IUE observations of comet Halley during the Vega and Giotto encounters

Abstract

The observations of comet Halley from the International Ultraviolet Explorer (IUE) satellite from 9 to 16 March 1986 allow a comprehensive study of gas and dust production by the comet in the week during which the fleet of six spacecraft from four space agencies encountered the comet. The gaseous output of the nucleus was found to vary over timescales of the order of one or two hours. The Vega 2 probe crossed Halley's coma at the time of, or shortly after, the onset of a strong outburst. The Giotto spacecraft performed its measurements while the cometary nucleus was near a minimum of activity. The ultraviolet spectrum of comet Halley differs from that of other comets observed by the IUE satellite in that the continuum emission is very strong relative to the gaseous emissions, and the CO₂⁺ violet bands, if present, are weak. The rate of water production was estimated to be 5.6×10²⁹ and 5.2×10²⁹ molecules per second at the time of the Vega 2 and Giotto encounters, respectively, while atomic carbon was a few per cent and CO ∼10–20% of these values.