Albedo and Colour of Dust Grains: Laboratory Versus Cometary Results

Large dust grains (1 - 100 μm) contribute considerably (≡50%) to the total surface area of cometary dust producing light scattering in the coma. Referring to laboratory results (laser and microwave scattering experiments) it is shown, that absorbing irregular and fluffy dust grains can have low (⪉0.05) geometric albedos and that they can produce colour effects, which have to be taken into account in any interpretation of photometric Halley measurements. Therefore modelling exclusively by Mie theory is dangerous, since the scattering properties of large irregular particles deviate from that of smooth spheres.