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The eye of Gaia

Billion-pixel camera

The largest digital camera ever built for a space mission has been painstakingly mosaicked together from 106 separate electronic detectors. The resulting ‘billion-pixel array’ will serve as the super-sensitive ‘eye’ of the European Space Agency (ESA) galaxy-mapping Gaia mission.

At its Toulouse site, Airbus Defence and Space is making final adjustments to the scientific instruments for the Gaia satellite, which from 2013 will start mapping more than a billion stars in the Milky Way. As well as drawing up a 3D map of our galaxy, this mission will also verify Einstein’s general theory of relativity with greater precision than ever before.

In mid-September, Vincent Poinsignon witnessed the culmination of years of work. He had before him the largest digital camera ever built, with a total of a billion pixels. Vincent, who heads up the Gaia project at Airbus Defence and Space, maintains that from the Earth this camera could have measured a freckle on Neil Armstrong’s face as he took his first steps on the moon. The camera has now been integrated in the payload module of the Gaia satellite, built by Airbus Defence and Space for ESA, and is being tested in Airbus Defence and Space’s cleanroom facility in Toulouse. In late 2013, it will be launched from the European Spaceport in French Guiana and placed in orbit to assume its role as the most powerful ‘eye’ ever to observe the Milky Way.

Once in orbit, Gaia will begin to precisely measure the characteristics of a billion stars which will then be used to generate a 3D chart of our galaxy

“In astrometric terms, Gaia will have a precision 100 to 1,000 times greater than its predecessor Hipparcos,” explains Vincent. When ESA launched Hipparcos in August 1989 it was the first satellite devoted to astrometry, a branch of astronomy involving measurement of the position and movements of celestial bodies as well as their distance from the Earth. Up until its ‘retirement’ in 1993, it amassed a catalogue of 120,000 stars with a precision 200 times greater than any other previous measurements. Now, Gaia’s eagle eye makes Hipparcos seem as blind as a bat. Once in orbit around the sun, the new satellite will begin to precisely measure the characteristics of a billion stars which will then be used to generate a 3D chart of our galaxy. This sensitivity will enable it to detect more than 250,000 objects in our solar system (mostly asteroids), 15,000 extrasolar planets, 50,000 brown dwarfs and around 20,000 supernovae.


A diamond in space

Gaia’s exceptional capabilities have come about through years of work by a team of over 500 people at Airbus Defence and Space, explains Vincent Poinsignon, who was previously project manager for Mars Express, the first European mission to the Red Planet. Gaia contains two telescopes which in combination are powerful enough to detect stars 400,000 times fainter than the human eye can see. The weak light of these celestial bodies is captured on a focal plane the two telescopes share, which consists of 106 charge-coupled devices (CCDs), an advanced version of the chips in standard digital cameras.

The mosaic has been assembled at Airbus Defence and Space’s Toulouse site

A key feature of the CCD support structure and the rest of the satellite is the use of silicon carbide, a material with ceramic properties originally developed as a diamond substitute which is incredibly light and resistant to deforming under temperature changes.

Airbus Defence and Space has already used this material on other satellites, including the Herschel space observatory launched in 2009. The 3.5-metre mirror of Herschel’s large telescope, made entirely of silicon carbide, weighs a mere 270 kilograms compared to the 1,500 kilograms it would have weighed using standard technology. Now the use of silicon carbide will enable Gaia to function at temperatures of -110°C to increase the sensitivity of its sensors. A deployable sunshield with a length of around 10 metres will keep its instruments in the shade at all times to protect them from the sun’s heat. “Gaia’s innovations include the micropropulsion system and the antenna, along with the generalised use of silicon carbide not only for the telescope mirrors but also for many parts of the payload module,” highlights Timo Prusti, Gaia Project Scientist at ESA.

As Timo indicates, the antenna plays a vital role. The satellite will detect and measure the characteristics of hundreds of stars per second practically uninterrupted during its five-year mission. Each star will be monitored around 70 times with a precision “that could divide the moon into 180 million slices from the Earth”, he says. Every day, Gaia’s antenna will need to send 50 gigabytes of data to the Earth over a distance of 1.5 million kilometres. At the end of the mission, scientists will have gathered one petabyte of data, equivalent to the information stored on 200,000 DVDs.


Relativity takes centre stage

The Gaia mission was proposed in 1993 by Swedish astronomer Lennart Lindegren of Lund University. His idea will take to the skies 20 years after it took shape in his mind. Lennart, one of the world’s foremost astrometry experts, points out that Gaia will also verify with greater precision than ever Einstein’s general theory of relativity, which states that mass causes spacetime to curve. Even light rays are bent when they pass near a massive object such as the sun. “Gaia will measure this effect a few orders of magnitude more precisely than ever before and will also observe the deflection of light rays caused by Jupiter, the Earth and other planets,” says Lennart. “We expect all these measurements to be in line with the general theory of relativity and accordingly to confirm our current understanding of the structure of spacetime. However, it is part of scientific method to constantly test accepted theories in the most rigorous manner possible and Gaia will be a hard taskmaster in this sense,” he continues.

The one billion stars to be mapped by Gaia make up barely one percent of the stars in our galaxy

In the early 17th century, the Florentine astronomer Galileo Galilei tried to convince the cardinals of the Inquisition that the universe is “written in the language of mathematics, and its characters are triangles, circles and other geometric figures, without which it is humanly impossible to understand a single word of it”. According to Galileo, without these characters “one is wandering around in a dark labyrinth”. Four centuries later, Gaia is preparing to shed light on this labyrinth with the ‘luminous’ mathematics of astrometry – although there is still much to be done. The one billion stars to be mapped by Gaia make up barely one percent of the stars in our galaxy. The remaining 99% will have to wait for Hipparcos’ and Gaia’s successors …