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Comets are kilometer-sized icy conglomerates of frozen gases and dust particles which move around the Sun in an assembly called 'Oort's cloud' in the outer parts of the planetary system. Only those which are ejected towards the Sun by the perturbations of passing stars will evaporate gases and dust while they are near the Sun. This material forms the large coma and the tails which make up the characteristic phenomenon of a comet. The spectral analysis has yielded that water is the main constituent of a comet, other molecules are simple compounds of H, C, O and N. - The ESA is planning a space mission to Comet Halley in 1986. [ABSTRACT FROM AUTHOR]

Non-periodic comets come from regions at the limit of our solar system and have conserved their original structure and composition since they have originated from a pre-solar nebula together with the sun and the planets about 4.5·10 years ago. They are icy bodies of kilometer size whose structure and chemical composition is of great interest, also with respect to the origin of the solar system. It is hoped to send a space craft to comet Halley in 1986 to get more detailed information. [ABSTRACT FROM AUTHOR]

The studies carried out in the U.S.A. and in Europe to investigate the possibilities and the scientific merit of a cometary probe are surveyed and reviewed. The scientific objectives of such a mission are given and the conditions are stated which a feasible comet must fulfill. Further, proposals of the different groups for the instrumentation of the probe are discussed. Though not all the groups propose the same comets for a first mission, due to different selection criteria, they agree that a mission to a 'new', non-periodic comet is out of consideration at present, and that a mission to a periodic comet, though possible in principle, presents various difficulties with respect to orbit accuracy and energy requirements. It is emphasized that a mission to Comet Halley in 1986 would be of special value. [ABSTRACT FROM AUTHOR]