ATTOGRAM DUST CLOUD A MILLION KILOMETERS FROM COMET HALLEY

Particle-impact ion mass spectrometers (PIA and PUMA) carried on the Giotto and two Vega spacecraft missions to Comet Halley in 1986 produced unexpected signals almost one million kilometers from Halley which can be attributed to high number densities of dust particles with masses of order 10-18 g. The data imply that up to several percent of the total mass lost during the entire apparition was present as such dust within a half-million kilometer radius if radial symmetry is assumed. The total volume mass density of particles observed varied with r-5 at large distance, and the mass of the largest individual particles observed varied with r-3, while the number density varied as r-1. The mass distribution was also determined. The results imply strong fragmentation processes. The spectrometers were not designed to operate with such small dust particles, and the signals were long thought to be spurious noise. Although atomic mass spectra (composition) of the very small dust could not be extracted from the data, a number of requirements on the particle properties are implied. The close detailed agreement among data from all three missions is used to verify that the signals were indeed produced by dust particles.