IN-ORBIT PERFORMANCE OF THE HIPPARCOS ASTROMETRY SATELLITE

Launched in August 1989 into a geostationary transfer orbit, the Hipparcos astrometry satellite failed to reach its intended geostationary orbit through the failure of its apogee boost motor. Present indications are that the possible operational lifetime should nevertheless extend beyond the end of 1992, with extremely high-quality scientific data being returned for some 65 per cent of the time. For an assumed 3-year operational lifetime, the original mission goals (positions, parallaxes, and annual proper motions with an accuracy of about 2 milli-arcsec) should be achievable (the pre-launch expectations of the planned 2.5-year mission would be achievable with a 3.5-year mission in the revised orbit). This paper describes the in-orbit performance of the satellite, summarising the system parameters (scanning law, observing strategy, data recovery fraction) affecting the scientific programme, the payload performances and their time-dependence, and the resulting expected mission accuracies.