The Nucleus of Comet Halley
The nucleus is very dark compared to the brilliant coma and tail (neither of which is displayed here). Halley's nucleus measures about 15 x 7 x 7 km. Jets of vapor and dust are clearly seen leaving the sunlit (left) side. This is the material that feeds the coma and tail. This photo, taken by the European Space Agency's Giotto spacecraft, is more effective than the next two comet images at conveying how dark comet nuclei really are. The albedo of this nucleus is about 3%, or about as dark as the darkest coal dust. Since the bulk of a comet is water ice, which has a very high albedo (up to 90%), the dark color is thought to come from dust and organic compounds left over when the ice sublimates (passes from solid to vapor) under the direct glare of sunlight.
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