Gaia (or Global Astrometric Interferometer for Astrophysics) is a European Space Agency (ESA) astrometry space mission, and a successor to the ESA Hipparcos mission. Arianespace expects to launch Gaia for the ESA in March 2013, using a Soyuz rocket from CSG. It will be operated in a Lissajous orbit around the Sun-Earth L2 Lagrangian point for a planned 5 year mission.
Gaia will compile a catalogue of approximately one billion stars to magnitude 20. Its objectives comprise:
- astrometric (or positional) measurements, determining the positions, distances, and annual proper motions of stars with an accuracy of about 20 µas (microarcsecond) at 15 mag, and 200 µas at 20 mag
spectrophotometric measurements, providing multi-epoch observations of each detected object
radial velocity measurements.
Gaia will create an extremely precise three-dimensional map of stars throughout our Milky Way galaxy and beyond, and map their motions which encode the origin and subsequent evolution of the Milky Way. The spectrophotometric measurements will provide the detailed physical properties of each star observed, characterising their luminosity, effective temperature, gravity and elemental composition. This massive stellar census will provide the basic observational data to tackle a wide range of important problems related to the origin, structure, and evolutionary history of our Galaxy. Large numbers of quasars, galaxies, extrasolar planets and solar system bodies will be measured at the same time.
Gaia will also be capable of discovering asteroids with orbits that lie between Earth and the Sun, a region that is difficult for Earth-based telescopes to monitor since this region is only in the sky during or near the daytime.
Gaia will be launched on a Soyuz-FG rocket and will fly to the Lagrange point L2 located approximately 1.5 million kilometers from Earth. The L2 point will provide the spacecraft with a very stable thermal environment. There it will describe a Lissajous orbit which will avoid eclipses of the Sun by the Earth, which would otherwise limit the amount of solar energy the satellite can retrieve through its solar panels and also disturb the thermal equilibrium.
Similarly to its predecessor Hipparcos, Gaia consists of two telescopes providing two observing directions with a fixed, wide angle between them. The spacecraft rotates continuously around an axis perpendicular to the two telescopes' lines of sight (LOS). The spin axis in turn has a slight precession across the sky, while maintaining the same angle to the Sun. By precisely measuring the relative positions of objects from both observing directions, a rigid system of reference is obtained.
Each celestial object will be observed on average about 70 times during the mission, which is expected to last 5 years. These measurements will help determine the astrometric parameters of stars: 2 corresponding to the angular position of a given star on the sky, 2 for the derivatives of the star's position over time (motion) and lastly, the stars parallax. The radial velocity of the star is measured using the Doppler Effect by a spectrometer, which is integrated into the Gaia telescope system.
The Gaia payload consists of
- a 1.4 x 0.5 square metre primary mirror for each telescope
A 1.0 x 0.5 m focal plane array on which light from both telescopes is projected. This in turn consists of 106 CCDs of 4500 x 1966 pixels.
Gaia contains 3 separate instruments:
- The astrometry instrument (ASTRO), which is dedicated to measuring the angular position of the stars of magnitude 5.7 to 20.
The photometric instrument, which allows the acquisition of spectra of stars over the 320-1000 nm spectral band, over the same magnitude 5.7-20.
The high-resolution spectrometer to measure the radial velocity of the stars by acquiring high-resolution spectra in the spectral band 847-874 nm (field lines of calcium ion) for objects up to magnitude 17 ,
The telemetric link with the satellite is about 1 Mbit/s on average, while the total content of the focal plane represents several Gbit/s. Therefore only a few dozen pixels around each object can be downlinked. This means that detection and monitoring of objects on board is mandatory. Such processing is particularly complex when scanning dense stellar fields.
The Gaia space mission has the following objectives:
- To determine the intrinsic luminosity of a star requires knowledge of its distance. One of the only ways to achieve without physical assumptions is through the star's parallax. Ground-based observations would not measure such parallaxes with sufficient precision due to the effects of the atmosphere and instrumental biases.
Observations of the faintest objects will provide a more complete view of the stellar luminosity function. We must observe all the objects up to a certain magnitude in order to have unbiased samples.
You need a large amount of objects to examine the more rapid stages of stellar evolution. Observing a large number of objects in the galaxy is also important in order to understand the dynamics of our galaxy. Note that a billion stars represents less than 1% of the content of our galaxy.
Measuring the astrometric and kinematic properties of a star is necessary in order to understand the various stellar populations, especially the most distant.
Gaia is expected to:
- Measure the astrometric properties of over a billion stars down to a magnitude of V = 20
Determine the positions of stars at a magnitude of V=10 down to a precision of 7 millionths of an arcsecond (μas) (this is equivalent to measuring the diameter of a hair from 1000 km away); between 12 and 25 μas down to V = 15, and between 100 and 300 μas to V = 20, depending on the color of the star
Determine the distances to the nearest stars within 0.001%, and to stars near the galactic center, 30,000 light years away, within 20%
Measure the tangential speed of 40 million stars to a precision of better than 0.5 km/s
Measure the orbits and inclinations of a thousand extrasolar planets accurately, determining their true masses
Among other results relevant to fundamental physics, Gaia will follow the bending of starlight by the Sun’s gravitational field, as predicted by Albert Einstein’s General Theory of Relativity, and therefore directly observe the structure of space-time.>>