First results of plasma and neutral gas measurements from Vega-1/2 near Comet Halley

Based on the ion, electron, and neutral-gas observations performed by five of the six sensors comprising the PLASMAG-1 experiment on board Vega 1 and 2, the following results are discussed: the existence of the bow shock and its location at about 1.1 million km for Vega 1 inbound; the existence of a cometopause and its location at about 160,000 km for the phenomena taking place within the cometary plasma region (including mass spectroscopy of cometary ions at distances less than about 15,000 km); the existence of keV electrons near closest approach to the nucleus; and the radial dependence of the cometary neutral gas and the comparison with model calculations, yielding a mean ionization scale length of about 2 million km and an overall production rate of about 1.3 x 10 to the 30th molecules/s for Vega 1 inbound. The results are also discussed in the context of the other (remote and in situ) observations performed on board the Vega and Giotto spacecraft. (Author)