Hipparcos

A European Space Agency satellite designed to carry out astrometric surveys to an unprecedented degree of accuracy. The launch in 1989 was only partially successful, in that the satellite went into a highly elliptical orbit instead of the intended geostationary orbit. However, it was still possible for the scientific objectives to be met. The cumulative effect of radiation damage forced the termination of observations on 15 August 1993. The name "Hipparcos" is an acronym for High Precision Parallax Collecting Satellite, chosen also for its similarity to the name of the Greek astronomer, Hipparchus (also spelt Hipparchos), who measured the Moon's parallax and made an accurate star map, which led to the discovery of the precession of the equinoxes. The basic instrument was an all-reflective Schmidt telescope with a 0.29-metre (11.5-inch) primary mirror and the programme required the advance compilation of an Input Catalogue, including specially made ground-based observations. The satellite observations resulted in the Hipparcos catalogue, containing the positions, parallaxes and proper motions of 118,000 stars with an accuracy of 2 milliarcseconds, together with the Tycho catalogue which lists these data for over a million stars with less precision but with systematic errors of only about one milliarcsecond. Hipparcos more than doubled the number of known variable stars and discovered many thousands of new double and multiple star systems.