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D. T. Young, F. J. Crary (University of Michigan), J. E. Nordholt (Los Alamos National Laboratory), J. J. Hanley, J. L. Burch, D. J. McComas, R. Goldstein, D. Boice (Southwest Research Institute), R. C. Wiens, D. R. Lawrence (Los Alamos National Laboratory), A Eviatar (Tel Aviv University), K Sauer (Max Planck Institut fur Aeronomie), R Meier (University of Hawaii)
On September 22, 2001, the Deep Space 1 spacecraft flew through the coma of Comet 19P/Borrelly, coming within 2154 km of the central nucleus at a relative velocity of 16.6 km/s. We report here on initial analysis of solar wind and cometary pickup ions measured by the Plasma Experiment for Planetary Exploration (PEPE) onboard DS1. PEPE is a novel energy-angle-mass/charge spectrometer that simultaneously measures ions and electrons over the energy range ~few eV to 32 keV/q and the M/q range 1 to ~100 amu/e. Ion and electron data indicate a very strong interaction between the solar wind and cometary pickup ions leading to slowing of the solar wind from 320 km/s prior to the encounter to ~10 km/s. A significant finding is that the ion flow field is not symmetric with respect to the nucleus-sun line, but rather is displaced a few thousand kilometers northward of it. It is possible that the strong jets observed in the DS1 images of the nucleus are related to the observed ion flow asymmetry.