TYCHO/HIPPARCOS A Successful Mission! First Results and Further Improvements

Abstract

The astrometric satellite HIPPARCOS has been launched by an Ariane IV rocket early in the morning of 9-August-1990. The intention of this mission to measure very accurate positions (2 milli-arcsec), trigonometric parallaxes and annual proper motions for about 118,000 selected stars seems to become impracticable after the failure of the apogee boost motor. However it turns out that the difficulties related with the highly elliptic geostationary transfer orbit can be mostly overcome with the socalled ’revised mission’. The degradation of the solar cells and the electronics due to the particles of the van Allen belts have been shown to be much less than even optimistic specialists stated during the first weeks after launch (see for example Jahreiß [1]). With a now expected lifetime of at least three years it is possible to get almost the same quantity and quality of input and output data as expected for the nominal mission. The additional TYCHO project will result in photometric and astrometric data for at least 500,000 possibly up to 1.000.000 stars in the range of 2 ≲ mag _B ≲ 11. This is even more than expected before launch and the values given in [1] for the revised mission are fortunately completely outdated.