Video: How the Gaia spacecraft will scan the entire sky
IT WILL be the biggest selfie of all time. When the launches next week, it is set to map a billion stars in our galaxy with unprecedented accuracy – and fundamentally transform our understanding of the cosmos around us.
If all goes according to plan, the European Space Agency’s bold mission will blast off from French Guiana on a Russian Soyuz rocket and travel 1.5 million kilometres into space. Far beyond the glow of Earth’s atmosphere Gaia will hover in orbit around the sun and start to spin slowly, capturing every celestial object that falls within its gaze for the next five years. As well as charting 1 per cent of the stars in the Milky Way – around one billion of them – the telescope will l ocate planets around other suns, warn us of asteroids in our solar system and pinpoint hundreds of thousands of new and distant galaxies beyond our Milky Way. It is a journey of discovery. “Gaia is going to revolutionise astronomy and I don’t say that lightly,” says Peter Allan of the in Didcot, UK, who is part of the Gaia team.
The accuracy of Gaia’s camera would allow it to pinpoint a flea on the moon (Image: ESA)
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An accurate map of the heavens is a powerful thing. The one made by Gaia’s predecessor, – which plotted the positions of more than 2.5 million stars between 1989 and 1993 – spawned thousands of papers on everything from the frequency of Earth’s ice ages based on the sun’s passage through the galaxy, to the dynamics of star clusters and the use of gamma-ray bursts in the search for extraterrestrial intelligence.
Hipparcos is a reminder that we have been charting the stars for more than 2000 years. The mission was named after Hipparchus, the Greek astronomer who lived from 190 BC to 120 BC and compiled the positions of around 850 stars visible to the naked eye. His star catalogue was not significantly bettered until the great celestial chart compiled by the Danish nobleman in the 16th century, decades before the invention of the telescope. Between 1576 and 1597, using just sextants and armillary spheres, Brahe and 100 visiting astronomers mapped over 1000 stars at Uraniborg, his grand observatory on Hven island between Denmark and Sweden.
Celestial selfie
The invention of the telescope shortly after Brahe’s death in 1601 brought further advances in the centuries that followed (see timeline). But Gaia’s map will be in a different league. At the heart of the spacecraft is a 1-billion-pixel digital camera, the largest ever flown into space. It is stable and sensitive enough to see objects 400,000 times fainter than the dimmest ones visible to the naked eye, allowing us to create the truest representation of our galaxy yet.
“Gaia can see objects 400,000 times fainter than the naked eye can, making for the truest picture of our galaxy yet”
At 100,000 light years across, the Milky Way is so vast that it is impossible for us to get outside it and photograph it from there. Despite this, previous observations have allowed us to figure out that the sun lies three-fifths of the way from the galaxy’s centre to its edge. They have also shown that our galaxy is a flat disc of stars, shot through with spiral arms of brighter stars. Gaia will pinpoint giant stars across most of the galaxy, even beyond its centre, where dust clouds obscure the view of smaller or dimmer objects, and so reveal the full details of their arrangement. “Gaia will let us build a picture of our own galaxy for the very first time,” says the project’s lead scientist Timo Prusti.
But it is not only the positions of the objects that are important – the way they move will help explain how our galaxy formed. English astronomer first noticed the movement of the stars in 1718 when comparing his own measurements for three stars – Arcturus, Sirius and Aldebaran – with those made by the ancient Greeks 1800 or so years earlier. Until then, the orbs of heaven had been referred to as the “fixed stars”. Halley proved this was not the case, and it meant that astronomers could extract more from stars than just their positions and brightness: their speed and direction of travel could provide important clues about the galaxy at large.
“The Milky Way is the only galaxy that we can study star by star. If you can measure positions and velocities, you can understand where all the forces are,” says Mark Cropper, an astronomer at University College London who works on the Gaia project. These forces have shaped the galaxy for billions of years.
It is thought that large galaxies such as the Milky Way grow through a process of merging. The first stage sees several smaller galaxies collide and coalesce, but as this process progresses, one of the colliding galaxies can become very large. Gravity pulls the minnows into streams of stars, which become incorporated into the general rotation of the larger galaxy, leaving only the smallest hint of their previous motion.
Gaia’s camera can pinpoint some stars to within a few millionths of an arcsecond – the equivalent of spotting a flea on the moon. By comparing observations over the five years of the mission, Gaia will be able to separate the stars into their respective streams on the basis of the similarity of their orbits around the galactic centre. This should reveal the remnants of the galaxies that have gone into making up the Milky Way.
