The goal of this proposal  is  to  study  dynamical  and  composition
 properties of grains emitted from comet nuclei and present  in  their
 trails. Comets are commonly considered the best reservoir of pristine
 cosmic dust in the Solar System. The importance of a  detailed  study
 of comets is testified  by  the  results  obtained  from  past  space
 missions such as Giotto (Grewing et al. Eds., A&A 187, 1987)  and  by
 the future ESA  programs, with Rosetta mission recently  selected  as
 cornerstone number 3 (ESA Publ., SCI(93)7, 1993). Preparing  for  the
 success of Rosetta,  the  present  knowledge  about  physico-chemical
 properties of comets has to be improved. The  observation  of  nuclei
 and cometary trails provides information on dynamical,  morphological
 and chemical properties of solid particles. Ground-based observations
 combined with appropriate model simulations have already  allowed  to
 perform important steps ahead (Newburn et al.  Eds.,  Comets  in  the
 Post Halley Era, Kluwer, Vol. 167, 1991). However, the possibility of
 exploiting the ISO facility will improve  enormously the accuracy  in
 observations. The combined spectral and imaging capabilities provided
 by ISO in the infrared are useful  to  analyse  faint  cometary  dust
 trails  and  comae,  not  detectable   by   means   of   ground-based
 observations.  These   data   will   be   complemented   with   other
 observations, possibly available in the visible range,  and  compared
 with, and/or used as inputs  of,  models  aimed  to  determine  grain
 dynamics, morphology and composition. Some of the comets included  in
 the present proposal are possible targets of the Rosetta mission. The
 selected  sample  appears  of  relevant  interest  for  the  possible
 scientific follow-up in the perspective of the Rosetta mission.