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The Hipparcos Space Astrometry Mission was a project of the European Space Agency (ESA) dedicated to the precise measurement of the positions, parallaxes and proper motions of the stars.
The program was divided in two parts: the Hipparcos experiment whose goal was to measure the five astrometric parameters of some 120,000 stars to a precision of some 2 to 4 milli-arcsec and the Tycho experiment, whose goal was the measurement of the astrometric and two-colour photometric properties of some 400,000 additional stars to a somewhat lower precision.

The project was accepted within the ESA scientific programme in 1980. The satellite was launched by an Ariane 4 launcher on August 18, 1989, and after collecting more than three years of extremely high-quality scientific data, communications were terminated with the satellite on August 17, 1993.

The final Hipparcos Catalogue (120,000 stars with 1 milliarcsec level astrometry) and the final Tycho Catalogue (more than one million stars with 20-30 milliarcsec astrometry and two-colour photometry) were completed in August 1996.
The catalogues were published by ESA in June 1997. The Hipparcos and Tycho data have been used to create the Millennium Star Atlas: an all-sky atlas of one million stars to visual magnitude 11, from the Hipparcos and Tycho Catalogues and 10,000 nonstellar objects included to complement the catalogue data.

The project and the experiments were named after the Greek astronomer Hipparchus and the Danish astronomer Tycho Brahe.

See also: The Hipparcos Space Astrometry Mission (http://astro.estec.esa.nl/Hipparcos/hipparcos)

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