The sky is getting crowded.

In the last few years, the number of satellite launches has increased by an order of magnitude as mega-constellations of internet-powering hardware crowd into low Earth orbit. The pace of both launching and retiring these units is creating new kinds of pollution, potentially upsetting the climate system and the protective ozone layer.

There were 14,300 active satellites orbiting at the start of January. Twenty years earlier, there had been 871, according to figures published by Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics. Of active units today, over 9,200 belong to SpaceX, which in December filed a request with the Federal Communications Commission to launch another 15,000. Amazon has plans for more than 3,000, while Amazon founder Jeff Bezos’s Blue Origin this month announced plans for a network of 5,408 more. China’s Qianfan and Guowang have also begun launching mega-constellations.

Because these satellites are relatively inexpensive and the technology is rapidly improving, many are scuttled after just five or 10 years: directed to reenter the atmosphere and burn up so they don’t clutter the valuable real estate a few hundred miles above Earth.

Those burning satellites release a mix of heavy metal and carbon particles that float down closer to Earth, into the otherwise chemically stable stratosphere – where airplanes fly, weather patterns are shaped, and ozone screens our planet.

“We’re really changing the composition of the stratosphere into a state that we’ve never seen before,” said John Dykema, an applied physicist at SEAS, who warns that scientists today poorly understand many of the impacts.

What is becoming apparent, he said, is that the rush to space risks disrupting the global climate system and further depleting the ozone layer, which shields all living things from DNA-destroying ultraviolet radiation.

How satellite soot can heat the sky

As the friction of reentry ignites a satellite, organic materials onboard – such as plastics or carbon-fiber composites – release particles of black carbon, the same fine soot in wood or coal smoke.

Physically, black carbon comes in different forms. Some particles follow repeating patterns and are “crystalline,” like graphite – the smooth flakey layers found in pencils. Others are random, or “amorphous.”

Those structural and chemical differences – including how the carbon atoms bond with elements like hydrogen – determine how the black carbon affects incoming sunlight. Some particles scatter sunlight; some absorb it. Some absorb infrared light, but not ultraviolet.

Exactly how this process plays out in the stratosphere, and the impact on the climate, remains uncertain. Does black carbon mostly alter Earth’s overall energy balance – the equilibrium between incoming solar energy and the outgoing heat radiated back to space? Or does it more strongly disturb stratospheric circulation – the slow conveyor belt that carries air, heat, and trace gases such as ozone and water vapor out of the tropics, toward the poles, and back down again, shaping how heat is shuffled around the climate system and how surface weather evolves?

“We’re putting thermal energy into the Earth’s climate system, but we’re putting it in new places,” Dykema said. “We don’t really understand the implications of changing stratospheric circulation. It could cause storm tracks to move. Maybe it could shift climate zones, or possibly be a new source of droughts and floods.”

Threats to the ozone layer

Measurements and modeling by Harvard researchers and Rajan Chakrabarty of Washington University in St. Louis suggest that one of the common metals in satellite construction, aluminum, can end up as aluminum‑containing particles in the stratosphere, where aluminum oxide on those particles provides surfaces for ozone‑destroying chemistry.

Sandro Vattioni at ETH Zurich has shown the large uncertainties in how this process can switch ambient chlorine at that altitude into a highly reactive form.  

“Chlorine is one of the key actors in the ozone hole,” said Dykema. “And so if you add a new surface that converts existing chlorine into reactive and free radical forms, that will also promote ozone loss.” This additional loss is not yet enough to create a new ozone hole, but it can slow the recovery that began after the 1987 Montreal Protocol phased out chlorofluorocarbons.

And what happens as humanity puts more and more material into orbit, much of it eventually returning as debris? If aluminum oxide continues to accumulate, ozone loss could become more serious. “It’s not the most efficient way to create an ozone hole, but it does cause additional ozone erosion, which slows the recovery,” Dykema said.

What should policymakers do?

For now, scientists say, the most important step for policymakers is to take the problem seriously without pretending to have all the answers. Researchers around the world are working to understand what is at stake to guide future policy.

Dykema suspects that changes to stratospheric circulation may ultimately prove more consequential than the additional ozone loss, because the outcomes are so uncertain and potentially far-reaching. For the moment, many questions “are not really amenable to straightforward, linear analysis,” he said, cautioning, “the ozone loss is significant, and we’re putting so much stuff up there that it could grow in ways that are not proportional.”

But even in the face of those unknowns, choices are already on the table. Governments and regulators can push for a more sustainable use of space, from the materials used in satellite construction to the types of propellants powering rockets. Solid fuels, for example, produce aluminum and aluminum oxide in exhaust, suggesting that decisions about solid versus liquid propellant, and about which regions and seasons to launch in, could have real atmospheric consequences.

The question, as the satellite age accelerates, is whether policymakers will act on those concerns before the invisible wake of our spacefaring ambitions becomes impossible to ignore.

–As told to David Trilling