2024-03-29 12:56:21
By François Mignard
Astronomer at the Côte d’Azur Observatory, emeritus research director at the CNRS, member of the Academy of Sciences and the Academy of Air and Space. He participated in the Hipparcos mission and, since its origin, in the Gaia adventure. He is a member of the Gaia Science Team of the European Space Agency (ESA).
In 1967, the French astronomer Pierre Lacroute (1906-1993), director of the Strasbourg observatory, had the idea of placing a small space telescope outside the atmosphere intended for the precise measurement of position and movement. stars. Identifying stars and building catalogs (lists of sources, organized by position in the sky or by physical properties of objects) is almost as old as astronomy itself, either as a pure science approach or because of strategic issues. and economical through navigation techniques on the high seas.
Today it is more regarding space navigation, but above all knowledge of the world of stars and galaxies, going well beyond simple census. Astronomers have always perfected their measuring instruments, improved the sights, collected more light with larger mirrors or larger lenses at the end of their glasses, but fundamentally the optical tube and its integral frame of graduated circles remained the tool to locate the position of the stars.
An instrumental revolution
And now Pierre Lacroute proposes an instrumental revolution by suggesting a completely new principle, which would upset the habits of astronomers and call into question their certainties. He proposed to them not only to place themselves outside the atmosphere (which we can understand ten years following the launch in 1957 of the first satellite, Sputnik 1), but above all to carry out simultaneous observations in two directions to measure angles separating two stars in the sky.
The advantage was that these angles would be much wider than what was permitted with the narrow field of view of a conventional telescope or refractor. The National Center for Space Studies (Cnes), the French space agency, positively evaluated the idea, but it was not immediately followed up due to the costs, the somewhat isolated aspect of the proposal compared to the community concerned, and probably also because it is too new.
However, good ideas never completely die and Pierre Lacroute’s anticipation was revived around 1980 with the Frenchman Jean Kovalevsky as the main advocate and the support of the European Space Agency (ESA). This will lead to the almost miraculous approval of the Hipparcos satellite supported by a community of astronomers aware of the contribution of fundamental distance measurements to approach the physics of stars and those of movements for the study of the structure of the Milky Way.
A historic activity is reborn
This mission, launched in 1989, aimed to carry out very high precision mapping of the celestial sphere by observing approximately 120,000 carefully selected stars (well distributed over the entire sky and well sampled in color, temperature and brightness) with precise in position of 0.002 arcsecond (angle at which we would see an ordinary car on the Moon), or 50 times better than the best position measurements achievable with specialized instruments on the ground.
Space Europe thus put very significant resources into carrying out in space what astronomers had done for centuries on Earth – which in 1980 was no longer considered as part of current science. Indeed, participating in planetary exploration, major advances in cosmology and the discovery of galaxies was much more attractive for young astronomers.
Finally, in the international competition for access to space, astronomers and the selection committees of the space agency, and first and foremost its scientific director, decided that the stakes were important for science and that the Modern techniques would revive this historic activity of astronomy consisting of locating stars and especially estimating their distances.
This was the major challenge of the mission chosen by ESA. Going from a batch of regarding 3,000 stars whose distance was reasonably known to nearly 100,000 in a few years was a giant leap, whereas it took a hundred and fifty years for the first 3,000. Despite a very big problem during the launch, and an announced failure, the Hipparcos mission observed the sky for three years and was a resounding success in the scientific world. The risk had paid off and had given space Europe a technical lead which it was able to capitalize on.
Measuring the thickness of a hair at 1,000 km
Mastering this new technique for carrying out global astrometry (observation of large angles over the entire sky, as opposed to the successive paths of classical astrometry in small fields) and aware of its founding role for the entire astrophysics, European astronomers have united around a new large-scale space project with the ESA.
In September 2000, three years following the publication of the Hipparcos catalog, the ESA selected the Gaia project as the cornerstone of its scientific program, for a launch at the end of 2011, which will finally take place in December 2013. The central objective was to detect nearly of 2 billion stars in the Milky Way and to give their very precise position, but also their distance from the Sun, as well as the characteristics of their movement.
Stars observed with the naked eye appear to occupy unchanging positions relative to each other over the course of a human lifetime. More exact measurements established from the beginning of the 18th century that over time small changes are clearly visible and would end up over thousands of years significantly modifying the shape of the constellations and breaking the alignments familiar to any diligent observer of the sky. .
Thus, for Gaia to be able to detect the movement of stars, it is necessary to determine their position on different dates, typically every two months during the life of the mission. To understand the height of the challenge, the precision with which the stars must be placed on the sky is equivalent to the angular dimension of the thickness of a hair placed at 1,000 km. Impossible to do this from the ground because of the degradation of the images by the atmosphere.
Even for the star with the largest annual movement (around 1,000 times faster than that of an average star), this can only be observed with meticulous measurements. When we talk regarding speed of movement in this context, we are not talking regarding kilometers per hour or meters per second, but changes in direction which correspond to angles and the speed becomes a fraction of an arc second per year.1.
In principle, this movement might be easily measured… by letting time pass. If a good quality catalog of stars produced 10,000 years ago fell into our hands, it would be easy to see differences in positions with current positions and to deduce the value of the movement per year, by doing the hypothesis of its regularity.
But it does not exist, and even the catalogs of Greek or Chinese astronomers are of little use: they are not precise enough to reconstruct the movement over 2,000 years and they only concern the thousand stars visible at the naked eye of a given place. Gaia, in a few years of observation, has done much better than the comparison of two catalogs which would have been established for example, one in 1900, the other in 2000, with the best techniques of their respective eras.
The most productive space mission
Gaia is much better equipped than her predecessor Hipparcos to scan the sky. It is equipped with a photometer to measure the brightness of stars and collect information on the distribution of their radiation in different wavelengths, as well as a spectroscope which provides the speed of the stars on the line of sight (approximation or distance) as well as details of their chemical composition.
This information is very valuable to the global astronomical community, due to the fundamental nature of the data collected, the precision of the measurements, the number of objects involved and the presence of all categories of stars (up to magnitude 20.7, i.e. a luminosity 700,000 times lower than that of the last stars visible to the naked eye).
The space instrument works hand in hand with a gigantic ground processing program, preparation for which began in 2006 with the training of a group of 450 European researchers and engineers within a dedicated consortium. This has already carried out three deliveries of results to the scientific world, covering increasingly large volumes at each stage.2. Astronomers around the world use these results freely and have published more than 10,000 articles in specialized journals, making Gaia the most productive space mission in terms of number of annual publications.
Scientific applications, which completely call into question the history of the formation of the Milky Way, range from objects in the solar system to exoplanets, outer galaxies and gravitational phenomena, including one of its major objectives: the structure, the genesis and evolution of our galaxy and its different components, over several tens of millions of years. The next delivery, the fourth, will take place in the first half of 2026 and is eagerly awaited.
The observation mission and the active life of the satellite will end at the beginning of 2025, when the gas necessary for its piloting runs out. Since July 25, 2014, it has sent an average of 600 million elementary observations every day covering 70 million stars detected as they pass in front of its two telescopes which scan the sky in a perfectly choreographed dance since day one.
Around 2030, the consortium will deliver its final catalog and the mission will truly be complete. It is therefore a great success for space Europe, a field of excellence recognized worldwide and which will probably have a continuation in around twenty years.3.
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