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The Gaia Satellite Will Map One Billion of the Milky Way's Stars

Gaia is ready to start the largest ever census of our galaxy.
Image: ESA

The European Space Agency's Gaia satellite was launched at the end of last year to start its journey into space for a pretty awesome mission: mapping around a billion stars. Now the pleasing flying saucer-shaped satellite has reached its destination 1.5 million miles away from Earth and is starting its stellar tally.

The basic aim is to measure the positions and movement of a billion stars in the Milky Way, which sounds comprehensive but is only around one percent of all the stars in the galaxy. Nevertheless, it'll create a 3D map of our local space surroundings unlike anything before.


As one of Motherboard's top space enthusiasts Amy Teitel explained on Gaia's launch, this will tell us a lot about the Milky Way's history; we'll be able to see how stars moved in the past, and thus learn about the early formation of the galaxy. On the side, it's expected to identify tens of thousands of other space bodies, like planets, asteroids and comets.

How it works is simple in theory: Gaia has two telescopes that focus their light on a single plane, effectively creating a near billion-pixel camera (the largest camera ever to have flown in space) that will repeatedly scan the sky around it as it orbits around a point. According to the ESA, "Gaia must be able to measure positions to a level equivalent to the width of a human hair seen at 2000 km."

A video introducing the camera in more detail explains that only the data from the pixels that actually contain information about the tracked stars (as opposed to the blank space or other stuff in between) is stored, to prevent total data overload. Five data processing centres, including one in Cambridge in the UK, will process the downlinked data.

The space agency explains that Gaia will observe each of the billion stars about 70 times over the five-year project, the point being that astronomers will be able to track small changes in their movement. In a blog post, astronomer Paolo Tanga compares the technique to the "photo finish" method used in sporting events. But for one billion stars, not to mention the "rebel" asteroids that are more eccentric in their movements. Sounds like they've got their work cut out.

Now that Gaia's ready to start scanning the skies, it's got a few difficulties to contend with. The commissioning didn't go off without a hitch, and it's had problems with water freezing around the optics. That water was probably trapped in the spacecraft before launch— all sorts of stuff can end up on spacecraft even after decontamination—and it's being removed by heating some of the affected parts.

Scientists are also working to mitigate the effects of "stray light" on Gaia's readings, as it seems more light than expected is getting past the satellite's sun shield. But despite the glitches, the ESA says it's now good to go, and to start the largest census ever of our own galaxy.