COMETS .1. CONCEPTS AND OBSERVATIONS

The status of cometary astronomy and astrophysics as of mid-1992 is reviewed, i.e. at a time when the first in situ observations of comets in 1985-86 have been thoroughly discussed, interpreted and compared with ground-based investigations. Many earlier ideas about comets were vindicated and a plethora of new discoveries resulted which have now led to reformulation of certain observational strategies and, in particular, to greatly improved possibilities for the detailed physical/chemical modelling of many cometary phenomena. A main purpose of this paper is to assess the current situation and to provide a reasonably complete, yet concise and critical evaluation of the most important questions and promising lines of research in this field. After a short introduction which defines the overall subject and the framework of the present review (Sect. 1), we take a look at some of the major past developments of our concepts about comets, in particular the crucial new insights which were gained during the past four decades (Sect. 2). The rapid advances in observational technology have greatly extended the realm of accessible problems and we next discuss the present possibilities and restrictions of the various techniques employed (Sect. 3). Part II of this paper discusses the modelling of cometary comae and tails (Sect. 4), the cometary nucleus (Sect. 5), the evolution (Sect. 6) and origin (Sect. 7) of comets and ends with an overview of the main questions now being asked by cometary studies (Sect. 8).