Hipparcos (for High Precision Parallax Collecting Satellite) was an astrometry mission of the European Space Agency (ESA) dedicated to the measurement of stellar parallax and the proper motions of stars. The project was named in honor of Hipparchus. Ideas for such a mission dated from 1967, with the mission accepted by ESA in 1980.

The satellite was launched by an Ariane 4 on 8 August 1989. The original goal was to place the satellite in a geostationary orbit above the earth, however a booster rocket failure resulted in a highly elliptical orbit from 507 to 35,888 km altitude. Despite this difficulty, all of the scientific goals were accomplished. Communications were terminated on 15 August 1993.

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 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.

There were questions over whether Hipparcos has a systematic error of about 1 milliarcsec in at least some parts of the sky. The value determined by Hipparcos for the distance to the Pleiades is about 10% less than the value obtained by some other methods. As of 2004, the controversy remained unresolved. [1].

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