The World’s Largest Telescopes at Risk Due to Industrial Development

[Un article de The Conversation écrit par Julien Milli – Astronome, Université Grenoble Alpes (UGA) ; et Fabien Malbet -Directeur de recherche CNRS en Astrophysique, Université Grenoble Alpes (UGA)]

Let’s set the scene. We are on Mount Paranal, at 2,600 meters above sea level, in the driest hot desert in the world. The Pacific Ocean is about ten kilometers to the west, and the first major city, Antofagasta, 120 kilometers to the north. As professional astronomers, we had the chance to survey this site during observation nights, and even to work there for several years. In the absence of the Moon and after a few minutes of adaptation to the darkness, the Milky Way, the galactic center and thousands of stars become visible above our heads, while the night landscape of the Atacama Desert seems lit by the starry vault alone.

This wonder that every visitor feels at the VLT translates into a quality of measurements unmatched in the world. The site was chosen in the 1980s by astronomers from the European Southern Observatory (ESO) after a long testing campaign. Mount Paranal and its neighbor Mount Armazones, where the ELT is currently being built, combine several unrivaled advantages, the first two being the low turbulence of the atmosphere, which improves the angular resolution (the ability to distinguish two very close in the sky); and the low humidity, conducive to infrared observations.

In 2004, the first image of an exoplanet was obtained at the VLT, the 2011 Nobel Prize in Physics on the acceleration of the expansion of the universe was awarded for observations made in part at the VLT, and the Nobel Prize in Physics 2020 was awarded for research carried out with the VLT telescopes on the supermassive black hole at the center of our Milky Way.

Why is the Paranal sky unrivaled?

It is no coincidence that the main astronomical observatories were built on the western facades of continents (Pic du Midi in France, West Coast of the United States, Chile) or at the top of volcanic hotspot-type islands (Hawaii). , Canary Islands) located to the west of these continents.

Indeed at a certain altitude (9-15 kilometers), the movement of air corresponds to the atmospheric circulation system generated by the earth's rotation which goes from west to east. Because of this rotation, the air arriving on the western facades has been very little disturbed by convection on land heated by the Sun, and therefore the flow is laminar, that is to say with less turbulence.

Additionally, observatories located above 2,000 meters above sea level, as is the case in Chile, are above the “thermal inversion” layer, which blocks clouds at lower altitudes. Thanks to their high perched position, they benefit from an unrivaled number of clear, low-turbulence nights.

In the case of the Atacama, this exceptional situation is further reinforced by the Andes mountain range, to the east, a rampart of more than 6,000 meters which blocks disturbances likely to enter from the east (Argentina) or from the northeast (Bolivia), hence the aridity of the region.

Strict rules limit human activities around the observatory

But these assets would be reduced to nothing without the darkness of the sky, due to the absence of human settlements within a radius of several tens of kilometers.

The observatory imposes strict rules on light emissions around the site: the few vehicles moving at night are only allowed to use their position or hazard lights, and curtains must be drawn in the rooms. residence and dormitories before turning on the lights.

Today, ESO's observatories at Paranal and Armazones are among the darkest places on Earth, with far less light pollution than other observatories.

And yet, even in this unspoilt landscape, we astronomers see on the horizon the light signature of the city of Antofagasta, and even that of the largest open-air copper mine in the world, La Escondida, a hundred miles away. kilometers to the northeast. Although telescopes generally do not point below 30º height due to high atmospheric absorption and greater turbulence, some phenomena like zodiacal light, which is a faint glow above the horizon in the plane of the ecliptic, are therefore already impacted by this halo, limiting possible research.

The expected impact of such an industrial project on light pollution

In December 2024, the company AES Andes launched environmental audit procedures with a view to the construction of a gigaproject, named INNA, whose footprint would be approximately 3,000 hectares and which would be located between 5 and 12 kilometers from Mount Paranal. This involves producing hydrogen and ammonia from the electrolysis of seawater (previously desalinated). This energy-intensive process would be powered by electricity produced by wind turbines and photovoltaic panels, supplemented by storage batteries, and would consume 1.7 gigawatts, according to AES Andes. In total, the investment represents around $10 billion and includes an industrial port to export production.

A new industrial site, certainly less spread out than the city of Antofagasta or the La Escondida mine, but ten times closer, would necessarily have a major impact on the quality of the sky at the VLT and ELT level. On Earth, the majority contribution to light pollution is often due to the private sector (mainly businesses but sometimes individuals), with public lighting representing only a minority fraction, even in cities. A study in the city of Tucson, Arizona, for example, showed that street lighting accounts for less than a fifth of the sky's brightness — floodlights installed in industrial sites, businesses and home gardens causing the stay.

Disastrous consequences for ground-based astronomy

With an increase in light pollution of two to three times the current value, not only would the Paranal and Armazones observatories lose their status as the purest skies in the world, but some objects in the night sky would simply no longer be visible enough from the terrestrial surface to be able to study them, like very distant galaxies, which are currently only visible by combining a hundred hours of observation.

Dust and aerosols released into the atmosphere by industrial activity (during the construction phase, or under the mixing action of wind turbines) would have an amplifying effect on light pollution, because these particles diffuse the artificial lights emitted from the ground.

But this also threatens the future network of Cherenkov telescopes (CTA) which has chosen the valley between the Paranal and Armazones mountains to detect gamma rays from the most energetic astrophysical phenomena (supernova explosions, black holes, etc.). This type of optical telescope uses the earth's atmosphere as a detector; the presence of aerosols or any source of light pollution is therefore particularly detrimental for these observations.

Can we prevent the economic development of a region in the name of scientific research?

The question is legitimate, but it is not certain that it is relevant here. The company AES Andes highlights that the construction phase will employ around 5,000 people, but only between 500 and 600 workers are expected to work on the site during normal operation. It announces that it wishes to employ at least 20% “local” workers from the small towns of Paposo and Taltal, and the large city of Antofagasta.

However, we can question the real impact on the region's economy. The example of the countless mines in the Atacama Desert shows that companies generally prefer to employ qualified workers who live in the center of the country, and who come to work by plane for periods of a few weeks — rather than having to train local staff.

Above all, we can wonder if the vast Atacama Desert would not offer other sites than this one, literally stuck to the astronomical observatories. That an industry that claims to be “green” begins by showing indifference to the degradation of a valuable environmental resource — the darkness of the night sky — raises questions.