ANTU, Kueyen, Melipal, Yepun. These four hulking figures dominate the summit of Cerro Paranal, a rust-red mountain in Chile’s Atacama Desert. Their home is among the most inhospitable places on Earth – a desolate, dusty terrain reminiscent of the surface of Mars.
As night falls, the giants slowly rotate and stir into life. Doors slide open, and within the structures vast mirrors begin to capture light from distant corners of the universe. Together they make up the world’s most powerful optical telescope: the .
You might have seen some of the VLT’s spectacular snapshots of swirling nebulae and far-away galaxies. But it was not built just to take pretty pictures. In the 20 years since the VLT saw first light, it has given researchers of the European Southern Observatory, a 17-nation astronomical collaboration, a clearer view of phenomena that could answer some of the universe’s greatest open questions, from how stars and planets form and whether there is life beyond our solar system, to how our underlying theories of the cosmos stand up. But enough is never enough: the giants are evolving, with the promise of even more spectacular discoveries to come.
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This corner of Chile is an astronomer’s paradise. The sky is cloudless for 330 days of the year, there is almost no light pollution and the air contains barely any moisture that would otherwise block certain wavelengths of light. “When the moon is down, the night sky is absolutely amazing,” says staff astronomer , who spends four months of the year on the peak. “The Milky Way is as clear as day.” But even in near-perfect conditions, you need a whopping amount of light-gathering power to peer deep into the universe.
Each of the VLT telescopes has an 8.2-metre-wide mirror. Bigger single mirrors do exist, such as the two 10-metre telescopes at the Keck Observatory atop Mauna Kea on Hawaii. But the VLT has a special feature that sets it apart. It can combine light from all four mirrors and computationally reconstruct them into a single image. Together with the output of smaller, auxiliary telescopes, this effectively creates a single mirror up to 200 metres wide. This unrivalled light-gathering apparatus can distinguish two car headlights at the distance of the moon.
Right from the beginning of its career in 1998, the VLT has been producing images that both confirm and challenge our understanding of the universe. Although the first planets orbiting other stars had been discovered in the 1990s, their presence had only been deduced by their influence on their parent stars. In 2004, the VLT snapped the first direct image of an exoplanet, opening an era in which we could not only see these alien worlds, but also infer what their atmospheres are made of. A few years later, it took the temperature of the distant universe by analysing carbon monoxide molecules 11 billion light years away. The result was in almost perfect agreement with what our theories predict for a universe that started with a big bang.
But there is always some answer that lies just out of sight, even of the most powerful telescope. So even if it ain’t broke, fix it.
Various factors beyond mirror size limit a telescope’s performance, such as the effects of atmospheric turbulence. The Paranal Observatory, home to the VLT, sits 2650 metres up. Here the air is thinner than at sea level, but the constant churning of atmospheric gases still randomly bends light, making stars twinkle to our naked eyes and blurring telescope images.
The VLT has always had a sophisticated system of adaptive optics designed to correct for such effects, but it has recently undergone a significant upgrade. In 2016, Yepun gained a deformable secondary mirror just 2 millimetres thick, studded with almost 1200 actuators that constantly reshape the surface in real-time in response to measurements of atmospheric distortion. The same year also saw the addition of GRAVITY, a system that more subtly controls the light from all the individual telescopes to sharpen the resulting images.
With these upgrades, the VLT has in the past 12 months captured the first image of a planet forming in the dusty disc around a young star, measured the chemical composition of an asteroid in the frigid outer solar system, observed the aftermath of a collision between two neutron stars and analysed the atmospheres of the seven Earth-sized worlds around the TRAPPIST-1 star, several of which appear to be rich in water.
But perhaps its most impressive recent observation is of a star sailing perilously close to Sagittarius A*. This monster black hole, 4 billion times the mass of the sun, sits at the Milky Way’s centre some 25,000 light years away. Astronomers have been watching this star, known as S2, since the early 1990s. They knew its elliptical orbit would eventually take it close enough to Sagittarius A* to enable the most stringent tests ever of Einstein’s general theory of relativity, which describes gravity’s effects.
S2 is very faint, making it tricky to observe with the necessary precision. “Both the star and the black hole are very far away, and there is a thick veil of dust between us and the galactic centre that allows precious little light to penetrate,” says Oliver Pfuhl at the Max Planck Institute for Extraterrestrial Physics in Germany. But in May 2018, as S2 made its closest pass of Sagittarius A*, Pfuhl and his colleagues used the VLT to clock its speed at 7600 kilometres per second and measure its wobble towards and away from Earth. The black hole’s immense gravitational field stretched the star’s light by almost precisely the amount that general relativity predicts.
The VLT has continued to track S2 in the hope that it can give us a clearer picture of what the fabric of the universe does around a supermassive black hole. “You can basically trace space-time in a place that nobody has traced it before,” says Pfuhl.
The next step is to trace it in a galaxy that’s not our own. That could become possible thanks to a gizmo fitted to Yepun in 2016. It shoots four powerful lasers into the upper atmosphere. “See the photos and you might think they were taken with a long exposure or Photoshopped, but it’s not true,” says Smoker. “You see the lasers with the naked eye.”
Now fully operational, the beams of the Four-Laser Guide Star Facility excite sodium atoms in the atmosphere to act as artificial guide stars to calibrate the instrument. The light they give out is analysed 1000 times per second to see how turbulence is distorting the atmosphere, and so what contortions the adaptive optics must perform to counter it. This should allow the VLT to achieve images with a sharpness close to the theoretical limit for a telescope its size, says Joël Vernet, a VLT researcher. It may even let us track stars around supermassive black holes that lurk in galaxies beyond our own.
And when that is no longer good enough? Just 23 kilometres from the Paranal Observatory, on another of northern Chile’s dusty mountaintops, construction recently began on an even larger telescope. The Extremely Large Telescope (ELT), slated to start imaging the universe in 2024, will build on the technology of its predecessor, but boast a main mirror almost 40 metres wide. “That is a huge boost in light-collecting power,” says Vernet. With a virtual mirror like the VLT’s, you can only see bright objects at the farthest distances.
The ELT should give us a view of the very first, faint galaxies, imparting new clues about how structure in the universe evolved. It could provide the first direct measurement of the accelerating expansion of the universe, shedding light on the mysterious dark energy that appears to be causing it. The telescope will even search for variations in the fundamental physical constants over time, and so challenge the very basis of the laws of nature.
The restless imaginations of astronomers and cosmologists are already stirred. “We can’t wait to see it,” says Smoker.
This article appeared in print under the headline “Land of the giants”
