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 aft er the encounter with GIOTTO spacecraft. We have analyzed two images o btained 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 come t. We have used the inverse numerical method of dust tail analysis (Fu lle 1989) to estimate the following cometary grain parameters: ejectio n velocity, anisotropy of the emission, size distribution and loss rat e. This from the observations back in time to the GIOTTO encounter. Th e model has been applied both to our images and to that taken by Jocke rs et al. (1993) 2 weeks before. The comparison between these data all ows 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=15 m s -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 < d < 12 mm; (d ) the power index of the size distribution was very close to -3.3 for dust diameters between 4 mum and 12 mm. The analysis of the V image in dicates, in addition, hemispherical gas ejection. Finally, infrared ph otometry 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.