GAIA’S ASTRONOMICAL LEGACY

The European Space Agency’s peerless space telescope continues to break new ground in astrometry, and the best is likely yet to come. Richard A. Lovett reports.

The European Space Agency’s awesome telescope continues to break new ground in astrometry. But RICHARD A. LOVETT discovers that they’ve only just begun to explore its potential.

Just over four centuries ago, the Dutch mathematician Willebrord Snellius measured the approximately 116 kilometres from Alkmaar, in North Holland, to Breda, in the country’s south, by breaking it up into quadrangles built upon a chain of 33 carefully constructed triangles.

Snellius – better known in the English-speaking world as Snell (as in Snell’s law, of light refraction) – underestimated the distance by 3.5%. Still, it wasn’t a bad first effort in modern times to use triangulation as a survey method, especially because the quadrant he used (an instrument for measuring angles), although revolutionary for its time, was only accurate to tenths of a degree.

People improved on Snellius’s work (largely by developing ever better methods of measuring angles) throughout the 18th and 19th centuries, eventually reaching a point of accuracy that was surpassed only when global navigation positioning systems became commonly used from the 1980s.

But GPS only works on Earth. If you want to look at and map objects much further out in space you need another method. European space telescope Gaia does this by going back to the future: it uses a process akin to how surveyors measure distances on Earth, but on a far grander scale.

“[Gaia] uses the Earth’s orbit to provide a long baseline to triangulate [on stars and] relies on making very accurate measurements of positions,” says Nick Rowell, a wide-field astronomer at the Royal Observatory of Edinburgh, Scotland.

And Gaia isn’t just doing this for a few stars. Its latest data release, announced last December at a press briefing by the Royal Astronomical Society, now maps the positions, brightnesses, distances and motions of 1.8 billion stars.

The whole process is a testament to the advancement of computer technology, not just while Gaia was being built, but afterwards. “They are absolutely dependent on Moore’s Law,” says George ‘Fritz’ Benedict, a retired astrometer from the University of Texas, Austin (citing the famous computer-tech dictum that processing capabilities double every two years). “When they built this, the computers absolutely weren’t fast enough to process the data. Now they can.”

Even though it is still collecting data, Gaia is already making its mark on astronomy: as of April 2021, the Astrophysics Data System at Harvard University listed a whopping 5172 refereed Gaiarelated studies.

“There is hardly a field of astronomy that isn’t revolutionised by Gaia,” says Dafydd Evans, a Gaia researcher at the University of Cambridge, UK.

Martin Barstow of the University of Leicester, UK, calls it “a tsunami rolling through astrophysics. You’ll be talking about astronomy before Gaia and after Gaia.”

Eyes in the sky

Astronomy is best known for the spectacular images produced by the best telescopes. But it is a field with many subdisciplines, one of the earliest of which is astrometry.

Stripped to its basics, astrometry is nothing more than the making of sky maps. Google Earth for the heavens: a tabulation of the positions, brightnesses, and colours of the stars. Modern astrometers have also included distances and motions, but the idea goes back to the ancient Greeks, who by the time of Hipparchus (~190 BCE to ~120 BCE) had created a catalogue of nearly 1,000 stars. “It’s one of the oldest sciences, ever,” says Leanne Guy, data management project scientist for the US’S Vera C. Rubin Observatory.

Such maps are, of course, incredibly important to backyard stargazers trying to figure out where to point their telescopes. But to the ancients they served more practical purposes, such as allowing sailors to navigate the ocean or farmers to track the seasons so they knew when to plant crops.

Today, GPS has replaced stars for navigation and astrometry has far outstripped the needs of people wanting to know where to point a telescope.

“The data is about 100,000 times more accurate than we need,” says Oregon-based Jerry Oltion, an amateur astronomer, telescope-maker and columnist for Sky & Telescope magazine. But that doesn’t mean astrometry is a relic of history.

Gaia Sky (above) utilises the data from Gaia (right) to create an open-source simulation of our local stellar neighbourhood, complete with planets, dwarf planets, some satellites, moons, asteroids, trajectories, locations and more. It’s used for both scientific and recreational purposes; anyone can download the map and data sets – including star clusters, nearby galaxies (NBG) or distant galaxies and quasars (SDSS) – and cruise round their favourite part of the Solar System and beyond.

At the heart of this is Gaia, launched in 2013. It’s the successor to a prior ESA space telescope called Hipparcos, which orbited the Earth from 1989 to 1993. Hipparcos wasn’t large as telescopes go – only 29 centimetres in diameter – but in space, free of the distorting effects of the Earth’s atmosphere, it was able to collect the most accurate information then available on 118,000 stars, measuring their positions to an accuracy akin to spotting a $1 coin at a distance of 2,500 kilometres.

Gaia took that groundbreaking effort and raised it exponentially. Hipparcos was “pretty good,” says Benedict. “Gaia is about 50 times better.”

Gaia does its magic via a pair of rectangular telescopes each measuring 145 x 50 cm, substantially larger than the one on Hipparcos. Rather than being in Earth orbit, it is in a location called the L2 point, about 1.5 million km from Earth (see box at right). There, the balance of forces from the Earth and the Sun keeps it on station, while also holding it in an orbit where Earth never blocks its view. (NASA’S upcoming James Webb Space Telescope, scheduled for launch on 31 October this year, will also be placed at L2.)

Gaia’s twin mirrors focus starlight onto a onebillion-pixel camera, the largest such detector ever launched into space. It’s so good that Gaia can do the

GAIA’S plots of the 14,099 asteroids orbiting between the Sun and Jupiter show the Sun at the centre, Earth as the inner white circle and Jupiter as the outer white circle. Orbits coloured by albedo (above) – the “lightness” of an asteroid’s surface –show dark asteroids (red) dominating the external regions, with the lightest bodies in the inner Main Belt. Orbits coloured by perihelion distance (right) – the minimum distance from the Sun – show the trajectories of Near-earth in blue and those of Main Belt asteroids in green. Jupiter’s gravitational perturbations are evident, except for Trojan asteroids (red) that precede and follow the planet in safe, stable regions. Coloured for their eccentricity (opposite), most asteroids’ orbits are nearly circular (green), while others take eccentric paths that zip close to the Sun (purple).

Just as atmospheric dust colours sunsets on Earth, interstellar dust affects the light we see from stars and galaxies. It preferentially scatters shorter wavelengths and leaves behind longer wavelengths, making stars appear dimmer and redder than they actually are. This 2D projection is a dust map of the whole sky, centred on the heart of the Milky Way. It was created using data from 88 million individual stars spied by Gaia, allowing astronomers to quantify the average “interstellar extinction” caused by dust – and therefore account for it in their measurements of stars and galaxies. AG refers to the magnitude of extinction along the line-of-sight: the brighter the colour, the more strongly light is attenuated by dust. The map reveals many of the Milky Way’s features, such as the disk of the galaxy and the fine structures of large dust clouds.