The sensitivity of the size distribution to the grain dynamics: Simulation of the dust flux measured by GIOTTO at P/Halley

A cometary dust emission model, based on rigorous keplerian dynamics, is developed and, for the first time, the dependence of fluence on the probability distribution of the dust ejection velocity vector is demonstrated. The results are compared with the fluences measured by the DIDSY experiment on board the GIOTTO spacecraft during the Halley's fly-by in 1986. A fit of the total fluence is obtained and an interpretation of the observed differences, between pre and post fly-by, is proposed. From the best-fitting process we conclude that the dust ejection from P/Halley was strongly anisotropic and mainly Sun-ward oriented with an angular dispersion of 18.4°, for the adopted Gaussian distribution. The most probable velocity at the fly-by is 50 ± 5 m s-1 for 1 mm sized grains and the power index of the velocity size-dependence is -0.5. Both these results agree with those of dust-gas drag models. Moreover, the dust velocity presents a wide dispersion (35 ±5 m s-1), which explains the velocity size-dependence derived by Neck-Line photometry. For grains larger than 20 fim, the power index of the differential size distribution is constant (a = -3.5 ± 0.2). Since a > -4, most of the dust mass is released in the form of large grains. The dust to gas ratio is x = 4 ± 1. The last two conclusions agree with the output of previous DIDSY fitting processes and are compatible with inverse dust tail models; they must be considered the best constrained results coming from the DIDSY experiment. Our results imply that future in-situ cometary experiments will have to measure both mass and velocity vector for each grain, in order to determine the dust size distribution.