In Situ Dust Measurements From within the Coma of 1P/Halley: First-Order Approximation with a Dust Dynamical Model

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Abstract
In situ measurements of the light locally scattered by cometary dust, as well as the local dust spatial density, are only available for one comet, 1P/Halley. These data were returned from the Optical Probe Experiment (OPE) and the Dust Impact Detection System (DID) aboard the European Space Agency spacecraft Giotto. Due to a detailed calibration of OPE at the time of Giotto's encounter with comet 26P/Grigg-Skjellerup, as well as improved analysis techniques, the similarities and correlation between the OPE and DID data sets can be reassessed. In this paper, we utilize this opportunity to compare these unique observations with a cometary coma dynamical model. Our results indicate that, to first order, the data can be fitted by a coma model that incorporates a grain size distribution index (at the nucleus), which need not be time dependent. Further, we find that the general shape of both the OPE and DID data can largely be explained by Keplerian dynamics alone, without recourse to fragmentation processes. The model is used to constrain the cometary dust bulk density, and a likely range of 50 < ρ < 500 kg m-3 is found, although a value of ρ = 100 kg m-3 is favored. In addition, the corresponding favored solution of the geometric albedo Ap(α = 0°) is found to be 0.04. Within the quoted density range, the ratio between density and albedo remains constant at ρ/Ap(α = 0°) ≈ 2500 kg m-3. The modeling also indicates that the grain size distribution power index at the nucleus is constant (in the range 10-12 < m < 10-3 kg) and has a likely value k = -2.6 ± 0.2 (i.e., cumulative mass distribution index b = -0.53 ± 0.1).