Coma opacity and surface contrast

Parameters which describe the visibility of the cometary nucleus within the dust coma are derived. The ratio of radiation scattered by a dust column to the radiation reaching the observer directly from the nucleus influences the contrast deterioration of features on the surface; the visibility of the whole nucleus depends on the ratio of light coming from the direction of the nucleus to that of an adjacent line of sight passing the nucleus. These ratios are calculated for the date of the Giotto spacecraft flyby of comet Halley. The single scattering calculations are based on dust particle densities in the coma derived from a hydrodynamical sublimation model. Mie theory is used to calculate the radiation properties of olivine dust particles. During the Giotto encounter with comet Halley, the contrast on the nuclear surface may be impaired, so imaging experiments must be geared to highest possible contrast resolution. Nucleus shape and size remain discernable under adverse conditions.