Evolution of the dust coma in comet 67P/Churyumov-Gerasimenko before the 2009 perihelion

Context. Comet 67P/Churyumov-Gerasimenko is the main target of ESA's Rosetta mission and will be encountered in May 2014. Because the spacecraft will orbit the comet nucleus before and after release of the lander Philae, it is necessary to know the conditions in the coma. Aims. We study the dust environment, including the dust production rate and its variations along the comet's pre-perihelion orbit. Methods. The comet was observed during its approach to the Sun on four epochs between early-June 2008 and mid-January 2009 over a large range of heliocentric distances that will be covered by the mission in 2014. Results. An anomalous enhancement of the coma dust density was measured toward the comet nucleus. The scale length of this enhancement increased with decreasing heliocentric distance of the comet. We interpret this as a result of an unusually slow expansion of the dust coma. Assuming a spherically symmetric coma, we derive the average amount of dust as well as its ejection velocity. The latter increases exponentially with decreasing heliocentric distance (r(h)), ranging from about 1 m/s at 3 AU to about 25-35 m/s at 1.4 AU. Based on these results we describe the dust environment at those nucleocentric distances at which the spacecraft will presumably be in orbit.