According to recent research, numerous satellites in low Earth orbit, such as those operated by Starlink, could pose a threat to our planet’s ozone layer once they re-enter the atmosphere.
Constellations of small satellites, such as those used for broadband, tend to have a relatively short lifespan. In the case of Starlink, it is expected to be around five years. After that, they re-enter the atmosphere and burn up, after which replacement satellites are sent into the atmosphere to take over their role.
But researchers at the University of Southern California (USC) say this leads to the buildup of aluminum oxides in the atmosphere, which are known to accelerate ozone depletion. Given the large number of satellites involved – Starlink alone is estimated to have 6,078 satellites in orbit by May 2024 – this could pose a serious risk.
A research letter published in Geophysical Research Letters entitled “Potential ozone depletion from satellite demise during atmospheric re-entry in the era of mega‐constellations” states that the demise of a typical 250 kg satellite can release about 30 kg of alumina nanoparticles that may persist in the atmosphere for decades.
The researchers assume that large satellite constellations could release more than 360 tons of aluminum oxide compounds into the atmosphere each year, which could lead to significant ozone depletion.
Aluminum is one of the most common materials used in satellites, the article says. Upon re-entry into the atmosphere, it reacts with oxygen to produce aluminum oxide, which can affect ozone chemistry. A chlorine activation reaction catalyzed on the surface of aluminum oxide particles enhances ozone depletion.
The effects of aluminum have been known for some time from studies of the emission of hundreds of tons of such particles by solid rocket engines during ascent into the Earth’s atmosphere. However, little attention has been paid to the re-entry of spacecraft from low Earth orbit, and the re-entry of satellites has not been considered at all, according to the researchers.
They estimate that it may take up to 30 years for the byproducts of reentry to pass from the upper mesosphere to the stratospheric ozone layer. Upon reaching an altitude of about 40 km, aluminum oxides catalyze chlorine activation, which promotes ozone depletion.
This would mean a noticeable delay between the start of the reentry process when the orbiting bodies are decommissioned and the effects of stratospheric ozone depletion. With satellite reentry rates increasing, it is critical to further investigate the concerns highlighted in this study, the researchers claim.
How concerned should we be about this? The short answer seems to be that we don’t know and more research is needed.
Until now, scientists were convinced that the ozone layer was slowly recovering from damage caused by chemicals such as chlorofluorocarbons (CFCs), thanks to the Montreal Protocol agreed in the 1980s.
Last year, a UN-backed scientific panel reported that around 99 percent of banned ozone-depleting substances had been phased out. The panel said that if current policies were maintained, the ozone layer in Antarctica could be expected to recover to 1980 levels (before the ozone hole) by around 2066 and in much of the rest of the world by 2040.
We asked the USC researchers whether the amount of aluminum oxide they expect to enter the atmosphere would have serious implications for ozone layer regeneration.
They told us that the current study simply compares the amount of aluminum oxide created by satellite re-entry to natural concentrations in the atmosphere. They are currently unable to compare this to the effects of other chemicals, but plan to investigate this in a future study.
Robyn Schofield, associate professor of atmospheric chemistry at the University of Melbourne, explained in an article that other researchers studying aerosol particles in the stratosphere had previously discovered traces of metal during spacecraft re-entry.
“We don’t know what the impact will be,” she said. “One likely consequence would be that the aluminum particles promote the growth of ice-containing particles. That means there would be more small, cold, reflective particles with a larger surface area on which chemical processes can take place.”
“We also don’t know how aluminum particles react with the sulfuric acid, nitric acid and water found in the stratosphere. So we can’t really say what effect this will have on ozone depletion.”
And there’s a certain urgency to find out. Starlink and other low-orbit communications operators alone are planning tens of thousands of new satellites by 2030, and the total worldwide could reach into the millions, according to some estimates. ®