The mission design that was established in order to meet these requirements considered a spinning satellite in geostationary orbit whose spin axis preceded around the Sun-satellite direction, or Sun axis, thus allowing a complete and uniform coverage of the sky. The two viewing directions, perpendicular to the spin axis and separated by an angle very close to 58 deg which is called the basic angle, were projected on the same detector by the beam combiner, a mirror that was cut in two halves and glued back at an angle equal to half the basic angle. In order to accurately calibrate the basic angle on the basis of the satellite's smooth rotation, the satellite in general and the beam combiner in particular required a mechanically and thermally very stable environment, which was to be achieved thanks to the large distance from Earth characterizing geostationary orbits and to the constancy at 43 deg of the Sun angle, i.e. the angle between the spin axis and the Sun axis. The images from the two fields of view were then superposed on the focal plane by an all-reflective Schmidt-like telescope working in the visible with rms mirrors and a small aperture of 29 cm. The detector system consisted of photoelectric detectors in conjunction with slit systems for signal modulation, and was in many ways similar to that which is used in meridian circles. In both cases the objects crossed the field of view, as a result of the satellite's and Earth's rotation, respectively. The central part of the focal plane was equipped with an image dissector tube and used for the main experiment, whereas at the two sides two photomultipliers were used for the star detection process that was necessary for the main experiment and for the Tycho experiment.
This revolutionary mission concept, however, posed some challenging problems that had never occurred before. At a very early stage of the mission, it became clear that the reduction of the large and complicated dataset which was to result for any star from the whole mission could not be accomplished in just one run, due to the huge computer resources that would be needed to do so. However, in 1976 already Lennart Lindegren designed a three-step reduction system which was to be adopted and which allowed to handle the problem with only a small loss of accuracy. Even so, the data reduction was to be very delicate, as there was no independent material available with which to compare Hipparcos results, and it was decided that it would be beneficial to the project if more than one group carried out the full data reduction process. This led to the establishment of two consortia, whose collaboration greatly improved the quality of the final catalogues. It was then necessary to select the stars to be observed, on the basis both of the desired scientific goals and of the mission's constraints, the most stringent of which being that the stars had to be uniformly distributed over the sky and that their positions at the epoch of observation had to be known a priori with an accuracy better than . Following the submission of observing proposals, a laborious selection process involving massive simulation of observations brought to the identification of an optimal observing list of about 120000 stars. All that was known about these in terms of astrometry, photometry, multiplicity information, spectral type and radial velocity was gathered in order to optimize the observation strategy and was later published in The Hipparcos Input Catalogue ([Turon et al. 1992]).
The satellite was designed and constructed by a European industrial consortium led by Matra Marconi Space and Alenia Spazio and on August 8, 1989 was launched into a geostationary transfer orbit by an Ariane 4 launcher. Despite the failure of the Apogee Boost Motor, which was to put the spacecraft into geostationary orbit but left it stuck in a highly elliptical one, the satellite acquired data for 3.5 years, significantly exceeding all the mission goals. In 1997, after painstaking verifications of the data quality and four years after the end of data acquisition, the mission results were finally published by ESA. The Hipparcos and Tycho Catalogues ([ESA 1997a]) contain high-quality astrometric data (positions, proper motions and parallaxes) with an accuracy1.7in the 0.5-2 mas range for about 120000 stars (Hipparcos), and in the 10-60 mas range for over 1000000 stars (Tycho). The astrometric data were complemented by photometric data in the band for Hipparcos stars and in the and bands for Tycho stars, and a great deal of additional data on variable and multiple stars was made available as well. The final results were about 1.5 to 2 times better than the original aims and improved the accuracy of orders of magnitude with respect to ground-based measurements.
The astrophysical significance of such a large and uniform dataset became apparent early during the mission, and was later confirmed by the high and steady rate of Hipparcos-based articles' publication1.8.
Besides, despite the careful reduction of the observations that had been
carried out, it became clear that important information was still ``hidden''
in the raw Hipparcos and Tycho data.
Therefore, even after the catalogues' publication, great care has been given
at fully exploiting the scientific potential of the data.
As a first step, proper motions with an accuracy of about 2.5 mas/yr
were obtained independently by [Urban, Corbin and Wycoff 1998] and [Høg et al. 1998a] for most Tycho stars
by comparing Tycho positions with the one-century old positions of the
Astrographic Catalogue stars obtained by [Urban et al. 1998].
The recently completed thorough re-reduction of Tycho raw data in combination
with a large number of ground-based catalogues has then allowed to increase by
a factor of 2.5 the number of observed stars.
The resulting Tycho 2 Catalogue ([Høg et al. 2000a] and [Høg et al. 2000b]) contains
positions and proper motions for the brightest 2.5 million stars in the sky,
complemented by components of 7500 resolved double stars with separation
down to 0.8 arcsec.
Further dedicated data reduction procedures are now being developed for
an optimal treatment of double and variable stars, which will result
in the largest duplicity and variability survey ever, the so called
Tycho 3 Project.
These improvements demonstrate a fundamental principle of astrometric
measurements, namely that they are a particularly valuable resource for future
analysis and comparison.
The content of the Hipparcos, Tycho and Tycho 2 Catalogues are summarized
in Table 1.1.
These must be considered as the present state-of-the-art in astrometry, and may
be compared with the measurement capabilities expected from GAIA, and described
in Section 2.7, in order to evaluate the achievable progress.
|Mean Star Density||3 stars/deg|
|Limiting Magnitude||12.4 mag|
|Median Precision of Positions (J1991.25) for||0.77 mas in RA|
|0.64 mas in Dec|
|Median Precision of Proper Motions for||0.88 mas/yr in RA|
|0.74 mas/yr in Dec|
|Median Precision of Parallaxes for||0.97 mas|
|Parallaxes determinaed to better than 10%||20853|
|Parallaxes determinaed to better than 20%||49399|
|Systematic Errors in Astrometry||0.1 mas|
|Solved Double or Multiple Systems||12195 (2996 new)|
|Median Photometric Precision in for||0.0015 mag|
|Periodic Variables||2712 (970 new)|
|Mean Star Density||25 stars/deg|
|Limiting Magnitude||11.5 mag|
|Median Precision of Positions (J1991.25) for mag||7 mas|
|Median Precision of Positions (J1991.25) for All Stars||25 mas|
|Systematic Errors in Astrometry||1 mas|
|Median Photometric Precision for mag||0.014 mag in|
|0.012 mag in|
|0.019 mag in|
|Median Photometric Precision for All Stars||0.07 mag in|
|0.06 mag in|
|0.10 mag in|
|Mean Star Density||60 stars/deg|
|Limiting Magnitude||12.4 mag|
|Completeness to 99%||11.0 mag|
|Completeness to 90%||11.5 mag|
|Median Precision of Positions (J2000.0) for||7 mas|
|Median Precision of Positions (J2000.0) for All Stars||60 mas|
|Median Precision of Proper Motions for All Stars||2.5 mas/yr|
|Systematic Errors in Positions||0.1 mas|
|Systematic Errors in Proper Motions||0.5 mas/yr|
In summary, the Hipparcos mission put an end to the long-standing strict separation between astrometry and astrophysics by dramatically demonstrating the deep astrophysical significance of extremely accurate global astrometric observations. A further improvement of orders of magnitude in terms of accuracy and number of objects is now expected from the next generation of astrometric satellite missions such as GAIA, proposed to ESA in the framework of its Horizons 2000 long-term scientific programme. These outstanding observational capabilities could be the key towards the solution of one of the major problems in contemporary astronomy, namely the understanding of the structure, formation and evolution of the Galaxy.