Pinpointing the star positions with respect to all the other stars and measuring their distances and motions are a historical key to the astronomy. In addition to the importance of the astrometry for the understanding of the 3-dimensional structure of the universe and the place the Earth is occupying, these observations served during a long time for the navigation, even if they were not many and to imprecise. The Hipparcos catalogue brought together 118,000 stars the 3-dimensional positions, speeds and luminosity of which were measured with an unprecedented accuracy. A real gold mine for the all-around-the-world astronomers.
Measuring the stars' positions in the sky
In the 2nd century before our era, the Greek Hipparcos measured the positions of a thousand stars with a precision of less than 0.4 degrees. Ulugh Begh (in the 13th century), Tycho Brah2 (in the 16th century) and others continued this difficult task. Their measurements were "relative": they gave the star positions with respect to all the other stars.
It was therefore necessary to wait for instrumental techniques' enhancement to have the first catalogue of absolute positions (compared with a given point) featuring 14 stars (Friedrich Bessel, 1818). The measurements' number and accuracy kept improving... but slowly.
When Hipparcos was launched in 1989, the positions of 1,536 stars were known with an accuracy of 50 milliarcseconds (mas).
Eight years later, the Hipparcos catalogue featured 118,000 stars with a positioning accuracy of less than 2 mas! In addition, thanks to the repeated surveys over the years, it has been getting richer adding the stars' trigonometric parallaxes and proper motions.
Measuring the stars' distances
The only direct calculation method (hypothesis-free) for a star's distance is based on the measurement of the trigonometric parallax: the apparent displacement of a star in the celestial sphere seen from the Earth during its revolution around the Sun (see next). The larger the distance, the smaller the displacement.
And the parallax of the nearest star, Proxima Centauri, at a distance of 4 light years, is only 0.77 angular seconds... Here again the first precise measurement did not arrived before the 19th century (Bessel, 1838).
Many surveys were thus performed but the accuracy was still a problem: when Hipparcos was launch, only a few very close stars' distances (less than 30 light years) had been quite precisely measured (better than 10%).
Thanks to Hipparcos, the distances of 20,000 stars in a radius of 500 light years around the Sun were measured at less than 10 %.
The satellite also measured the apparent magnitude of the stars in two spectral ranges. Combined with distances, this data allowed the calculation of the stars' absolute magnitudes, i.e. the intrinsic luminosity.
The long story of Hipparcos
First proposed by the French scientist Pierre Lacroute to CNES in 1966, the Hipparcos mission benefited from a feasibility study at CNES before being selected by ESA in 1980.
The principle -and power- of the method was based on the simultaneous observation of two clearly separated (in those instances 58º) fields of view brought back on the same focal plane by an optical device. This device featured an image dissector and received the star images from the two fields which were then modulated by a highly regular grid.
Since the satellite was constantly spinning, the star images were in the focal plane. Moreover, the analysis of the received light emissions and their variations allowed the investigators to pinpoint the star positions within the grid and then to rebuild the relative positions of the stars with respect to all the other stars.
The satellite's spinning (axis and speed) were determined to cover the whole sky several times. Every star of the catalogue was thus observed on average 110 times over three years. The great many measurements of angular deviations between star couples provided at last a set of 4 millions equations with 600,000 unknown values which after their resolution provided the specifications (position, trigonometric parallax and proper motion) of each star.
Launched on August 8th, 1989 by an Ariane IV Rocket from Kourou, the satellite followed a geostationary transfer orbit after its apogee motor failed. The ESA's operational teams and the scientists achieved the juggling act of reconfiguring the mission in a few weeks time, to ultimately obtain better results than expected. The observations lasted until August 1993. Data were processed by two independent systems developed by two scientific consortia before being delivered to ESA. CNES played a major role in one of the two consortia.
The scientific impact of the Hipparcos' catalogue was huge for all branches of astronomy, from solar system study to cosmology, including fundamental physics and the study of our galaxy. Only a few examples are mentioned.
The definition of an accurate inertial reference frame is essential in astronomy. The International Celestial Reference France (ICRF) was initially defined based on radio observations which suffered little deteriorations by the atmosphere using a high-precision instrument, the Very Large Base Interferometer (VLBI). Nevertheless, recalibrating the observations in other wavelengths in relation to this frame of reference was not easy: for instance, the radio and optical photometric centroids of the supernova SN1987 seemed to present a 0.5 mas deflection.
The Hipparcos catalogue allowed the definition of the ICRF in optics and showed that the radio/optical deflection of SN1987 was the result of an error in the optical frame of reference. It is still a reference.
Thanks to Hipparcos, positions and orbits of 48 little planets of the solar system were pinpointed 10 times more accurately than ever. The measurements helped to predict, and thus observe in details, the impact of the Shoemaker-Lévy 9 comet on Jupiter in July 1994 (see next).
Even if the high-precision measurements were only for the stars at a distance of less than 500 light years (when the Milky Way Galaxy extends on 100,000 light years), the catalogue made the calibration of other long-distance measurement methods possible. It is the case for the Cepheids. These stars host luminosity periodic variations since period and intrinsic luminosity are linked. The distance could thus be deduced by measuring the period and apparent bright of a Cepheid. Hipparcos identified the period/luminosity relation of these stars and so revealed that the until then admitted distance scale in the universe had to be increased by 10 to 15 %.
An embarrassing contradiction existed between the estimated age of the Universe and the age of the globular cluster of our galaxy: these seemed older than the Universe! By showing that their distances and so their luminosity had been under-evaluated, Hipparcos solved the problem: these clusters are, in fact, younger than the Universe.
... but also application spin-offs
On August 8th, 1993, just like the Early Ages mariners, the star positions were again used for navigation... But this time into space: the Galileo probe, the operators of which had been organizing its meeting with the asteroid Ida using the Hipparcos' measurements.
Hipparcos: precursor for GAIA
The success of the first satellite dedicated to astrometry led the European science community and ESA to prepare its successor: starting from 2013, the GAIA mission will measure the positions of a billion objects over almost approximately half the galaxy with an accuracy of a few tens microarcseconds (less than 10 for the brightest!) It will also provide spectroscopic measurements on 200 millions moons.
- Astrophysics scientist at CNES: Olivier La Marle