Mission Summary
Hipparcos is a pioneering space experiment dedicated to the precise measurement of the positions, parallaxes and proper motions of the stars. The intended goal was to measure the five astrometric parameters of some 120,000 primary programme stars to a precision of some 2 to 4 milliarcsec, over a planned mission lifetime of 2.5 years, and the astrometric and two-colour photometric properties of some 400,000 additional stars (the Tycho experiment) to a somewhat lower astrometric precision. The project was accepted within the ESA scientific programme in 1980. The satellite was launched by Ariane 4, in August 1989, and after collecting more than three years of extremely high-quality scientific data, communications were terminated with the satellite in August 1993. All of the original mission goals have been significantly exceeded.

The final Hipparcos Catalogue (118,218 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 first phase of the transfer of data to Principal Investigators was initiated on 8 August, and the second and final phase of data transfer to the remaining Principal Investigators took place in January 1997. The catalogues were published by ESA in June 1997.

The Tycho-2 Catalogue, an astrometric and photometric reference catalogue of the 2.5 million brightest stars on the entire sky, was released on 8 February, 2000. The catalogue contains positions, motions, brightness and colours for 2,539,913 stars, more than doubling the number of stars in the original Tycho Catalogue.