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(hi-par -koss) (High-Precision Parallax Collecting Satellite) An astrometric satellite of the European Space Agency (ESA) launched in Aug. 1989 to gather modern data on the position, brightness and other properties of an input catalog of selected stars with an unprecedented level of accuracy. Although its name was an acronym, it was deliberately chosen to recall the ancient Greek astronomer Hippachus, who compiled an important star catalog in the 2nd century BC. A faulty booster motor failed to lift Hipparcos into its intended geostationary orbit, but, following a revision of its mission, it was operated successfully from its 10-hour orbit and completed its mission in 4 years (rather than the planned 2.5 years). Prolonged exposure to radiation eventually caused it to cease working in 1993.

The satellite's 29-cm Schmidt telescope plotted very exactly the position, magnitude, parallax, and proper motion of a large number of stars. The resulting Hipparcos catalog (published 1997) gave the positions of 118 218 stars with a precision (0.002 arcsec) not possible from the ground. It was complete to magnitude 7.5 but included many fainter stars down to magnitude 12.5. The mean epoch was 1991.25. The less accurate Tycho catalog was prepared using the Hipparcos satellite's less sensitive star tracker instrument and was published the same year. Tycho gave positional date and magnitudes for the 1 058 332 brightest stars, including many as faint as magnitude 11.5. Tycho 2, a collaborative venture between the United States Navy Observatory and the University of Copenhagen, Denmark, was released on CD-ROM in 2000. It extended the number of the brightest stars listed to 2 539 913, complete to magnitude 11, with their positions precessed to J2000.0.

Among its other achievements, Hipparcos helped to forecast the impacts on Jupiter of the fragments of Comet Shoemaker-Levy 9 and identified stars that in the far future will pass close to the Sun.

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