Large dust particles along the Giotto trajectory

Abstract

Images of Comet Halley taken by the Halley Multicolour Camera (HMC) onboard ESA's Giotto spacecraft provide highly accurate information on the viewing direction. As a part of the spacecraft dynamical system, HMC was very sensitive to attitude variations that occurred during the final approach phase. HMC identified several discrete events that were caused by dust particles in the 1–50 mg range. Together with impacts reported from the Japanese Suisei spacecraft by Uesugi (1986, 20th ESLAB Symp., ESA SP-250 II, 219–222), this is the first independent observation of particles in this mass range. The well-calibrated measurements unequivocally set firm lower limits of the mass of large dust particles. These particles dominate the impact history of the spacecraft and, therefore, the dust mass distribution. The data suggest the existence of a halo of heavy old dust grains surrounding the cometary nucleus. The statistics of the observed impacts are discussed in the context of other dust observations and models. The HMC measurements support models that assume a dust mass distribution dominated by large grains.