Nature and Origin of Comets

IN so many fields of astrophysics the textbooks and the ordinary review articles have a tendency to present only the ‘generally accepted’ point of view, often not mentioning alternative theories, and even sweeping under the rug those observational facts which are adverse to the dominant view. This also applies to the theory of comets. As the arrival of Comet Kohoutek, and also the space missions to comets which are now being planned, is focusing the interest on comets, we may have unique opportunities in the near future to clarify which views are correct. This calls for a survey of the controversial points. A partial list of these, with which we will be concerning ourselves here, is the following: (1) What is the physical nature of the cometary nucleus? (a) Does it invariably consist of a single monolith ? or (b) could it, in some instances at least, constitute a bunch of particles having no physical coherence ? (2) What is the genetic relationship between comets and the meteor streams with which they are associated ? (a) Do the stream meteoroids invariably derive from the associated comets ? or (b) is the reverse process also possible ? (3) Is there a net dissipation or accretion during the lifetime of a comet ? (4) What is the origin of short-period comets ? (5) What is the origin of long-period comets ? Several other questions with regard to the chemical composition of the nucleus, the mechanisms for the production of the observed radicals and ions, and the nature of the comet solarwind interaction merit discussion but are outside the scope of this letter.