Some Predictions on The Nature of Comet Halley

Visible comets like Halley's differ from the great majority of comets in that they exist in adjacent pieces in gentle motion with respect to each other. From time to time the pieces rub together, causing a dusty surface which otherwise would be very dark and difficult to observe to be temporarily swept clean. The cleansed patches are then subject to evaporation, so making such multinuclear comets visible, unlike the majority of comets which have single nuclei and which stay dark. The existence of a large number of dark comets could have an interesting relation to the past history of the Earth. This revised version was published online in July 2006 with corrections to the Cover Date.