A kind of card IGN of a small portion of the sky

In a few hours, the Corot satellite will left land burdens to beat to the rhythm of the stars. Sacred relief for the parents of the orbiting telescope which, since 1984, live a way of cross. COROT is one of the might of the vicissitudes of the CNES, its master programme.

Originally, astronomers had designed the satellite around the only measurement of stellar seismology mission. To strengthen the chances of the program, his supporters added him along the way a more mission, identification of extrasolar planets. "In the late 1990s, these planets have become very fashionable", rit today Annie Baglin, scientific head of the mission to the Observatory of Paris, this scheme "very marketing" in which scientists have more use. Since then, about 15 missions and dozens of terrestrial projects captured the subject. The scientific head of the mission is not fully the Woods. II it is a test, the launch Soyuz 2 b, the more stressful perhaps (see box).

Once in orbit, and after a few weeks of warm-up, Corot will open the cache which protects her eye and will begin his two and a half years of observation. Stethoscope of the stars, Corot will record before their pulse. Twenty years ago, they realized that the light of the sun beats following millions of oscillation modes. Cracked plate sounds differently from another, a star, according to its chemical composition, according to the behavior of ionized gases (convection and rotation for example). This small music that plays gas agitation is a beautiful gift to astronomers. These changes in brightness are actually only exploitable signals of the internal activity of the stars. This is the opportunity to dissect these corps known until here the skin. New physical phenomena may emerge in.

The antenna of Corot will patiently follow a sample of stars, or rather two samples. Because the "modesty" of the budget of the mission (160 EUR) prohibits him from venturing beyond the Earth's suburbs. The satellite will turn to 900 km above the Earth on a polar orbit, the only one whose plan is fixed relative to the stars. COROT should change direction of sight every six months to turn its back on the Sun and to avoid glare. That is why, in the summer, it will point to the center of the milky way, our Galaxy, and the anticentre in winter.

Map IGN of the sky

Annie Baglin explains that the target of the seismic observations has been voluntarily limited to 100 stars: "the objective is not to much but long follow." Five periods of continuous 150 days will be recorded, lasting impossible to obtain a terrestrial observatory, from which can monitor an area from the sky during two to three months, during the night. The light will be analysed on two of the four sensors to charge transfer (CCD). More than their resolution of 2.048 x 2.048 pixels each, they are distinguished by their sensitivity the lights of the stars are a hundred billion times less powerful than the Sun. By reliability also "usual cameras include many dead pixels", said Pierre Bodin, head of the instrument.

The other two sensors scruteront with more patience still a field of 100,000 stars with the hope of detecting small planetary systems (a star and its planets). It was not until 1995 to observe (indirectly) the first planet outside our solar system. According to the inventory of the French Jean Schneider, to have been recorded 209 at the present time. Only the larger planets soft, the size of Jupiter, could be listed. Astronomers now want to hold on to their hunt for exoplanets of smaller table, in particular of the soil body (not gas), ideally an ersatz of land which could rule the life...

To justify their research, scientists less explicitly brandish the fantasy of the alien, who will remain in the background of Corot, his instrument being inadequate to exobiology. The telescope is a Scout of the future science of the 21st century by identifying a few beautiful specimens to examine later. A kind of card IGN of a small portion of the sky.

The satellite will use the method of the transits. Its sensors register not the planet itself it emits no light. They identify the variation in brightness when it passes between the star and Corot. The discretion of the phenomenon (one-tenth of the luminosity of the star) and the probability of being on the right axis greatly reduces the chance to attend. Statistically, the researchers hope to tens of small planets through the monitoring of the 100,000 stars.

The Corot mission has shown in more than twenty years its qualities of Marathon. It must now shine in the sprint. Because, in the meantime, Nasa launched Kepler, a late cousin who started his delay. Its launch is planned in 2008 and the American promise "the heavy". The one-tonne probe will leave the Earth's orbit, last ten years and inventory hundreds of exoplanets. The trump card: a lower noise level. But Corot may yet him grilling the priority of the first publications: "We have another two years in advance to play," warns Annie Baglin.