The World's Largest Telescope Has a Problem

The Very Large Telescope or VLT is a project of the European Southern Observatory or ESO, and has  been in service since the 1990s.  It’s one of the world's most sensitive sky-watching instruments, capable of observing the most intriguing objects in the universe.  The ESO, knowing they were putting together the world's largest optical telescope, chose a remote, ultra-dark-sky location,  Mount Paranal, an 8,740-foot-high (2635m) peak in the Chilean Atacama desert. The VLT is built from four large aperture (27-foot or 8.2 meter diameter) telescopes that can act as one.   

While I could personally geek out on the optical design details and watch the hours of video I'm sure I could find, that's really not the story here.  

The story is that astronomers have started sounding alarms because an American energy company called AES Energy wants to build a large renewable hydrogen manufacturing complex only a few kilometers from the VLT.  The astronomers are concerned that light pollution from this plant will ruin their images. Xavier Barcons, ESO's Director General, told Space.com that the observing potential of this astronomical powerhouse will be significantly curtailed if the hydrogen project, called INNA, goes ahead.

"The brightness of the sky is going to increase by up to 10% from this project," Barcons said. "And that is enough to make a difference between the best observatory in the world and an average observing place."

Thanks to the unique geography of the area in the North Chilean Andes mountains, the night sky above Mt. Paranal is perfectly clear more than 11 months per year, providing perfect conditions for the most challenging astronomical research.  Barcons added, "It's the darkest place where we have ever set an observatory in the world, by a large margin." 

A survey published in 2023 found that among the world's 28 most powerful astronomical observatories, telescopes on Mount Paranal suffered from the lowest levels of artificial light pollution.  

A complication here is that the ESO is planning the successor to the Very Large Telescope, called (brace yourself) the 1.5 billion dollar Extremely Large Telescope or ELT.  Construction has begun on the ELT, and reports are it's more than halfway built; it's also in the Chilean Andes, just on a different peak, called Cerro Armazones.  The ELT website says it's "in the central part of Chile's Atacama Desert, some 130 kilometres south of the town of Antofagasta and about 20 kilometres (12 miles) from Cerro Paranal, home of ESO's Very Large Telescope (VLT)."   

If the VLT is going to be compromised by this hydrogen project, it's reasonable to think the ELT will be compromised as well.  

My first take on this is that we know why the big telescopes are in this area of Chile: the dark and transparent skies are the best in the world.  The question then is why the AES Energy complex can't be somewhere else.  My bet is it's probably the cost of the land for the project and getting it done in Chile compared to doing it elsewhere, like in Ivanpah, California, home to one of the biggest bird incinerators in the world.  It's not clear what the ESO is looking for in doing this "press release" (that might not be the best word).  Are they trying to get a groundswell of public opposition to the INNA project, or are they trying to convince Chile to deny approval to build it?

I know humanity isn't ready for this option just yet, but I think the far side of the moon is the best place for the VLT, ELT, or the "Oh My God You Won't Believe How Large Telescope" that they'll build for the next generation.  God willing, I'll live to see that.

A photo of one of the VLT optical telescopes at work.  The four bright lines are the most powerful lasers ever put on a telescope and are used to provide a "synthetic star" that can be used to correct atmospheric distortion in the telescopic image - a technique called adaptive optics.  Image credit: ESO/S. Lowery.