It will conduct a census of a billion stars in our galaxy, monitoring each of its target stars about 100 times over a five-year period, precisely charting their distances, movements, and changes in brightness. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and failed stars called brown dwarfs. Within our own solar system, Gaia should also identify tens of thousands of asteroids.
Additional scientific benefits include detection and characterisation of tens 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 about 10 million galaxies and 500,000 distant quasars. It will also provide stringent new tests of general relativity.
The spacecraft will use the global astronomy concept successfully demonstrated on Hipparcos, also built by Astrium, which successfully mapped 100,000 stars in 1989. Gaia will be equipped with a latest-generation payload integrating the most sensitive telescope ever made. This cutting-edge technology draws on Astrium’s extensive experience particularly on silicon carbide (SiC) telescopes, used on the Herschel telescope and Aladin instrument as well as on three Earth observation satellites (Formosat, Theos and Alsat 2). Gaia’s measurement accuracy is so great that if it were on the Moon, it could measure the thumbnail of a person on Earth!
Gaia will be placed in orbit around the Sun, at a distance of 1.5 million kilometres further out than Earth, at the L2 Lagrangian point of the Sun–Earth system.
The Lagrange points are points in space at which a body can remain fixed in relation to two other bodies. Joseph-Louis Lagrange determined a five-point position for the Sun-Earth system, at which solar attraction and terrestrial attraction are precisely offset by the centrifugal force induced by Earth's movement round the Sun. A satellite placed in orbit in relation to one of these points rotates round the Sun at the same speed as Earth, and is consequently fixed in relation to the two stars.
By convention, the Lagrange points are denominated L1 to L5.
Of the five Lagrange points, only L4 and L5 are stable. This means that matter tends to accumulate at these points. The other points, such as L2, are consequently unstable and little perturbation is required for them to move out of position. It is for this reason that the two satellites will describe Lissajous orbits round L2 so as to minimise their fuel consumption.
The choice of point L2 is explained by the fact that the satellites will be protected from the Sun by Earth due to their alignment, thus providing excellent conditions for astronomic observation purposes. Furthermore, in the case of Herschel, the instruments carried by the satellite will not be perturbed by the strong infrared emission from Earth and the Moon, nor will its observations be perturbed by the Earth's radiation belts. As for Planck, this satellite will thus avoid emission from Earth, the Moon and the Sun, such as could otherwise perturb the CMB (Cosmic Microwave Background) radiation signal.
Space missions essentially use the L1 and L2 points:
SoHO (Solar and Heliospheric Observatory) has been positioned at the L1 point, 1.5 million km from Earth (between the Earth and the sun) since 1995.
The L2 point, 1.5 million km from Earth on the opposite side from L1, is particularly well adapted for observing the cosmos. Planck Surveyor and Herschel, are positioned at the L2 point, as will the James Webb Space Telescope. By the way: it takes about 10 seconds for Herschel to communicate with Earth (two-way).