The GAIA payload

Gaia is a cornerstone mission of the European Space Agency, approved with launch by end of 2012, but studied with a programmatic assumption of a launch in the 2010 time frame. Gaia will build an extraordinary precise 3-D map of about one billion stars throughout our Galaxy with an astrometric accuracy of about 10 μas (micro-arcseconds) for a star magnitude V = 15. The initial Gaia design was made in 1998 by Matra-Marconi Space, today EADS-Astrium (France) under ESA contract, awarded in open competition. The concept included two separate telescopes each having a very large focal plane made of about 250 CCDs. The target accuracy of 10 μas for a star magnitude V = 15 was shown to be achievable with available CCD technology and with an instrument envelope compatible with Ariane 5 launcher. In 2002, the initial design was deeply revisited by EADS-Astrium again under ESA contract in view of reducing the programme cost while preserving the science objectives. The reassessed concept still makes use of the same measurement principle but with some major modifications: the size of the instrument has been scaled down so as to enable a Soyuz-Fregat launch, and the two astrometric focal planes were superimposed by using a new optical configuration, therefore reducing the total number of CCDs from 500 to about 200. The paper describes the major evolution of Gaia payload.