Overviews of the European Space Agency's Hipparcos astrometry mission, its history, scientific objectives and observing programme, the payload design, and plans for the satellite operations and data reductions, have been given frequently before the satellite launch. The most comprehensive description of the satellite, observing programme, and methods foreseen for the data reduction are contained in a 3-volume ESA publication, ESA SP-1111 (Perryman et al. 1989). The project was accepted within ESA's mandatory scientific programme in 1980, with a planned operational lifetime of 2.5 years. The goal of the Hipparcos project was to measure the astrometric parameters of about 100000 stars with an accuracy of some 2-4 milli-arcsec (depending on magnitude) for the main mission and, as subsequently incorporated during the design phase, the astrometric and two-colour photometric properties of a further minimum of some 400000 or more stars with somewhat lower accuracy for the Tycho experiment. By placing the observing platform above the perturbing atmosphere, and exploiting the all-sky visibility and freedom from gravitational flexure and thermal fluctuations, differential angular measurements are built up over large angles, at many different orientations and at many different epochs. From these measurements, relative positions, annual proper motions and absolute parallaxes, free from regional or systematic errors at the milli-arcsec level, can be derived.
The Hipparcos satellite was launched by Ariane 4, flight 33, on 8 August 1989. At the time of publication of this `Hipparcos Input Catalogue', the satellite is more than two years into its measurement programme, the on-board instrumentation is functioning according to specification and, despite the necessary revisions to the mode of operation brought about by the non-nominal orbit, the target accuracies of the original mission should be attained.