Gaia is a global space astrometry mission, and a successor to the ESA Hipparcos mission. Part of ESA’s long-term scientific programme, the spacecraft has been built by Airbus Defence and Space and has been launched on 2013 December 19 on a Soyuz vehicle.
Gaia has been placed in orbit around the Sun, at a distance of 1.5 million kilometers further out than Earth, at the L2 Lagrangian point of the Sun–Earth system.
Gaia's resulting scientific harvest is of almost inconceivable extent and implication.
It is expected to discover hundreds of thousands of new celestial objects.
Within our own solar system, Gaia should identify tens of thousands of asteroids and comets.
Far beyond the solar neighbourhood, some 20,000 Jupiter mass exoplanets should be discovered by the time it completes its survey.
It is 10 times the number of the known exoplanets when Gaia was launched.
Thanks to Gaia's unprecedented measurement precision, our galaxy will be mapped, for the first time, in 3D.
It will conduct a census of a billion stars in our galaxy, monitoring each of its target stars about 80 times over a 5-year period, precisely charting their distances, movements, and changes in brightness.
It will provide detailed information on stellar evolution and star formation.
It will clarify the origin and formation history of our galaxy.
Additional scientific benefits include a comprehensive survey of objects ranging from 1 million galaxies in the nearby Universe, through 500,000 distant quasars.
It will also provide stringent new tests of general relativity and a detailed mapping of the galactic dark-matter distribution.
The spacecraft uses the global astronomy concept successfully demonstrated on Hipparcos, also built by Airbus Defence and Space, which successfully mapped 100,000 stars after its launch in 1989.
Gaia is equipped with a latest-generation payload integrating the most sensitive telescope ever made.
This cutting-edge technology draws on Airbus Defence and Space’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!
The commissioning of Gaia came to its formal end on 18 July 2014 when the board members of the mission in-orbit commissioning review confirmed the readiness of the space and ground segments to start routine operations.
The review board took note that, considering the current performances, the vast majority of the expected Gaia science objectives will be achieved.
11 months after its launch, Gaia's eye had already observed 10 billion transits leading to a collection of 100 billion astrometric images, 20 billion photometric images and more than 3 billion spectroscopic images.
Besides additional improvements, Gaia has already proven its ability to reach in 1 day the astrometric performance (1 milliarcsecond) that its predecessor Hipparcos performed after its entire 4-year mission.
Moreover, Gaia demonstrated how incredibly sensitive it is, being able to discern the 240 billion-year galactic rotation period with only 3 months of stellar position measurements.
Gaia has also discovered by the end of August its first supernova, a stellar explosion in another galaxy far, far away … some 500 million light-years away.
The scientific community is eagerly waiting for the first intermediate catalogue of stars which is expected in the summer of 2016.
It has already recognized all the fantastic work ESA and Airbus Defence and Space put into building, launching, and commissioning Gaia.
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).