Humanity’s messy fingerprints, from disposable grocery bags adrift in the deepest oceanic trenches to microplastic-laced snowfall on the tallest mountaintops, litter Earth—and each contaminant poses potential environmental hazards that remain poorly understood. Now scientists have found another pollutant to worry about: vaporized metals from burned-up space junk that are floating around in Earth’s stratosphere, the same atmospheric region that holds our planet’s fragile, protective ozone layer.
In a series of high-altitude research flights over Alaska and the U.S. Midwest in March and April, researchers sampled stratospheric air using specialized mass spectrometers. They discovered surprising amounts of many metals commonly used in rockets and satellites, often in ratios mirroring those found in specific high-performance aerospace alloys. The investigation revealed that the metals are accumulating within sulfuric acid particles, which constitute most of the stratosphere’s particulates and influence our world’s ozone layer and climate.
Although only about 10 percent of the sampled sulfuric acid particles contained spacecraft-sourced metals, the researchers forecast that value could grow to 50 percent or more in the coming decades because of skyrocketing numbers of launches and satellites. The work was sponsored by the National Oceanic and Atmospheric Administration, and a paper reporting the findings appeared in the Proceedings of the National Academy of Sciences USA on October 16.
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In some respects, none of this is unexpected. Since the dawn of the Space Age, scientists have understood that the heat of atmospheric reentry vaporizes rocket stages, derelict satellites and other infalling spacecraft debris. Aerosolized metals from this human-sourced material add to those from the estimated 50 to 100 tons of space dust that falls into the atmosphere each day. But only very recently—in the past few years—has the contribution from falling space junk come to rival, or in some cases perhaps even exceed, that from this natural background. Data from the NOAA flights suggest that most of the aluminum, copper and lithium now found in the stratosphere is from space junk.
Altogether, the NOAA flights found more than 20 distinct elements from spacecraft and satellite reentry, including silver, iron, lead, magnesium, titanium, beryllium, chromium, nickel and zinc.
An Unquestionable Link
To date, most modeling of reentering space junk has focused on whether any pieces will endure to threaten bystanders on the ground, says Daniel Murphy, the study’s lead author and a research chemist at NOAA’s Chemical Sciences Laboratory in Boulder, Colo.
Earlier researchers “didn’t think a lot about what happens to things that vaporize during reentry. Of course they can’t disappear. They’ve got to go somewhere,” Murphy says. “And now with these measurements, we know where they go. They go into particles in the stratosphere.”
A link between the metals and spacecraft reentry is indisputable, says study co-author Daniel Cziczo, an atmospheric scientist at Purdue University. The results can’t be explained by pollution from rocket launches or aircraft passing through the stratosphere, which yield particles with very different sizes and chemical signatures. Nor can they be ascribed to ground-based industrial processes such as metal smelting, which also produce distinctively different particles that are limited to lower altitudes. “What we are seeing is due to reentry of material—a mixture of burned-up meteors and spacecraft, which slowly coagulates to form particles that settle through the atmosphere,” he says.
Small traces of lithium were the first eye-catching anomaly in the high-altitude flight data, Murphy says, followed by high concentrations of aluminum that far exceeded the abundance found in meteoric space dust. “What really clinched it [was observations of] niobium and hafnium, both of which are really exotic. You absolutely don’t expect to find them in the stratosphere,” he says.
But the NOAA flights weren’t deliberately seeking out vaporized metal from space junk. Rather they were targeted at studying sulfuric acid particles and other stratospheric aerosols. The complex photochemistry of these tiny particles has outsized, planetary-scale effects. Aerosols can tweak Earth’s temperature by modulating the formation of sunlight-reflecting clouds and influence our planet’s natural “sunscreen” by either spurring or suppressing ozone-destroying chemical reactions.
Reining In a Reign of Fire
The ultimate impacts of spacecraft-sourced metals upon Earth’s climate and habitability remain unclear.
What is clear is that this metallic pollution is set to accelerate in years to come as the numbers of rocket launches and atmospheric reentries continue to grow. Thanks in large part to proliferating plans for satellite “mega constellations” such as SpaceX’s Starlink and Amazon’s Project Kuiper, the global launch industry is on track to loft as many as 50,000 new satellites into orbit by 2030. And it doesn’t take a rocket scientist to realize that most of what goes up must come down—in this case as showers of fiery debris pumping more metals into the stratosphere.
“With all these launches, the amount of material vaporizing during reentry could become roughly comparable to the amount of meteoritic material coming in. And it’s different stuff, a different mix of metals,” Murphy says. “When you have potentially 50,000 satellites in orbit, and they have a five-year lifetime, that’s 10,000 reentries a year—something like 30 a day. That is very different than the situation in the past, and that’s one of the things that is really changing.”
Leonard Schulz, a researcher at the Technical University of Braunschweig’s Institute of Geophysics and Extraterrestrial Physics in Germany, who was not involved with the new research, considers the NOAA measurements a groundbreaking empirical validation of his own earlier theoretical work. In 2021 Schulz and his Braunschweig colleague Karl-Heinz Glassmeier published a paper estimating how much material humanity’s space industry might inject into the atmosphere now and in the future. The researchers found that the amounts were likely to be significant, compared with natural sources.
“They confirm the concerns that our modeling of the human-made contribution to atmospheric injection has raised,” Schulz says. “As a result, there is the concrete possibility that this alters the atmosphere of our home planet and has detrimental environmental effects, especially with the current strong growth of the space sector and spacecraft mega constellations.”
Clarifying the nature and extent of space junk’s effects on the stratosphere, Schulz says, depends on many things—more thorough modeling and better observational coverage of spacecraft reentries chief among them. To help that happen, he argues, launch providers and spacecraft manufacturers should publicly disclose information otherwise treated as a trade secret, such as the specific structural recipes and simulated reentry profiles for satellites.
Such secrecy “makes it really hard for scientists to get an idea about the composition of spacecraft, element-wise, and thus determine their impact” upon reentry, Schulz says.
Outside of more industry cooperation, Cziczo sees ample opportunities for further NOAA follow-up flights and lab-based studies.“We should expand the range and seasons, for example, [by obtaining] measurements in the tropics and other places to understand the sources of other types of particles in the stratosphere,” he suggests. “There also needs to be laboratory research to investigate the consequences of adding these metals to sulfuric acid particles. Can the particles nucleate ice and impact clouds and chemistry in the stratosphere?”
Answers won’t come easily but will be essential for properly assessing what risks, if any, humankind’s ongoing expansion into space poses for life down on Earth.
“It’s uncomfortable not knowing whether or not it’s a problem,” Murphy says. “How important is it? Maybe it is not completely important. Or maybe it’s really important. It’s [something] people haven’t, as yet, thought very much about.”