Gaia: The 'impossible space mission' ready to fly

A machine to provide "all the answers to all the questions we have about the stars"

"It blows your brain. I remember when we went and asked for this stuff we just fell about laughing. We said: 'Well, let's try; let's see what happens. These engineers are smart'. And, boy, they're smart - they did it!"

Cambridge University's Gerry Gilmore says astronomers thought they were requesting the impossible when the spec was put forward in the early 1990s for a space mission to make a far-reaching census of the Milky Way.

The desire was to map very precisely the position, motion and properties of hundreds of thousands of the brightest stars all the way to the galactic centre and beyond.

If that could be achieved, the scientists argued, we could unlock remarkable information about the structure and history of our corner of the Universe.

Today, this mission impossible is built and ready to go.

On Friday, the European Space Agency's Gaia telescope will be flown to French Guiana to begin the preparations for launch on a Soyuz rocket in mid-November.

The satellite is going to make the ultimate map of the sky. For certain, its catalogues will underpin pretty much all of astronomy for decades to come.

Gaia will pinpoint about a billion stars and build a profile on each and every one, including:

• exact coordinates, including distance

• brightness

• temperature

• composition

• age, and so on.

And for about 150 million of these stars, the telescope will determine their velocity not merely across the sky but their movement either towards or away from us.

It's this part that blows my brain, because these three-dimensional markers then allow us to trace the evolution of the Milky Way, long into the future and deep into the past.

"It will be a time-lapse movie and we're going to watch it," says Prof Gilmore.

"We will see the remnants, the debris streams, of the first shards that became what is today the Milky Way. We can run the process right back to the first things that ever happened. We will see the entire history of the Milky Way unfolding before our eyes."

If all that were not enough, Gaia will log simply anything that flashes across its field of view.

It will build lists containing thousands upon thousands of asteroids, comets, new planets, failed stars, variable stars, exploded stars and visible galaxies (many of them billions of light-years distant).

It will churn out discovery after discovery - so many in fact that it will take astronomers years to follow up on every new piece of information.

To achieve all this has required some very smart engineering indeed.

Gaia will have to track huge numbers of objects simultaneously, and even though the satellite spins to acquire its data, it must be a rock-steady platform from which to make exquisite measurements.

This super-performance is derived from a number of features, but it's worth picking out just a few, says the mission's project manager, Giuseppe Sarri.

"With Gaia, we can see down to magnitude 20, which is 400,000 times fainter than what we can see with our eyes," he explains.

"That's important because a star can be faint not just because it's small but because it is very distant. And being able to see a long way allows us to see lots of stars - a billion stars.

"We can go so faint because we have a very large, state-of-the-art focal plane. These are the CCDs - essentially like the chip you have in your mobile phone camera. But whereas that chip is maybe 1cm by 1cm, in Gaia we have a mosaic of chips that make a focal plane of half a square metre."

I've written previously about the extraordinary British camera detector on board. Built by e2v in Chelmsford, it will be the largest focal plane ever sent into space. The current record holder is the company's 95 megapixel system in Nasa's Kepler planet-hunting telescope. Gaia's array of 106 charge-coupled devices (CCDs) boasts almost a billion pixels.

This unit, the three instruments that feed off it and the telescope optics (there are actually two telescopes on Gaia) are mounted on a stiff silicon carbide frame. "It is absolutely stable. It will not move no matter what the variation in thermal environment," says Mr Sarri.

The camera detector lies at the heart of Gaia's super-performance

There are also no moving parts anywhere on Gaia. Even the antenna to communicate with Earth has been designed to point electronically, not mechanically.

And if the satellite does need to make fine adjustments, it has been equipped with thrusters that can squirt just 1.5 micrograms of nitrogen gas a second. Tiny volumes. It would take a thousand of these thrusters simply to hold up a piece of paper.

Gaia - The discovery machine

• Gaia will make a very precise 3D map of our Milky Way Galaxy

• It is the successor to the Hipparcos satellite which mapped some 100,000 stars

• The one billion to be catalogued by Gaia is still only 1% of the Milky Way's total

• But the survey's quality promises a raft of discoveries beyond just the star map

• It will find new asteroids and planets; It will test physical constants and theories

• Gaia's sky map will be the reference to guide future telescopes' observations

When the astronomers wrote their mission spec back in the 90s, they worked out the number of stars they needed to see to get a representative sample of the Milky Way (it could contain perhaps 100 billion stars in total), and that determined the level of accuracy they needed to reach in Gaia's measurements.

The satellite will employ the stellar parallax method, which involves tracking the way a star appears to move on the sky when viewed from opposite sides of the Earth's orbit around the Sun. Because we know the Sun-Earth distance so well, it's possible to work out the distance to the star from the angle it draws on the sky over six months. It's a relatively simple piece of trigonometry. But these are small angles, typically just fractions of an arcsecond.

• As the Earth goes around the Sun, relatively nearby stars appear to move against the 'fixed' stars that are even further away

• Because we know the Sun-Earth distance, we can use the parallax angle to work out the distance to the target star

• But such angles are very small - less than one arcsecond for the nearest stars, or 0.05% of the full Moon's diameter

• Gaia will make repeat observations to reduce measurement errors down to seven micro-arcseconds for the very brightest stars

• Parallaxes are used to anchor other, more indirect techniques on the 'ladder' deployed to measure the most far-flung distances

Gaia's ultra-stable state means its measurements will reach an astonishing accuracy of about seven micro-arcseconds for the nearest stars. That angle is akin to seeing the button on the suit of an astronaut standing on the Moon.

"Gaia is an astronomer's dream," says Esa's director of science, Alvaro Gimenez.

"Gaia is the machine that has been designed so that we will get all the answers to all the questions we have about the stars."

Esa has a blog to follow events leading up to the launch.