Thousands of satellites have been deployed in “mega-constellations” to meet the growing need for worldwide internet services, with many more planned to be launched soon. However, these compact satellites have short operational lifetimes and are known to emit pollutants that can damage the ozone layer when they disintegrate on re-entry. A recent study published in Geophysical Research Letters quantified the extent of this pollution for the first time. Credit: SpaceX/public domain
When internet-providing satellites — now launched by the thousands — reach the end of their lives, the residue from their burn-up in Earth’s atmosphere starts chemical reactions that damage stratospheric ozone.
When obsolete satellites re-enter Earth’s atmosphere and disintegrate, they release tiny particles of aluminum oxide that deplete Earth’s ozone layer. A recent study shows that the presence of these particles increased eightfold from 2016 to 2022, and is expected to rise further as the number of satellites in low Earth orbit increases.
The 1987 Montreal Protocol successfully regulated ozone-depleting CFCs to protect the ozone layer and shrink the ozone hole over Antarctica, with recovery expected in the next fifty years. However, an unexpected increase in aluminum oxides could interrupt ozone restoration progress in the coming decades.
Of the 8,100 objects in low Earth orbit, 6,000 are Starlink satellites launched in the last few years. The demand for global Internet coverage is leading to a rapid increase in launches of small communications satellites. SpaceX is a pioneer in the venture with an additional 12,000 Starlink satellites authorized to launch and up to 42,000 planned. Amazon and other companies around the world are also planning constellations of 3,000 to 13,000 satellites, the study authors said.
Internet satellites in low Earth orbit have a short lifespan of about five years. Companies must then launch replacement satellites to maintain Internet service and continue the cycle of planned obsolescence and unplanned pollution.
Aluminum oxides cause chemical reactions that destroy stratospheric ozone, which protects the Earth from harmful UV radiation. The oxides do not react chemically with ozone molecules, instead triggering destructive reactions between ozone and chlorine that damage the ozone layer. Because aluminum oxides are not consumed by these chemical reactions, they can continue to destroy ozone molecule by molecule for decades as they drift down through the stratosphere.
Yet little attention has been paid to the pollutants produced when satellites fall into the upper atmosphere and burn up. Previous studies of satellite pollution have largely focused on the consequences of launching a launch vehicle into space, such as leaking rocket fuel. The new study by a research team at the University of Southern California Viterbi School of Engineering is the first realistic estimate of the extent of this long-term pollution in the upper atmosphere, the authors said.
“It’s only in recent years that people have started to think that this might be a problem,” said Joseph Wang, an astronautics researcher at the University of Southern California and corresponding author of the new study. “We were one of the first teams to look at the implications of these facts.”
The study was published in the open access journal AGU Geophysical Research Letterswhich publishes high-impact reports in a short format with immediate implications spanning all Earth and space sciences.
A sleeping menace
Because it is virtually impossible to collect data from a spacecraft that is on fire, previous studies have used analyzes of micrometeoroids to estimate potential contamination. But micrometeoroids contain very little aluminum, a metal that makes up 15% to 40% of the mass of most satellites, so these estimates did not apply well to the new “swarm” satellites.
To get a more accurate picture of the contamination from satellite re-entry, the researchers modeled the chemical composition and bonds in the satellite materials as they interact at the molecular and atomic level. The results allowed the researchers to understand how the material changes with different energy inputs.
The researchers found that in 2022, re-entering satellites increased the aluminum content of the atmosphere by 29.5% above natural levels. Modeling showed that a typical 250-kilogram (550-pound) satellite, with 30% of its mass being aluminum, would generate about 30 kilograms (66 pounds) of alumina nanoparticles (1-100 nanometers in size) on its re-entry. Most of these particles are created in the mesosphere, 50-85 kilometers (30-50 miles) above the Earth’s surface.
The team then calculated that, based on particle size, it would take up to 30 years for aluminum oxides to reach stratospheric altitudes, where 90% of Earth’s ozone resides.
Researchers have estimated that before the currently planned satellite constellations are completed, 912 metric tons of aluminum (1005 US tons) will fall to Earth each year. This releases approximately 360 metric tons (397 US tons) of aluminum oxides annually into the atmosphere, an increase of 646% over natural levels.
Reference: “Potential ozone depletion due to satellite extinction during atmospheric return in the megaconstellation era” by José P. Ferreira, Ziyu Huang, Ken-ichi Nomura, and Joseph Wang, 11 Jun 2024, Geophysical Research Letters.
DOI: 10.1029/2024GL109280
This work was funded by NASA.