Building on the unique experienced gained with the Hipparcos mission, and using the same principle of measurement to obtain absolute parallaxes, the Gaia satellite project intends to extensively map the Galaxy in positions and velocities, covering more than one billion objects down visual magnitude 20. As Hipparcos 30 years earlier, the Gaia project is ambitious and will be ground breaking in many areas (Luri et al. 2004; Bailer-Jones 2005). It will affect nearly all aspects of astronomy, from solar-system studies, through stellar structure and evolution, galactic dynamics and reconstruction of the history of our Galaxy, to fundamental physics and cosmology.
The current Hipparcos study has already been contributing to the data processing approach for the Gaia mission, testing and applying aspects of the iterative astrometric solution planned for Gaia. In this section various instrumental and data processing aspects of the Hipparcos and Gaia missions are compared, with an emphasis on where parallels can assist in solving the rather large data processing task for Gaia. We start with a brief summary of the instrument and mission plan, and then look at the various data processing challenges ahead.
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