In situ dust measurements from within the coma of 1P/Halley: First-order approximation with a dust dynamical model

In situ measurements of the light locally scattered by cometary dust, as we ll as the local dust spatial density, are only available for one comet, 1P/ KalIey. These data were returned from the Optical Probe Experiment (OPE) an d the Dust Impact Detection System (DID) aboard the European Space Agency s pacecraft Giotto. Due to a detailed calibration of OPE at the time of Giott o's encounter with comet 26P/Grigg-Skellerup, as well as improved analysis techniques, the similarities and correlation between the OPE and DID data s ets can be reassessed. In this paper, we utilize this opportunity to compar e these unique observations with a cometary coma dynamical model. Our resul ts indicate that, to first order, the data can be fitted by a coma model th at incorporates a grain size distribution index (at the nucleus), which nee d not be time dependent. Further, we find that the general shape of both th e OPE and DID data can largely be explained by Keplerian dynamics alone, wi thout recourse to fragmentation processes. The model is used to constrain t he cometary dust bulk density, and a likely range of 50 < p < 500 kg m(-3) is found, although a value of rho = 100 kg m(-3) is favored. In addition, t he corresponding favored solution of the geometric albedo A(p)(alpha = 0 de grees) is found to be 0.04. Within the quoted density range, the ratio betw een density and albedo remains constant at rho/A(p)(alpha = 0 degrees) appr oximate to 2500 kg m(-3) The modeling also indicates that the grain size di stribution power index at the nucleus is constant (in the range 10(-12) < i n < 10(-3) kg) and has a likely value k = -2.6 +/- 0.2 (i.e., cumulative ma ss distribution index b = -0.53 +/- 0.1).