Launched from Kourou on the 19th of December 2013, Gaia has been, since March 1st 2014, in orbit around the L2 Lagrange point, a million and half km from the Earth, on the Sun-Earth line.
During the last eight months, all parts of the service module and the payload of Gaia have been run, tested, calibrated and validated.
During this commissioning phase, many tests and calibration runs have been carried out, to improve not only the scientific return of the satellite, but also to circumvent a number of unexpected problems, such as the detection of light pollution from the Sun and from bright celestial objects, and the frosting up of the mirrors.
A complete review of all the work done at this stage took place on July 18th 2014; as a consequence, scientific observations were able to start.
These opened on July 25th 2014: for 28 days, observations consisted of scanning along all the great circles crossing the ecliptic poles. During this period, the data obtained was calibrated by comparing them with many ground based observations obtained previously for just this purpose.
Since August 22nd, the spinning satellite has been scanning the sky continuously, at a speed of 60 seconds of arc per second of time. It takes 6 hours to do a full great circle on the celestial sphere, and it will take about 6 months to do the whole sky.
After the launch, the good news
The first good news was that injection into the L2 point was completely successful, leaving a large fuel margin for future manoeuvres. Thanks to this fuel reserve, one can envisage an extension to the length of the mission.
The system which controls the satellite spin is working perfectly, and, in particular corrects with no difficulty the effects of micrometeoritic bombardment. Gaia’s antenna and the system for receiving and transmitting data are working perfectly.
The key elements of the payload are working perfectly: the 106 CCD and their associated electronics; the 7 onboard computers which control the CCD; the data acquisition system, the on-board data handling and storage system; the telescopes, whose excellent image quality was confirmed after alignment and focusssing; the spectrophotometer and the spectrograph, whose resolution are as per specifications.
As far as observations are concerrned, a first class improvement is the new possibility to observe the 6000 brightest stars in the sky. The parameters of the on-board software have been modified so as to lower the threshold from magnitude 6 to magnitude 3.
Furthermore, a special observation mode is being developed, so that the 230 stars brighter than magnitude 3 can also be observed. The astrometric accuracy expected for these very bright stars is on the order of a few tens of microseconds of arc.
A few surprises
The luminosity of the Gaia satellite itself is significantly smaller than was expected before the launch. Gaia’s orbit must be known very accurately: 150 m for its position and 2,5 mm/s for its motion. Classic radar monitoring is inadequate for this level of precision, and additional ground based observations are required. The system thus had to be completely reviewed before lauch, in order to guarantee the availability of somewhat larger telescopes (on the order of 2 m diameter mirrors) as well as very long base line radio interferometric (VLBI) observations. This is all now under control.
Rather more ice than was foreseen, coming from water trapped in the satellite before launch, condensed on certain mirrors, It evaporates when the mirrors are heated. This decontamination procedure was foreseen (the mirrors are equiped with heaters), but it was not thought that it would have to be repeated. This has already be done three times, and will surely have to be done again during the coming months, but this time choosing which mirrors to treat. After each such operation, the mirrors regain their normal reflectivity.
Undesirable parasitic light has been observed : its intensity is variable, and depends on position in the focal plane. It is clearly correlated with the motion of the satellite itself. Two sources have been identified : the Sun and the brightest objects in the sky. Their impact is completely negligible as far as the brightest sources are concerned, but the observations, and in particular spectrophotography, of dim sources are affected. The on-board software is in the process of being optimized, in order to reduces as much as possible the effects of this background noise.
Finally, the angle between Gaia’s two base mirrors is a key parameter of the instrument. It varies with temperature changes induced by the rotation of the satellite. So that these changes can be properly taken into account in the data analysis, this angle is regularly measured by the «base angle monitor», an extremely accurate interferometric device. The variations are larger than foreseen, and special care is taken to monitor and calibrate them, in order that they can be eliminaed as far as possible during the data analysis.
Mission Objectives
The objective of Gaia is to obtain a 6D description of the whole Galaxy, including the physical characterization of a very large variety of its bodies.
To do this, Gaia has has an instrument with three functions: astrometry, photometry and spectroscopy, each making use of the three large mirrors on the satellite.
During its cointinuous scan of the sky, Gaia will measure systematically all objects brighter than magnitude 20 for at least 5 years: positions, distances, motions on the sky and spectrophotometry for a billion objects (essentially stars, but also asteroids, QSOs and galaxies), radial velocities for 150 billion objects, and even a detailed description of the chemical composition of the atmospheres of the 5 million brightest ones.
Such a systematic observation of the sky, giving not only the positions of objects along the line of sight and their motions, as well as characterizing their physical properties and their ages,, will lead to a very detailed description of our galaxy, the Milky Way, of its different populations, of all the kinds of stars of which it is made, in all the possible stages of their evolution, even the most rapid ones.
Gaia will also lead to the discovery of many exoplanets, of many variable stars and supernovae, of many asteroids.. Finally, its unprecedented astrometric accuracy will open the door do new tests of general relativity.
25 years after the Hipparcos mission
With a wink of the historical eye, the scientific observations of Gaia will begin, give or take a few days, just 25 years after the launch of the Hipparcos satellite, which took place on the 8th of August 1989.
Based on an idea of Pierre Lacroute, at that time Director of the Strasbourg Observatory and thanks to the audacity of the European Space Agency, Hipparcos was the first - and until Gaia’s launch, the only – astrometric satellite.
It led to a quantitative and qualitative leap in astrometric measurements: 118 000 stars were observed for 3,5 years, and of these the distance of 30 000 are now known to an accuracy better than 10%. Before this, this was so for less than 1000 stars.
This data produced an unprecedented 3D description of the solar neighbourhood, and has been used in numerous branches of astronomy and astrophysics, ranging from reference systems and fundamental physics to stellar and galactic physics, not forgetting the comic distance scale and the orbits of small Solar System bodies