Does a continuous solid nucleus exist in comets

The implication of actual cometary observations for the physical nature of comets is briefly reviewed and brings out the complete conflict with observation of the ice-dust solid nucleus model put forward in recent years as representing the fundamental structure of comets. That under increasing solar heat the nucleus develops an expanding atmosphere is incosistent with the well-established phenomenon that the comacontracts with decreasing distance from the Sun. Several comets remaining always beyond Mars have nevertheless been strongly active and produced fine tails. The some comets show at times a star-like point of light is readily explicable on the dust-cloud structure and by no means establishes that a solid nucleus exists. With the nucleus-area corresponding not to a small solid mass but to an optical phenomenon, there would be no reason to expect that it would describe a precise dynamical orbit. On the hypothesis of a nucleus, it is necessary to postulate further some internal jet-propulsion mechanism to account for the orbital deviations.In planning a space-mission to a comet, and if search for a nucleus is included, it will be of the highest importance for its success to ensure beforehand that the equipment carried with not fail to discover a kilometric-sized body if one is present, otherwise a null result could be interpreted simply as a failure of this part of the mission and not a proving the absence of any nucleus.