March 21, 2022 3:02 PM EDT

It’s hard not to love the Kepler Space Telescope. Launched in 2009, the venerable spacecraft discovered nearly 5,000 suspected or confirmed exoplanets—or worlds orbiting other stars—during its 11-year lifetime. Built and launched at a relative bargain price of $600 million, it generated 4,306 scientific papers written by 9,606 authors. So all good, right? Well, not entirely.

In that same 11 years, the telescope that discovered so many other worlds did no favors for our own, generating an annual total of 4,784 tons of carbon dioxide emissions, or a hefty 52,620 tons over its lifetime, mostly as a result of the electricity and supercomputing power it took to keep it operating. That also comes out to 12 tons of CO2 per paper and five tons per author.

Astronomy, in some ways, seems like the cleanest of sciences. After all, it costs nothing to look at the sky. But both ground-based and space-based observatories extract a huge environmental toll—in terms of construction, launch, energy generation and consumption, and even, at least before the pandemic, in the air miles burned as the world’s estimated 30,000 astronomers flew from conference to conference around the globe.

Now, a new paper in Nature Astronomy has taken the full measure of the greenhouse gas footprint of the skygazing discipline. For the study, researchers analyzed the total CO2 output of 46 space-based missions and 39 ground observatories, dating as far back as the the 62-year old Observatoire de Haute Provence, in southeastern France and as recently as the new InSight observatory in New Mexico, which went online in 2017. In that time, the researchers—affiliated with the Institut de Recherche en Astrophysique et Planétologie (IRAP), in Toulouse, France—concluded that the 85 observatories have generated a prodigious 20.3 million tons of CO2, or an average of 1.2 million tons per year.

“Astronomers get caught up in the day to day—the next funding grant, the next new project,” said Annie Hughes, IRAP astronomer and a co-author of the paper, at a March 17 news conference announcing the results. “Our colleagues are aware of climate change as a problem, but there’s an enormous amount of inertia in the system.”

The paper, which the researchers had been working on for three years, used 2019 as the reference year for data, and thus does not include such new observatories as the James Webb Space Telescope, or the massive Square Kilometer Array now under construction in Australia and South Africa. Other spacecraft not included in the work include the trio of ships from China, the U.S. and the United Arab Emirates which arrived at Mars in 2021. But even without those new entries to the field added, the numbers are troubling enough.

When it comes to ground-based observatories, the straightforward business of construction dominates the first phase of the CO2 emissions—with the pouring of concrete, which releases 600 kg (1,100 lbs.) of CO2 for every ton of the material used. All by itself, the cement industry is responsible for 8% of annual greenhouse gas emissions.

“The VLT [Very Large Telescope] and the ALMA [Atacama Large Millimeter/submillimeter Array] are just tremendously big and expensive infrastructure and they come with a huge carbon footprint,” said IRAP astronomer and paper co-author Jürgen Knödlseder at the press conference.

Location makes a difference too. Once a ground-based telescope is built, its biggest CO2 contribution comes from the electricity it takes to conduct its observations. Chile’s Atacama Desert is home not only to the VLT and the ALMA, but to 14 other observatories, thanks to its exceedingly dry air and its 330 nights of clear skies per year. That makes for great viewing, but Chile ranks only in the middle range of countries when it comes to the cleanliness of its electricity grid.

“Chile has a kind of average emission factor for electricity,” said Knödlseder. “So it’s not as high, for example, as Australia, which burns a lot of coal, but it’s not as low as Sweden and France, which use a lot of renewable energy.”

Worse, extremely remote observatories may not even be connected to their home countries’ electrical grids and thus must run on their own diesel-generated electricity. ALMA, one such observatory, emitted nearly 300,000 tons of CO2 during its construction phase and continues to pump out an average of 56,154 tons per year.

Space-based observatories have, on the whole, a smaller carbon footprint, since they are extremely small compared to terrestrial observatories and their construction generally takes place in climate-controlled clean-rooms and hangars. Their biggest climate impact comes from the electricity used in their annual operation, which adds up over time. The Hubble Space Telescope, which is still operating and had been in service for nearly 30 years as of the 2019 data used in the paper, has released 1.2 million tons of CO2 over its lifetime, or roughly 21 tons for every one of the 52,497 papers that have been written about its findings. The James Webb Space Telescope, launched on Christmas Day 2021, could have a similar carbon footprint of more than 1.22 million tons of CO2 over the course of its anticipated 20-year lifetime, the authors estimate.

The paper stresses that the astronomy community must take dramatic steps to address its carbon footprint and not simply consider it the cost of doing business. The 20.3 million tons of CO2 emitted overall by the 85 observatories is, after all, the equivalent of the annual greenhouse gas output of entire countries such as Croatia, Bulgaria or Estonia. There are ways to bring those numbers down.

“The first step,” said IRAP astronomer and co-author Lyigi Tibaldo, “is that existing structures are decarbonized, by switching to renewable energy sources.” Sun is abundant in the Atacama, making solar power a viable option. And the more the overall energy grid, especially in Europe, comes to rely on renewables, the more the telescopes located there will be able to operate without so much of a greenhouse impact. Most space-based observatories already rely on solar panels to keep them going, but a cleaner grid means their observations can be conducted and their data analyzed with a smaller carbon footprint too.

Another answer, the authors argue, is to slow down the current building boom in new observatories in the Atacama and elsewhere, relying more on the astronomical infrastructure that already exists. “The strong reduction of emissions that are required in the next decade will not be achieved if we continue building new infrastructure at the pace that is occurring now,” said Tibaldo. “That will also give us more time to perform more comprehensive exploration of the data we have from existing infrastructure.”

That, the authors acknowledge, will not be a popular prescription. “Some of our colleagues are a bit shocked at the idea of slowing down,” said Knödlseder. “But the [climate] emergency we are facing is so big that we think this option must indeed be on the table.”

For now, the telescopes will keep on working and the astronomers will keep on observing and the paper will continue to be churned out by the thousands and—academically at least—that is a very good thing. But as the authors noted in the very first paragraph of their paper, United Nations Secretary General António Guterres has warned that the most recent International Panel on Climate Change (IPCC) report is nothing less than a “code red for humanity.” It is well and good to go searching for and studying new worlds. But it’s even more important that we preserve and protect the one we’ve got.

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Write to Jeffrey Kluger at [email protected].

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