COMETS .1. CONCEPTS AND OBSERVATIONS

The status of cometary astronomy and astrophysics as of mid-1992 is re viewed, i.e. at a time when the first in situ observations of comets i n 1985-86 have been thoroughly discussed, interpreted and compared wit h ground-based investigations. Many earlier ideas about comets were vi ndicated and a plethora of new discoveries resulted which have now led to reformulation of certain observational strategies and, in particul ar, to greatly improved possibilities for the detailed physical/chemic al modelling of many cometary phenomena. A main purpose of this paper is to assess the current situation and to provide a reasonably complet e, yet concise and critical evaluation of the most important questions and promising lines of research in this field. After a short introduc tion which defines the overall subject and the framework of the presen t review (Sect. 1), we take a look at some of the major past developme nts of our concepts about comets, in particular the crucial new insigh ts 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 a nd restrictions of the various techniques employed (Sect. 3). Part II of this paper discusses the modelling of cometary comae and tails (Sec t. 4), the cometary nucleus (Sect. 5), the evolution (Sect. 6) and ori gin (Sect. 7) of comets and ends with an overview of the main question s now being asked by cometary studies (Sect. 8).