Gaia is an ambitious mission to chart a three-dimensional map of our Galaxy, the Milky Way, in the process revealing the composition, formation and evolution of the Galaxy. Gaia will provide unprecedented positional measurements for about one billion stars – about 1 per cent of the Galactic stellar population – in our Galaxy and Local Group, together with radial velocity measurements for the brightest 150 million objects.
Combined with astrophysical information for each star, provided by on-board multi-colour photometry, these data will have the precision necessary to quantify the early formation, and subsequent dynamical, chemical and star formation evolution of the Milky Way Galaxy.
Additional scientific products include detection and orbital classification of thousands of extra-solar planetary systems, a comprehensive survey of objects ranging from huge numbers of minor bodies in our Solar System, through galaxies in the nearby Universe, to some hundreds of thousands of distant quasars. It will also provide a number of stringent new tests of general relativity and cosmology.
Gaia was adopted within the scientific programme of the European Space Agency (ESA) in October 2000. The mission aims to:
- measure the positions of about 1 billion stars both in our Galaxy and other members of the Local Group, with an accuracy down to 24 microarcseconds
- perform spectral and photometric measurements of all objects
- derive space velocities of the Galaxy's constituent stars using the stellar distances and motions
- create a three-dimensional structural map of the Galaxy
The gathered large datasets will provide astronomers with a wealth of information covering a wide range of research fields: from Solar System studies and galactic astronomy to cosmology and general relativity.
Gaia originally stood for Global Astrometric Interferometer for Astrophysics. As the project evolved, the single interferometer concept was replaced by a new payload design. The mission name remained, however, even though it no longer reflects the methods used to perform the science operations.
The Gaia spacecraft consists of three major functional modules: the payload module, the mechanical service module, and the electrical service module. It carries a single integrated instrument that comprises three major functions: astrometry, photometry, and spectrometry. The three functions use two common telescopes and a shared focal plane, with each function having a dedicated area on the large 0.5 m × 1m CCD detector array.
The payload consists of a single integrated instrument the design of which is characterised by:
- A dual telescope concept, with a common structure and a common focal plane. Both telescopes are based on a three-mirror anastigmat (TMA) design. Beam combination is achieved in image space with a small beam combiner.
- Silicon-carbide (SiC) ultra-stable material is used for mirrors and telescope structure
- A highly robust measurement system for the Basic Angle between the two telescopes' pointing directions
- A large common focal plane with an array of 106 CCDs. The large focal plane also includes areas dedicated to the spacecraft's metrology and alignment measurements
- Three instrument functions:
Astrometry accurate measurements, even in densely populated sky regions of up to 3 million stars per square degree Photometry continuous spectra in the band 330-1050 nm for astrophysics and chromaticity calibration of the astrometry Spectrometry high resolution, grating, narrow band: 845-872 nm
Gaia operates in a Lissajous-type orbit, around the L2 point of the Sun-Earth system, which is located 1.5 million km from the Earth in the anti-Sun direction. The orbit is not impacted by Earth eclipses. The orbit period is about 180 days and the size of the orbit is typically 340 000 × 90 000 km. An operational lifetime of 5 years is planned.
Science operations are conducted from the European Space Astronomy Centre (ESAC, Villafranca del Castillo, Spain).