OPTICAL PROBING OF DUST IN COMET GRIGG-SKJELLERUP FROM THE GIOTTO SPACECRAFT

The optical probe experiment (OPE) has provided the first in situ high-resolution optical probing of the very inner coma of a short period comet, during the 10 July 1992 encounter of the Giotto spacecraft with Grigg-Skjellerup. Preliminary results of the encounter are presented below. Indications of entering the dust coma occurred around 17,000 km before closest approach. From the rapid brightening of the signals detected under the observational geometry, there is good evidence that the distance of Giotto's closest approach to the nucleus was below 200 km, and that Giotto passed the nucleus on the anti-sunward side. The scanning of the dust coma at about 70-degrees from the line of motion clearly shows the transition from the coma on the dark side to the coma on the sunny side, where changes in the optical properties of grains occur. The evolution of the polarization degree at about 90-degrees phase angle demonstrates that the optical properties of dust grains differ in the very inner coma, in the dust jets and in the outer coma.