A QUANTITATIVE INTERPRETATION OF THE IN-SITU OBSERVATIONS OF THE DUSTCOMA OF COMET P GRIGG-SKJELLERUP BY THE OPE PHOTOPOLARIMETER/

On July 10, 1992, the Giotto spacecraft passed by comet P/Grigg-Skjell erup. The eight-channel OPE photopolarimeter provided data throughout the flyby, excepting very brief data losses. The time profile of the d ata of all channels during the flyby was rather unexpected, characteri zed, in particular, by four ''sudden increases'' of extremely small ri setime and large amplitudes, followed by decreases exhibiting an inver se square dependence upon time. We here provide a quantitative fit to all data acquired by the channels sensitive to dust emissions. This fi t is based on the following assumptions: (1) the four transients are d ue to light scattered by shells of fine dust or debris ejected by four impacts on the foremost part of the spacecraft; this interpretation i s shown to be fully consistent with the previous analysis of radio-sci ence data by Patzold et al. (1993); (2) the rest of the signal is due to a classical spherically symmetric inner dust coma. The following re sults are obtained: (1) the spacecraft closest approach to the nucleus was 95 km, and the OPE line of sight missed the nucleus by 7 km; (2) the brightness of the inner dust coma obeys a strict proportional to r (-1) radial dependence law for 7 km < r < 3000 km; (3) the P/Grigg-Skj ellerup dust loss rate is found to be in strict agreement with that de rived a few hours later from ground-based observations by Jockers et a l. (1993).