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The HIPPARCOS (HIgh-Precision PARallax COllecting Satellite) mission was the first space experiment dedicated to Astrometry, and was accepted within the ESA scientific programme in 1980. The satellite was designed and constructed under ESA responsibility by a European industrial consortium led by Matra Marconi Space (France) and Alenia Spazio (Italy), and was launched by Ariane 4 on August 8th 1989 from the ground base of Kourou (French Guyana). The payload consisted of a small telescope (29 cm diameter) with the task of scanning the celestial sphere several times in order to obtain positions, parallaxes (i.e., distances) and proper motions of about 120,000 stars with magnitude lesser than 13 and a precision of about 2 milli-arcsecs.

The satellite was meant to reach a geostationary orbit at 36,000 Km from Earth; instead, due to a failure of the apogee booster ignition it was put on a very elliptic orbit with a perigee of 526 Km and an apogee of 36,000 Km. Because of this particular orbit, both scientists and flight technicians worked very hard to review critical mission parameters and the observation strategy in order to make this mission successful.  

Therefore, the number of ground stations for data acquisition was increased. Moreover, because of such a low perigee, the satellite encountered several problems, such as periodical crossing of the van Allen belt, resulting in possible deterioration of the solar panels, multiple solar eclipses phenomena, and more frequent occultations of the celestial sphere by the Earth. Notwithstanding these problems, HIPPARCOS started collecting scientific data on November 26, 1989, and it continued - even with sporadic interruptions - until August 15, 1993.  

In this 4-year period, adding to a total of 37 months of scientific data acquisition, HIPPARCOS has been capable of producing positions, proper motions and parallaxes of 118,218 stars with an accuracy which is comparable to the angular dimension of the metallic star on top of the Mole Antonelliana monument in Torino, as it would be seen from the Moon.  

In parallel, the satellite HIPPARCOS has produced a second catalogue named TYCHO containing high accuracy photometric data (about 2%) of 1,058,332 stars.  

HIPPARCOS has also observed 23,882 double/multiple stars, which are now part of an ad hoc catalogue, which contains physical and astrometric parameters of these objects (Söderhjelm S. et al., 1997, ESA SP-1200, vol. 10). 

Furthermore, it has produced a catalog of photometric data for 11,597 variable stars.

A first HIPPARCOS scientific result, which is of fundamental importance to both astrophysics and cosmology, is the recalibration of the distance of stars and globular clusters. Being the latter farther away from us, they are intrinsically more luminous and therefore younger than what it was thought before. Using this findings it was possible to correct the age of the Universe from 15-20 billion years to 12 billion years only!  

Another relevant scientific result obtained with the HIPPARCOS data is the discovery of a slight distortion of the Galactic plane. In other words, the stars revolve in a curved plane around the Galactic center (Smart R.L. et al., 1997, ESA SP-402, 715-720 and Smart R.L. et al., 1998, Nature, 392, 471-473). This phenomenon, called Galactic Warp, is probably due to the gravitational effect of dark matter present in the Galactic Halo.  

The Astrometric Group of OATo has actively participated to the accomplishment of this mission, in collaboration with other Italian and European Institutes, with main contributions in the following areas:  


realization of the celestial sphere model for the determination of the stellar astrometric parameters;


realization of a catalogue of known and newly discovered double and multiple stellar systems;


scientific utilization of the satellite data, with results published in the astronomical literature.

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© OATo - Morbidelli, Sarasso 2002