The Milky Way at Chile’s Paranal Observatory, a site with the darkest skies out of 28 leading observatories assessed by researchers.Credit: ESO/P. Horálek
A massive green hydrogen plant proposed for construction in Chile could increase light pollution at one of the world’s most powerful telescopes by at least one-third, says the European Southern Observatory (ESO), the consortium that operates the telescope and will either host or operate others being built nearby.
An ESO analysis released on Monday found that light pollution would increase by at least 35% at the Very Large Telescope (VLT) ― one of the most advanced optical telescopes in the world ― and by at least 55% at the southern array of the Cherenkov Telescope Array Observatory (CTAO), which is now under construction and will be the largest ground-based observatory for very-high-energy γ-ray astronomy. The analysis also found that the green-energy project would increase atmospheric turbulence at the telescopes and cause vibrations that will damage the sensitive equipment.
Together, the effects will cause “devastating, irreversible” damage that cannot be mitigated, astronomer Itziar de Gregorio-Monsalvo, the ESO’s representative in Chile, said at a media briefing on Monday. “It will reach a point where it is highly likely that we won’t be able to operate these telescopes.”
Before the analysis was released, the developer of the green-energy plant, AES Andes in Santiago, said that its project “aims to use the best technologies and the strictest standards”, and that a company analysis found that the project will not have a “significant impact” at the observatories. On Monday, the company said in a statement, “We are still working on gathering data from the ESO document to understand the discrepancies between ESO’s numbers and our own analysis.”
Perfect conditions
The green-energy project would span 3,000 hectares in Chile’s Atacama Desert, the driest on Earth. The arid climate and lack of cloud cover there create ideal conditions for stargazing. The region also has extremely dark skies: a 2023 study1 compared levels of light pollution at the 28 leading astronomical observatories worldwide and found that the Paranal Observatory, site of the Very Large Telescope, had the darkest location of them all, followed by the nearby Armazones Observatory, where the ESO is building the Extremely Large Telescope (ELT), which the organization says “will be the largest visible and infrared light telescope in the world”.
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The Atacama’s dark skies and other qualities lured organizations to build cutting-edge instruments such as the VLT, the ELT and the southern array of the CTAO. “The darker the sky, the fainter the astronomical object that can be examined,” notes a summary of the ESO’s report on the green-energy project.
The conditions in the Atacama also lured energy company AES Andes, a subsidiary of the AES Corporation in Arlington, Virginia. The company has proposed using the region’s intense sunshine and strong winds to generate solar and wind power. These would provide energy for the extraction of hydrogen from water, which could be collected from the nearby Pacific Ocean. Hydrogen made with renewable energy is often called green hydrogen.
AES has proposed building a green-hydrogen complex that would include a port, ammonia and hydrogen production plants, and thousands of electricity generators. The project’s main unit would be located 11 kilometres from Paranal Observatory, 5 km from the CTAO and 20 km from the ELT.
Duelling numbers
The ESO’s latest analysis was conducted by Martin Aubé at the Center for Research in Astrophysics of Quebec in Sherbrooke, Canada, who studies light pollution at astronomical facilities, and other researchers led by ESO director of operations Andreas Kaufer. It is based on information in the environmental-impact report that AES submitted in early January to Chile’s Environmental Evaluation Service agency (SEA). The SEA will decide whether the AES project goes ahead.
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The ESO analysis found that the increase in artificial light from the AES project would make it harder for instruments to distinguish between celestial bodies. “Increasing the brightness of the sky is like reducing the size of the telescope mirrors,” said Eduardo Unda-Sanza, an astronomer at the University of Antofagasta in Chile.
AES Andes’s findings contrast sharply with those of the ESO. The company said that its calculations showed that the facilities would increase sky brightness by at most 0.27% at Paranal, 0.09% at the ELT and 0.45% at CTAO, wrote Luis Sarras, the firm’s vice-president for international green hydrogen, in an e-mail to Nature. Those numbers are well within Chilean regulations, the company noted.
AES was in contact with the ESO throughout all stages of the project until the environmental file was completed, Sarras says. The company’s outreach included even inviting the ESO ”to participate in the lighting design and seeking their opinions and analysis results”, he says.
