is the dust mass distribution. We summarise the mass distribution functions
derived from the in-situ measurements at comet 1P/Halley in 1986. For 67P/C-G,
constraining the mass distribution is currently only possible by the analysis
of astronomical images. We find that the results from such analyses are at
present rather heterogeneous, and we identify a need to find a model that is
reconcilable with all available observations.
