Unique to Europe was the very first space mission for measuring the positions, distances, motions, brightness and colours of stars - for astrometry, as the experts call it. ESA's Hipparcos satellite pinpointed more than 100 000 stars, 200 times more accurately than ever before. As astrometry has been the bedrock of the study of the Universe since ancient times, this leap forward has affected every branch of astronomy.
The primary product from this pioneering and successful mission was a set of stellar catalogues, The Hipparcos and Tycho Catalogues, published by ESA in 1997.
The following goals were those adopted at the time of, or in the case of the Tycho experiment soon after, the formal adoption of the project by ESA (i.e. around 1980).
Number of stars 100 000
Limiting magnitude V = 12.4 mag
Complete to V = 7.3 - 9.0 mag (*)
Positional accuracy 0.002 arcsec (B=9 mag)
Parallax accuracy 0.002 arcsec (B=9 mag)
Proper motion accuracy 0.002 arcsec per year (B=9 mag)
Systematic errors <0.001 arcsec
Number of stars >400 000
Limiting magnitude B = 10 - 11 mag
Positional accuracy 0.03 arcsec (B=10 mag)
Photometric accuracy 0.05 mag in B and V (per observation)
Observation per star approx. 100
*depending on galactic latitude and spectral type
Additional results on photometry, variability and double and multiple stars, not foreseen in detail at the time of the mission's acceptance, also appear as part of the final mission products.
The actual results achieved by the mission in all case supersede these expected performances.
Hipparcos is an acronym for High Precision Parallax Collecting Satellite. This name, and that of Tycho, honour great astrometrists of classical and early modern times, Hipparchus the Greek (190-120BC) and Tycho Brahe the Dane (1546-1601).
The satellite has been described in the literature on several occasions, and a comprehensive description of the satellite, the Input Catalogue and the data reductions has been given in ESA SP-1111. The payload was centred around an optical all-reflective Schmidt telescope. A novel feature of the telescope was the beam combining mirror, which brought the light from the two fields of view, separated by about 58 degrees and each of dimension 0.9 x 0.9 degrees, to a common focal surface, and thus achieved both large- and small-field measurements simultaneously. The satellite swept out great circles over the celestial sphere, and the star images from two fields of view were modulated by a highly regular grid of 2688 transparent parallel slits located at the focal surface and covering an area of 2.5 x 2.5 cm².
The satellite was designed to spin slowly, completing a full revolution in just over two hours. At the same time, it was controlled so that there was a continuous slow change of direction of the axis of rotation. In this way the telescope was able to scan the complete celestial sphere several times during its planned mission.
Hipparcos operated in a highly elliptical orbit, following the failure to reach its nominal geostationary orbit.
The Hipparcos satellite was controlled by the ESOC operations control centre. The satellite was tracked by three ground stations - Odenwald (Germany), Perth (Australia) and the NASA Goldstone station (USA) - compared with the single ground-station originally foreseen.