The Hipparcos astrometry satellite was launched on 8 August 1989, and after spacecraft and payload commissioning, commenced the routine data acquisition phase on 26 November 1989. Having failed to reach its planned geostationary orbit, major revisions in the mission operations were made, and the post-launch expectations of the mission were strongly degraded with respect to the original goals - principally due to the greatly reduced observational efficiency (caused by the lack of ground station coverage) and the anticipated degraded mission lifetime (as a result of the high-energy particle degradation of the solar arrays in the geostationary transfer orbit).
The final astrometric accuracies attainable by the Hipparcos mission will be influenced by the spacecraft and payload performances on the one hand, and by fraction of useful data and mission lifetime on the other. It will be shown that the elemental observational measurements correspond very closely to the predictions, and the data recovery fraction now stands at around 60–70 per cent, so that the ultimate scientific value of the Hipparcos results will be tied directly to the satellite lifetime. A measurement duration of at least 18 months is mandatory if the astrometric parameters (parallaxes and proper motions) are to be decoupled through the data reductions procedures. A somewhat longer lifetime (2.5–3 years) is necessary in order to reduce the errors on the astrometric parameters to the astrophysically-significant accuracies of around 2 milli-arcsec.
It will be shown that the present indications of the satellite performances, and the significant progress already made in the data reductions, indicate that the difficulties of the ‘revised’ Hipparcos mission have been largely overcome, and that these target accuracies could still be achievable.