A billion stars in sight

What is our galaxy made of? How did it form and evolve? GAIA, for Global Astrometric Interferometer for Astrophysics, will help us address these and other questions. Initiated by the European Space Agency (ESA), the satellite was successfully launched yesterday.

The mission’s primary goal is to meticulously scan the sky and survey more than one billion stars in our galactic neighborhood – representing only about one percent of the whole stellar population. Among many other goals, it will also study the Kuiper belt, observe Venus and its moons, track thousands of asteroids, comets and other space objects, look for brown dwarfs and search for planets in other solar systems. GAIA will thus collect a huge amount of data and build a precise 3D map of the Milky Way.

The satellite will take four months to reach its definitive destination, an orbit called L2, or one of the Lagrangian points. This is a position located at 1, 5 million kilometers from our planet where gravity forces of two large objects – here the Sun and the Earth – combine in such a way that it gives a smaller third object the centripetal force required to stay in orbit. Practically, GAIA will turn around the Sun, accompanying the Earth and staying constantly in its shade.

“This orbit is very particular and interesting, says Federico Belloni, scientific assistant at the Swiss Space Center. It is the best place to observe the Universe, because the sunlight and the Earth perturbations are minimal. The main challenge is to bring a satellite to such a remote place. It needs a lot of power and some orbital transfers and trajectory corrections that are not easy to implement.”

To accomplish its mission, Gaia is armed with two highly performing telescopes, able to see a single hair from a thousand kilometers away. With a billion pixels each, its cameras are a hundred times more powerful than standard ones. It also uses innovative and particularly resistant materials, guaranteeing tremendous stability.

The color tells the age

“Precision is the keyword in this mission,” points out Christophe Paccolat, also a scientific assistant at the Swiss Space Center. “To maximize the remarkable performance of the instruments, the satellite definitely needs to be as stable as possible, in terms of position and temperature.” That why the satellite is equipped with a large shield, measuring ten meters in diameter, that will protect it against the Sun’s light and radiation and cool down its instruments. “This shield is a central piece of the satellite, but it also is very fragile,” adds Federico Belloni. “Spreading it out will be a very delicate operation.”

During a five-year period, the satellite will measure the position and velocity of the stars and then observe how these change over time. Gaia will also capture and calculate the color, or wavelength, that each one reflects. Since a star's luminosity and color vary as it passes through different evolutionary stages, this information will reveal their age and deliver important clues on the galaxy’s genesis.

“This is a unprecedented approach,” comments Christophe Paccolat. “While the Hubble telescope offers pictures of the deep Universe as a whole, Gaia will study individual stars by focusing on them and scanning them one after another in detail.”