Mission Objectives Overview  ===========================    One of the Vega program's aims was the exploration of Comet    Halley. The scientific objectives of the Vega cometary    investigation are:  (i)  determination of the physical parameters of the nucleus:      dimensions, shape, temperature and surface properties  (ii) study of the structure and dynamics of the coma around the      nucleus  (iii)definition of the gas composition in the close vicinity of the      nucleus and the nature of the parent molecules  (iv) study of the dust particles' composition and mass distribution      as functions of the distance to the nucleus, and  (v)  study of the interaction of the solar wind with the atmosphere      and ionosphere of the comet.  The Vega mission to Halley's comet was not only scientifically highly successful, it also helped to forge international cooperation in space science at an unprecedented level, both within the Vega Project and in the worldwide cooperation with the other major space organisations. In fact, it led to the formation of the Inter-Agency Consultative Group for Space Science (IACG) which from 1981-86 coordinated the six Halley space missions from its four member agencies: Intercosmos, NASA, ESA, and the Japanese ISAS. Since 1986 the IACG has been coordinating a number of projects from the four space agencies in solar-terrestrial science.  The in-situ observations by the various experiments onboard the flyby spacecraft were complemented by a large number of remote observations from space and from the ground, the latter being coordinated by the International Halley  Watch  (IHW).  The  IACG and  its counterpart on the ground, the IHW, formed the cornerstones of a global effort to explore Halley's comet as completely as possible during its 1985/86 apparition. By the end of the 1980's when Halley disappeared again into the outer Solar System, it was the most thoroughly studied comet ever with more data having been collected on it than on all other comets put together.  The cameras onboard the Vega spacecraft made the most important observation of all: the discovery of the nucleus of comet Halley.