“The accuracy of Gaia’s camera would allow it to pinpoint a flea on the moon”
At present, such galactic archaeology, as astronomers call it, is little more than a dream. Just a small handful of streams are currently known. “Gaia takes us three orders of magnitude above the sensitivity we currently have to do this,” says Cropper. “It allows much of the galaxy to be traced out and makes galactic archaeology possible. We will be catapulted into a new regime.” That will tell us not only how our own galaxy formed, but will feed into our understanding of the evolution of the entire universe.
Similar searches might even reveal the sun’s siblings. The sun was just one of hundreds of thousands of stars born 4.5 billion years ago when a cloud of gas collapsed in the Milky Way. Time and gravity were not kind to the newly formed star cluster. They tore it apart and scattered the other stars in the sun’s litter far and wide across the galaxy. Despite this mayhem, sibling stars retain similar orbits to the sun – so Gaia’s ability to track stars’ movements could give us a chance of identifying them.
“The ability to track stars’ movements could help us identify the ones born in the sun’s litter”
A star with the same orbital characteristics is still not conclusively a sibling. But Gaia has another trick up its sleeve to aid identification. It will split the light from the celestial objects it observes into a spectrum that allows a reasonable analysis of each star’s composition. Because each gas cloud has its own chemical subtleties, the sun’s siblings should all have identical chemical fingerprints. It is a needle in a haystack, but Cropper is optimistic: “If we put it all together, we may be able to identify the solar siblings even though by now they will be spread out all across the sky.”
Gaia won’t just help us to understand our own galaxy. It will also allow us to make more precise measurements of the phenomenon known as parallax, which is crucial to our wider understanding of the cosmos. “How accurately we gauge distance in the universe depends upon the precision to which we can measure the parallax distances to the nearest stars,” says Prusti.
Every six months, Earth finds itself on opposite sides of its orbit around the sun. This movement of the planet means that the position of nearby stars appears to shift in relation to background stars that are further away. (It’s the same effect you get when you look at a nearby object with one eye, then the other and see the image move.) The parallax is quantified by the angle of this shift. Once the parallax has been measured, it is possible to calculate the distance to the star using simple trigonometry.
The parallax angle is tiny, even for the nearest stars: less than 0.05 per cent of the diameter of the moon. Gaia’s keen eye will be able to measure it for all 1 billion of the stars it will see. This is important because parallax distances are the foundation stone for the so-called cosmological distance ladder. This is a scale of inferences that allows us to estimate distances to further stars, which, in turn, allows the distances to be gauged to nearby galaxies, and then to more and more distant ones. Eventually, the scale reaches across the whole universe.
The more accurately we can measure the distance to a star using the cosmological distance ladder, the more accurately we know its brightness, and so the amount of energy it is producing and what is going on inside it. The most important rung on the cosmological distance ladder is the first, because an error that creeps in here will affect all further rungs. More accurate distance measurements will also allow us to refine our understanding of dark energy, the mysterious force that is accelerating the expansion of the universe. “Gaia is going to get us out into the wider universe much more accurately than ever before,” says Allan.
With so much riding on the mission, it is little wonder that Gaia has been decades in the making. It was first proposed in 1993, the year that Hipparcos stopped working, but it wasn’t until 2006 that ESA began to build Gaia,whose price tag is a cool 650 million euros.
All the science now depends on the launch going smoothly. The moment of ignition is always a heart-stopper. “It is only one moment in the mission but it is a pretty concrete moment,” says Prusti, who will be at the Kourou space port in French Guiana to witness the ascent. “It is a moment of silence when the mission walks a very thin line.”
But if all goes well, Gaia will give us a whole new appreciation of the galaxy and the universe. “Gaia will show us all sorts of amazing and unexpected things,” says Cropper. Which is more than you can say for most selfies.
Wobbly worlds
Astronomers estimate that Gaia will discover about 2000 planets around other stars. The spacecraft will not be able to see those planets directly because they are too small and dim. Instead, it will infer their presence by the effect they have on their central stars.
As the gravity of a mighty star pulls on a planet, the puny planet tugs back, causing the star to perform a small pirouette. Gaia will be able to measure these motions for stars within about 500 light years of Earth. Its haul of planets should span everything from Earth-size rocky worlds to gas giants like Jupiter and Saturn.
These planets aren’t much bigger than the exoplanets discovered by NASA’s Kepler mission. But Gaia will search “nearby” stars across the entire sky, rather than distant stars in a tiny patch of the galaxy as Kepler has done.
With astronomers currently planning missions to analyse the atmospheres of nearby planets, the resulting catalogue will be invaluable.
This article appeared in print under the headline “Top of the charts”
