Launched in August 1989 Hipparcos was 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. Having achieved the mission goals, communications with Hipparcos were terminated in August 1993.
About the mission
The final products of the Hipparcos mission - the Hipparcos and Tycho Catalogues - were published by ESA in June 1997.
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.
Besides giving an unprecedented 3-D picture of the distances and movements of stars in the vicinity of the Sun and the Earth, Hipparcos results have, for example:
Built for ESA by the European aerospace industry, and launched by an Ariane-4 rocket, the 1.4-tonne Hipparcos satellite operated in space from 1989 to 1993. The spacecraft turned slowly on its axis and repeatedly scanned right around the sky on different slants. It measured angles between widely separated stars, and recorded their brightness, which were often variable from one visit to the next. Each star selected for study was visited about 100 times over four years.
A million million bits of information, radioed from Hipparcos to ground stations in Germany, Australia and the USA, went into the biggest computation in the history of astronomy. Multinational teams created a super-accurate frame for the whole sky. Within it they found the shifts in direction of individual stars as the Earth orbited about the Sun - the parallax that enables a star's distance to be measured. The computations also revealed the 'proper motions' of stars across the sky, and orbital motions in the case of double stars circling around each other.
The resulting gift to the world's astronomers was a set of catalogues published by ESA in 1997. As well as making 16 bound volumes, including a three volume Millennium Star Atlas, the catalogues are also available in CD-ROM and online formats. In 1999, ESA said "thank you" to the leaders of the teams of astronomers who undertook the colossal work of generating the catalogues, by awarding the very first Director of Science Medals to Catherine Turon (Paris), Jean Kovalevsky (Grasse), Lennard Lindegren (Lund), and Erik Høg (Copenhagen).
Computations from observations by the main instrument generated the Hipparcos Catalogue of 118 218 stars charted with the highest precision. An auxiliary star mapper pinpointed many more stars with lesser but still unprecedented accuracy, in the Tycho Catalogue of 1 058 332 stars. The Tycho 2 Catalogue (2000) brings the total to 2 539 913 stars, and includes 99% of all stars down to magnitude 11, almost 100 000 times fainter than the brightest star, Sirius.
The names, Hipparcos and Tycho, honour great astrometrists of classical and early modern times, Hipparchus the Greek (190-120BC) and Tycho Brahe the Dane (1546-1601). Hipparcos is also an acronym for High Precision Parallax Collecting Satellite.
The directions and motions of stars in the Hipparcos Catalogue are precise to about one milli-arcsecond, or a quarter of a millionth of a degree. This corresponds to the height of an astronaut standing on the Moon, as seen from the Earth. Such accuracy is unattainable by ground-based observatories, because stars twinkle in the variable air, telescopes droop under gravity, and no one station can see the whole sky. Although Hipparcos was 200 times better, it will come to be seen as just the pioneering effort in space astrometry. ESA is developing Gaia, as the successor of Hipparcos, which promises to be 200 times better still, equivalent to measuring the thumbnails of that astronaut on the Moon!