No CrossRef data available.
Published online by Cambridge University Press: 12 April 2016
Hipparcos, the astrometric satellite of the European Space Agency, launched in August 1989, will provide absolute trigonometric parallaxes for about 100 000 stars brighter than the limiting V-magnitude 12.5. In spite of the difficulties caused by a faulty orbit, the mission is successfully going on. A lifetime of more than 4 years is expected which will allow to reach the original objectives of the mission: 2 milli-arcsec for stars brighter than V-magnitude 9 and 4 to 5 milli-arcsec for fainter stars. The observing programme is based upon a single, uniquely-defined Input Catalogue. Field and open cluster stars of almost all effective temperatures, chemical compositions and gravities are being observed. Most types of binaries and variable stars are well represented, but Hipparcos is discovering new ones. Significant parallaxes (relatives errors smaller than 20%) will be obtained for stars closer than 100 pc (about 36 000 stars). Reliable stellar structure models should fit all the possible observational constraints: the main contribution of Hipparcos parallax data will be the huge improvement in the individual luminosities. The determination of stellar masses from the study of visual binaries will also be improved. The preliminary results of the first-year data reduction are an encouraging indication of the expected quality of the final results. On the basis of a mission duration of 4 years, the final results are foreseen for 1995.