The Hipparcos catalog - Commentary on: Perryman M. A. C., Lindegren L., Kovalevsky J., et al., 1997, A&A, 323, L49

In July 1997 four letters were presented on the occasion of the publication of the Hipparcos catalog (Perryman et al. 1997; Lindegren et al. 1997; Høg et al. 1997; van Leeuwen et al. 1997), covering the astrometric data, the double stars, the Tycho catalog, and the photometric data. The first of these papers received about 10 times more references than the other three together, an indication of where the main impact of the Hipparcos catalog has been, namely in the parallax and proper motion data for the nearly 118 000 stars it contained. The history of the Hipparcos mission covers approximately the same time span as the journal A&A, starting at the submission of the first modest proposals in France in 1967 (Lacroute 1982). Studies for a European Space Agency (ESA) led mission started in the mid-1970s. The mission was approved by ESA at the end of 1981, and in 1982 the preparations for the input catalog (Turon et al. 1992) and the data processing software started, uniquely divided between two consortia processing the same data in parallel (Lindegren et al. 1992; Kovalevsky et al. 1992). The Tycho mission was early on established as a unique opportunity to scientifically utilize the star mapper detectors onboard the satellite (Høg et al. 1992). The launch took place in August 1989. The failure of the Apogee Boost Motor left the satellite trapped in its geostationary transfer orbit. After increasing the perigee height of the orbit to around 450 km, the observations were started in November 1989. In 1992, part 1 of Vol. 253 of A&A was dedicated to our first experiences with the actual mission data, as well as the exploration possibilities for the Hipparcos data. In 1995 a large part of Vol. 304 was dedicated to the first preliminary results for the mission, based on 3 years of accumulated and processed data. This provided the first glimpse at the wealth of data being produced and the accuracies that had now been realized. The catalog was finally made public in 1997, and the paper reviewed here with the three papers mentioned above formed part of the official presentation of the catalog in the refereed literature. A fifth, more extensive paper published in the same Vol. 323, established the Hipparcos catalog as the optical realization of the International Celestial Reference System (ICRS) (Kovalevsky et al. 1997). But this was not the end of the data processing. Over the years that followed it became clear that other significant improvements to the astrometric data were still possible. Subsequent studies (van Leeuwen 2005; van Leeuwen & Fantino 2005) culminated in the complete re-reduction of the astrometric data and the presentation of a new, and significantly improved for the brighter stars, catalog in October 2007 (van Leeuwen 2007a,b). However, the paper by the Hipparcos Project Scientist at ESA, Michael Perryman, the Hipparcos Science Team of 1997, and a couple of other names closely associated with the Hipparcos data reductions, marked what can be seen as the most important milestone in the history of the Hipparcos project, as well as an important milestone for astronomy in general (see also Perryman 2008, online). For the first time accurate absolute parallax measurements became available, and for relatively large numbers of stars. In the latest version of the Hipparcos catalog there are nearly 30 000 stars with parallax accuracies better than 10 per cent, allowing for a wide range of studies to take place. Since 1996 there have been over 1600 refereed papers (and a further 2000 non-refereed publications) relying at least partly on the Hipparcos data. The paper by Perryman et al. (1997) is little more than a rather dry summary of achievements by the Hipparcos mission and products from it. How remarkable those achievements were is by now almost hidden in the mist of time. Concentrating on the most crucial of all measurements, those of the absolute parallaxes, it is worth noting that the original aim of the mission was to reach a mean error of 2 mas. The value presented in 1997 was just below 1.0 mas, with best values at 0.4 mas, while in the latest 2007 reductions the mean value is about 0.7 to 0.8 mas and the best values have accuracies of 0.09 mas, more than 20 times the goal of the mission (Fig. 1). This was achieved despite the mission operating in the wrong, 10.6 h geostationary transfer orbit. The latter subjected the satellite to strongly varying torques, which created difficulties in the reconstruction of the satellite attitude. The reconstructed attitude provided the reference system with all positional measurements. To achieve the accuracies that were obtained were, to say the least, remarkable under these conditions. The anomalous orbit also caused frequent exposure to strong radiation at the crossings of the Van Allen belts, which gradually and thoroughly destroyed the electronic equipment onboard and reduced the power that could be extracted from the solar panels, forcing an end to the mission in May 1993. The improvements in the final results were made possible by both a better performance of the instrument than expected (leading to a higher gain factor) and a very detailed understanding of the dynamics of the satellite, significantly reducing calibration noise. Considering the limited space available for a Letter, the paper could not cover the extensive tests that had taken place at earlier stages in the preparation of the catalog on the reliability of the data and the errors assigned to it (Lindegren 1995; Arenou et al. 1995). In many ways these tests, which showed that the