The dust environment of comet P/Grigg-Skjellerup as evidenced from ground-based observations

Comet P/Grigg-Skjellerup was observed with the 2.2 m ESO telescope after the encounter with GIOTTO spacecraft. We have analyzed two images obtained in the V and R bands. Radial enhancement of the images allows to conclude that the R image mainly concerns the dust tail of the comet. We have used the inverse numerical method of dust tail analysis (Fülle 1989) to estimate the following cometary grain parameters: ejection velocity, anisotropy of the emission, size distribution and loss rate. This from the observations back in time to the GIOTTO encounter. The model has been applied both to our images and to that taken by Jockers et al. (1993) 2 weeks before. The comparison between these data allows us to exclude both plasma contamination and significant errors of the image calibration. We have been able to estimate dust parameters, at the time of the GIOTTO encounter: (a) dust ejection velocity = 15ms-1 for grains with a diameter d = 0.15 mm; (b) hemispherical emission; (c) dust mass loss rate = 200 kg s-1 for grains with 0.12< 12 mm; (d) the power index of the size distribution was very close to -3.3 for dust diameters between 4 urn and 12 mm. The analysis of the F image indicates, in addition, hemispherical gas ejection. Finally, infrared photometry in the J and H bands, performed during the same observing run, gives a color index J - H = 0.4 consistent with previous measurements of the comet.