The Contribution of Hipparcos to Fundamental Astronomy

Abstract

The European Space Agency’s Hipparcos satellite was launched in August 1989, and has now been operated for more than its planned 2.5-year lifetime, with consumables for up to a year of further observations. It is yielding high precision astrometric and photometric data for 120 000 pre-selected programme stars, and lower precision astrometric and two-colour photometric data for more than a million stars, typically brighter than 12 mag. The five astrometric parameters of each star (position and proper motion components, and parallax) are determined from the ensemble of one-dimensional relative coordinates along great circles, each constructed from up to 10 hours of satellite observations. More than 2000 such circles have been accumulated so far, and more than 1000 circle sets (corresponding to the first 18-months of satellite data) have been reduced and combined into a global astrometric solution. Independent reductions, and numerous internal and external analyses, indicate that more than 50 per cent of the programme stars already have corresponding formal positional and parallaxes accuracies better than 2 milli-arcsec, with 90 per cent better than 3 milli-arcsec. There is thus substantial confidence that the target accuracies of 2 milli-arcsec for the positions, parallaxes and annual proper motions for stars of 9 mag or brighter will be obtained, and in many cases exceeded, by the end of the mission.