DUST PARTICLE IMPACTS DURING THE GIOTTO ENCOUNTER WITH COMET GRIGG-SKJELLERUP

IN the European Space Agency's 1992 Giotto Extended Mission, the Dust Impact Detection System operated successfully during a fly-by that took the spacecraft within about 200 km of the nucleus of comet Grigg-Skjellerup. During the encounter, three meteoroid impacts were detected on Giotto's front shield. The particle masses were found to be 100(-50)+105 mug, 2(-1)+4 mug and 20(-10)+25 mug, suggesting that the mass distribution of the cometary dust was dominated by larger particles. This is supported by the independent detection of a very large meteoroid (14(-4)+40 mg) by the Giotto Radio-Science Experiment, and is consistent with data over the same mass range from the 1986 encounter with comet Halley. The results indicate a higher rate of mass loss from the nucleus than previously thought, and hence a higher dust-to-gas mass ratio.