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Spatial and mass distribution of low-mass dust particles (m < 10- 10 g) in comet P /Halley's coma ... Send offprint requests to: O. Vaisberg for n = 1, ... 6, and M7 ...
ASJRONOMY

Astron. Astrophys. 187, 753-760 (1987)

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ASJROPHYSICS

Spatial and mass distribution of low-mass dust particles (m < 10- 10 g) in comet P /Halley's coma O.L. Vaisberg, V. Smirnov, A.Omelchenko, L. Gorn, and M.Iovlev Space Research Institute, USSR Academy of Sciences, Profsoyuznaya 84/32, SU-11781 0 Moscow GSP-7, USSR Received February 2, accepted July 4, 1987 Summary. The spatial distribution of cometary dust particles in the mass range ~ 10 -1 0 to ~ 3 10 - 17 g was obtained with the plasma impact detectors SP-1 on Vega-1 and Vega-2. The mass spectrum continues to rise in the range 10- 14 _310- 17 g. The evolution of the mass spectrum with distance suggests radiation pressure separation and some other effects. Several dust shells are easily distinguished by spatial gradients and by different properties of the grains. The spatial mass dispersion in the prominent jet observed on Vega-2 is used to evaluate the rotation of the nucleus and to calculate the velocity dispersion of the dust. Key words: comets distribution

dust mass spectrum -

dust spatial

1. Introduction

First direct measurements of dust in comet Halley's coma showed some differences with respect to the models developed from astronomical observations (Divine et ai., 1986). A large number of small particles, m < 10- 14 g, with rising mass spectrum down at least to ~ 10- 16 g, was found (Vaisberg et ai., 1986; Mazets et ai., 1986; McDonnell et ai., 1986). Moderate depletion of particles in the mass range 10- 14 _10- 12 g was observed and tentatively attributed to radiation pressure (Vaisberg et ai., 1986), as proposed in theoretical models (Divine et ai., 1986; Bertaux et ai., 1982). This paper is a more systematic discription of the first results of an experiment with the plasma impact detector SP-l on the Vega-1 and -2 spacecraft (Vaisberg et ai., 1986). 2. Instrumentation The SP-1 detector (Gom et ai., 1987) is based on the measurements of the charge of a plasma cloud originating at hypervelocity impact of grains on a thick target (McDonnell, 1978). The total charge in the plasma cloud is approximately proportional to the grain mass (Griin et ai., 1984) constituting ~ 10 3 Coulombs per gram at impact velocity ~ 80 km s - 1, near the Vega's encounter velocity. With this conversion factor the decade mass-ranges mn of the SP-1 detector are: (1)

Send offprint requests to: O. Vaisberg

M. Grewing et al. (eds.), Exploration of Halley’s Comet © Springer-Verlag Berlin Heidelberg 1988

for n = 1, ... 6, and M7 = (10- 16 _3 10 -17) g. Charge thresholds were set to predetermined values with an accuracy of about 5 %. Stability of thresholds determined by temporal and thermal stability of resistors was about 10 %. The target was made of gold and had an open area of81 cm 2 oriented normal to the encounter velocity vector. The collector system consists of gold coated bars suspended above the target and forming elongated "topless" boxes with approximately square cross section of 0.5 cm x 0.5 cm dimension. The collectors are shielded from direct impacts. One of two identical sensors is shielded by a thin plastic foil. The collectors are biased in a geometric succession by + 30 V and - 30 V. Charge pulses were amplitude-separated, accumulated during 2 sec in decade counters, and transmitted through the memory of the APV-N plasma wave instrument to the spacecraft telemetry. Data from SP-l were obtained on two Vega spacecraft. Near the closest approach (CA) both instruments suffered temporary overloadings that apparently resulted in a change of some thresholds of SP-1 on Vega-1. A minor fault of the SP-1 electronics on Vega-lied to the summing of counts of channels 1 and 3, 4 and 6, 5 and 7. Instrumental noise before and after the encounter with the comet has not been observed.

3. Spatial and mass distribution The first dust particles were recorded at ~ 260,000 km on Vega-1 and at ~ 320,000 km on Vega-2, both in the lower mass channels. Profiles of the dust counting rates are shown in Fig. 1. Only inbound data are given for Vega-1 in a different format due to the specific performance of the instrument. Dust counting rates were ~2.5 times higher for Vega-1 (March 6, 1986) than for Vega-2 (March 9, 1986) at comparable distances, but the dust envelope during the Vega-1 passage was more compact. Radial profiles seem to show similar and identifiable features with boundaries Pi indicated in Fig.l and in Table 1. These boundaries were identified by stronger gradients in the counting rate, coinciding with changes in the mass spectra (see below). Some of these boundaries coincide for a wide mass range. The radial profiles show strong differences varying from ~ R - 1 dependence (between P 2 and P 3) to ~ R - 4.5 dependence (beyond P 3)' Small particles are observed well beyond the outer identifiable boundary P 1 • Differences in the dust envelopes are also seen in the mass spectra. Typical averaged spectra for Vega-2 are shown in Fig. 2 for different dust shells (indicated in Fig. 1). Spectrum A was obtained from fluence counts beyond approximately the P 1

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