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The ESA Rosetta spacecraft, currently orbiting around comet 67P, has alreadyprovided in situ measurements of the dust grain properties from severalinstruments, particularly OSIRIS and GIADA. We propose adding value to thosemeasurements by combining them with ground-based observations of the dust tailto monitor the overall, time-dependent dust-production rate and sizedistribution. To constrain the dust grain properties, we take Rosetta OSIRISand GIADA results into account, and combine OSIRIS data during the approachphase (from late April to early June 2014) with a large data set ofground-based images that were acquired with the ESO Very Large Telescope (VLT)from February to November 2014. A Monte Carlo dust tail code has been appliedto retrieve the dust parameters. Key properties of the grains (density,velocity, and size distribution) were obtained from Rosetta observations: theseparameters were used as input of the code to considerably reduce the number offree parameters. In this way, the overall dust mass-loss rate and itsdependence on the heliocentric distance could be obtained accurately. The dustparameters derived from the inner coma measurements by OSIRIS and GIADA andfrom distant imaging using VLT data are consistent, except for the power indexof the size-distribution function, which is $\alpha$=--3, instead of$\alpha$=--2, for grains smaller than 1 mm. This is possibly linked to thepresence of fluffy aggregates in the coma. The onset of cometary activityoccurs at approximately 4.3 au, with a dust production rate of 0.5 kg/s,increasing up to 15 kg/s at 2.9 au. This implies a dust-to-gas mass ratiovarying between 3.8 and 6.5 for the best-fit model when combined withwater-production rates from the MIRO experiment.

astronomy and astrophysics

The dust environment of comet 67P/Churyumov-Gerasimenko from Rosetta  OSIRIS and VLT observations in the 4.5 to 2.9 au heliocentric distance range  inbound