Free Astronomy Magazine May-June 2025

MAY-JUNE 2025 T his captivating image was taken from ESO’s Paranal Observatory. Home to the Very Large Telescope, the observatory sits proudly atop the 2635-metre-high Cerro Paranal in the Atacama Desert of northern Chile. The four individual VLT tele- scopes, located just right of centre in this panorama, pose in front of the enormous expanse of the Milky Way, which appears almost like a rainbow of stars arching over the site. Three of the four small Auxiliary Telescopes are also visible in the image. Astronomers use different configurations of these telescopes, which can be moved around on special tracks. [ESO/P. Horálek] According to the new, detailed anal- ysis, the industrial complex would in- crease 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 in- crease by a minimum of 5%. This in- crease 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, ob- serve faint galaxies, and even moni- tor asteroids that could cause damage to our planet,” says Itziar de Gregorio-Monsalvo, ESO’s Repre- sentative in Chile. “We build the largest and most powerful tele- scopes, 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.” For its technical analysis, a team of experts led by ESO Director of Oper- ations Andreas Kaufer joined forces with Martin Aubé, a world-leading expert on sky brightness at astro- nomical sites, to run simulations using the most advanced light-pol- lution models. As input, the simula- tions used publicly available infor- mation provided by AES Andes when submitting the project for en- vironmental assessment, which states the complex will be illumi- nated by over 1000 light sources. “The light-pollution figures we are reporting assume that the project will install the most modern avail- able luminaries in a way that min- imises 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 re- sults would underestimate the po- tential impact of the INNA project on the Paranal sky brightness,” Kaufer explains. He adds that the calculations assume clear-sky condi- tions. “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 observa- tions 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.”