Threat to the world’s darkest and clearest skies

New ESO analysis confirms severe damage from industrial complex planned near Paranal

An in-depth technical analysis by the European Southern Observatory (ESO) has evaluated the impact of the INNA megaproject on the facilities at the Paranal Observatory, Chile — and the results are alarming. The analysis reveals that INNA would increase light pollution above the Very Large Telescope (VLT) by at least 35% and by more than 50% above the south site of the Cherenkov Telescope Array Observatory (CTAO-South). INNA would also increase air turbulence in the area, further degrading conditions for astronomical observations, while vibrations from the project could seriously impair the functioning of some of the astronomical facilities, like the Extremely Large Telescope (ELT), at the Paranal Observatory.

In January, ESO publicly raised the alarm about the threat posed to the world’s darkest and clearest skies, those of ESO’s Paranal Observatory, from the industrial megaproject INNA. The project — by AES Andes, a subsidiary of the US power company AES Corporation — includes multiple energy and processing facilities, spread over an area of more than 3000 hectares, the size of a small city. Its planned location is just a few kilometres from the Paranal telescopes.

This eye-catching Picture of the Week was captured from ESO’s Paranal Observatory. Home to ESO’s Very Large Telescope, the observatory sits proudly atop the 2635-metre-high Cerro Paranal in Northern Chile’s Atacama Desert. Photo: ESO/P. Horálek

A preliminary analysis done at the time revealed that, due to its size and proximity to Paranal, the INNA project posed significant risks to astronomical observations. Now, a detailed technical analysis has confirmed that INNA’s impact would be devastating and irreversible.

Blinding light pollution

According to the new, detailed analysis, the industrial complex would increase light pollution above the VLT, which is about 11 km from the planned INNA location, by at least 35% above the current artificial-light baseline levels. Another of the Paranal facilities, ESO’s ELT, would see the light pollution above it increase by a minimum of 5%. This increase already represents a level of interference incompatible with the conditions required for world-class astronomical observations. The impact on the skies above the CTAO-South, located just 5 km from INNA, would be the most significant, with light pollution going up by at least 55%.

“With a brighter sky, we severely limit our ability to directly detect Earth-like exoplanets, observe faint galaxies, and even monitor asteroids that could cause damage to our planet,” says Itziar de Gregorio-Monsalvo, ESO’s Representative in Chile. “We build the largest and most powerful telescopes, in the best place on Earth for astronomy, to enable astronomers worldwide to see what no one has ever seen before. Light pollution from projects like INNA doesn’t just hinder research, it steals our shared view of the Universe.”

The Milky Way arches across this rare 360-degree panorama of the night sky above the Paranal platform, home of ESO’s Very Large Telescope. The image was made from 37 individual frames with a total exposure time of about 30 minutes, taken in the early morning hours. Credit: ESO/H.H. Heyer

For its technical analysis, a team of experts led by ESO Director of Operations Andreas Kaufer joined forces with Martin Aubé, a world-leading expert on sky brightness at astronomical sites, to run simulations using the most advanced light-pollution models. As input, the simulations used publicly available information provided by AES Andes when submitting the project for environmental assessment, which states the complex will be illuminated by over 1000 light sources.

“The light-pollution figures we are reporting assume that the project will install the most modern available luminaries in a way that minimises light pollution. However, we are concerned that the inventory of light sources planned by AES is not complete and fit for purpose. In that case, our already alarming results would underestimate the potential impact of the INNA project on the Paranal sky brightness,” Kaufer explains.

He adds that the calculations assume clear-sky conditions. “We would get even worse light pollution if we considered cloudy skies,” he says. “While Paranal is cloud-free most of the year, many astronomical observations can still be performed when there are thin cirrus clouds — and in this case the light-pollution effect is amplified since nearby artificial lights strongly reflect off the clouds.”

The dark skies above ESO’s Paranal Observatory, home to ESO’s Very Large Telescope (VLT), yield breathtaking views so clear and so full of stars that you could almost touch them. Credit: ESO/P. Horálek

Turbulence ahead

The technical analysis looked into other impacts from the project, such as the increase in atmospheric turbulence, the effects of vibrations on the delicate telescope equipment, and dust contamination of the sensitive telescope optics during construction. All of these would further deepen the impact of INNA on Paranal’s capabilities for astronomical observation.

In addition to the dark and clear skies, Paranal Observatory is the world’s top site for astronomy thanks to its exceptionally steady and stable atmosphere – it has what astronomers call excellent seeing conditions or very low “twinkling” of astronomical objects caused by turbulence in Earth’s atmosphere. With INNA, the best seeing conditions could deteriorate by up to 40%, in particular due to the air turbulence caused by the project’s wind turbines.

Another worry is the impact of the vibrations caused by INNA on the VLT Interferometer (VLTI) and the ELT, which are both extremely sensitive to micro-seismic noise. The technical analysis reveals that INNA’s wind turbines could produce an increase in these micro-vibrations of the ground that is large enough to impair the operations of these two world-leading astronomical facilities. Dust during construction is also problematic as it settles on the telescope mirrors and obstructs their view.

***The Laser Guide Star (LGS) is launched from the VLT’s 8.2-metre Yepun Telescope and aims at the centre of our galaxy, in the heart of the brightest part of the Milky Way.***

“Taken together, these disturbances seriously threaten the current and long-term viability of Paranal as a world-leader in astronomy, causing the loss of key discoveries about the Universe and compromising Chile’s strategic advantage in this area,” says de Gregorio-Monsalvo. “The only way to save Paranal’s pristine skies and protect astronomy for future generations is to relocate the INNA complex.”

Furthermore, INNA’s infrastructure is likely to encourage the development of an industrial hub in the area, which could turn Paranal into an unusable site for top-level astronomical observations.

“ESO and its Member States are fully supportive of energy decarbonisation. For us Chile should not have to make a choice between hosting the most powerful astronomical observatories and developing green-energy projects. Both are declared strategic priorities by the country and are fully compatible — if the different facilities are located at sufficient distances from one another,” says ESO Director General Xavier Barcons.

Florentin Millour captured this breathtaking wide-field image of comet C/2024 G3 (ATLAS) on January 21 from ESO’s Paranal Observatory in Chile. The Very Large Telescope sits atop Cerro Paranal to the left, while the comet sets in the western horizon right after sunset. Photo: F. Millour/ESO

Citizen Participation Process

The full technical report will be submitted to the Chilean authorities later this month, as part of the Citizen Participation Process (PAC) in INNA’s environmental impact assessment, and made public at that time, before the deadline of 3 April. ESO is making an executive summary of the report public in advance.

“We are extremely grateful for the support we’ve received from the Chilean and worldwide research communities, and from our ESO Member States. We also thank the Chilean authorities for looking into this matter. We are more committed than ever to working together to protect the irreplaceable Paranal skies,” concludes Barcons. (ESO)