Properties of dust in the coma of Churyumov-Gerasimenko
Résumé
The last apparition of comet 67P/Churyumov-Gerasimenko before the Rosetta spacecraft ren-dezvous in 2014 has taken place in 2008-2009. Observations have therefore been carried out to optimize the mission. In this context, observations of solar light scattered by dust, with em-phasis on polarimetric data, provide comparisons between the dust properties within different regions of the coma and with other comets. Efforts carried out both in India (December 2008 and April-May 2009) and in France (March 2009) have allowed us, with polarimetric imaging techniques, to follow the variations of the intensity and polarization of the scattered light from about 2 months before perihelion to 2 months after perihelion [1,2]. The interpretation of the observational results i) favors the presence of relatively large absorbing particles in the coma before perihelion, ii) reveals an increase in intensity and polarization a couple of weeks after per-ihelion, tentatively related to a significant ejection of small micron-or submicron-sized grains (possibly in the form of fluffy aggregates), and iii) suggests that these different types of particles originate from different parts of the nucleus that suffers noticeable seasonal effects. Although it needs to be kept in mind that remote observations do not allow the detection of local changes in the innermost coma, the implication of these results will be discussed in the context of the Rosetta mission strategy. E.g. during close-up approach, large absorbing particles could be harmful to the spacecraft, especially the optics; also, it would be of major interest to study the increase in activity after perihelion. [1] Levasseur-Regourd A.C., Hadamcik E., Sen A.K., Gupta R., Lasue J. In: Icy bodies in the solar system, IAU 263, Cambridge University Press, in press 2010. [2] Hadamcik E., Sen A.K., Levasseur-Regourd, A.C., Gupta R., Lasue J., AA, submitted, 2010