A cometary and interplanetary dust experiment on the Vega spacecraft missions to Halley's Comet

Abstract

An instrument from the University of Chicago which measures the mass and flux of dust particles as a function of time and position in the dust coma of Halley's Comet has been included in the two USSR Vega spacecraft launched 15 and 21 December 1984 for encounters with the comet on 6 and 9 March 1986 respectively. The dust measurements are based on the new detection principle reported by Simpson and Tuzzolino (Nucl. Instr. and Meth. 1985) which employs a polarized polyvinylidene fluoride polymer film (PVDF) and fast electronic pulse techniques. The instrument, called the dust counter and mass analyzer (DUCMA) is described along with its expected performance for the measurement of both cometary and interplanetary dust. Since the electronic PVDF detector signal, resulting from an impacting dust particle of mass m and relative velocity v, is a known function of m and v, measured detector signals at Halley's Comet encounter will directly determine dust particle masses, since v is the known (78 km/s) velocity of the spacecraft relative to the dust particles. DUCMA measures dust particle masses by making use of a single PVDF detector of area 75 cm 2. The dust mass thresholds selected for the encounter include 3 ranges between 1.5 × 10 −13 and 9 × 10 −11 g, and the integral mass flux above 9 × 10 −11 g. The dynamic range of the instrument counting rates exceeds 5 × 10 4 s −1 for each mass interval and, since the detector remains in calibration even when punctured by large particles, it is expected that the instrument should function properly under the most extreme dust bombardment conditions which could occur during the encounters. The two spacecraft travel around the sun before the March 1986 encounters. Therefore, a search will be undertaken after Venus encounter in June 1985 for possible enhanced streams of interplanetary dust crossing the spacecraft trajectories associated, for example, with known meteor or cometary showers. The opportunity to contribute the dust experiment built and tested by the University of Chicago for the Vega Halley's Comet missions was made possible through the invitation from the USSR Academy of Sciences, Space Research Institute (Moscow) and with assistance from the Max-Planck Institute (Lindau) and the Central Research Institute (Budapest) for arranging this cooperative venture.