Numerical simulation of the dust flux on a spacecraft in orbit around an aspherical cometary nucleus - I

This study is the first investigation of the dust collection by a spacecraf t orbiting a cometary nucleus, which is based on a physically consistent ab -initio model of the dust distribution in the vicinity of an aspherical com et nucleus. The homogeneous bean-shaped nucleus of Crifo & Rodionov (1997a) is used, with updated parameter values adapted to comet 46P/Wirtanen, targ et of the Rosetta mission, but the conclusions of the study have a general significance. The near-nucleus dust distribution is computed from the dusty gasdynamic model of the above reference, except that a power-law size dist ribution with differential exponent alpha = -3.5 is used here. The more dis tant distribution is computed from a Keplerian fountain model. Dust flux an d fluences are evaluated for surfaces with various orientations, taking or not into account flux collimation, so that both the signal from dust analys ers and the spacecraft contamination can be assessed. The results are compa red to previous evaluations based either on the unphysical, spherically sym metric coma assumption, or on the highly asymmetric "effective nucleus dust source" derived from the Giotto Halley flyby in-situ dust measurements by Fulle et al. (1995), scaled appropriately to the present problem. The main results are the following: (1) The results based on the spherical assumptio n can at best be used for a global (benchmark) test of the correctness of a more realistic models, but otherwise do not represent the fluxes or fluenc es undergone by any spacecraft surface during any realistic sequence of spa cecraft orbits; (2) there is a strong difference between the results from t he present model, and those based on Fulle et al. (1995), due in part to th e fact that the present dust source is less anisotropic that the "effective " source derived there; the two evaluations thus provide for the first time an estimate of the prediction uncertainties associated with the absence of a precise knowledge of the nucleus shape; (3) other differences between th e present and previous results are due to the non-radiality of the near-nuc leus motion, and to the total absence of a biunivocal mass-terminal velocit y relation in the present model; (4) the "reflected" component of grains re turned towards the nucleus by radiation pressure appears particularly sensi tive to the dust ejection model, and is therefore a potentially important q uantity to measure, to constrain such models; (5) the strongest dust irradi ation is generally obtained for orbits located in the dawn-dusk meridian; ( 6) proper allowance for the non-radial near-nucleus dust motion is extremel y critical to the strategy of dust fluence reduction on critical spacecraft systems.