Cometary nuclei.

The current understanding of cometary nuclei is reviewed with the aim of gathering initial data for an interpretation of the Tungusk phenomenon as the explosion of the nucleus of a large comet. The hypothesis according to which the Tungusk body has a snowy-coma structure and an anomalously low density (0.01 g/cu cm) is questioned. It is concluded that cometary nuclei are ice conglomerates consisting of water ice with admixtures of other ices and silicate and metallic inclusions. Causes of comet flares are not yet clear, though they are probably connected with internal processes in the nuclei.