Satellite mega-constellations could jeopardize the recovery of the ozone hole

When old satellites fall into Earth’s atmosphere and burn up, they leave behind tiny particles of aluminum oxide, which eat away at Earth’s protective ozone layer. A new study shows that these oxides increased eightfold between 2016 and 2022 and will continue to accumulate as the number of satellites in low Earth orbit skyrockets.

The research has been published in the journal Geophysical research letters.

The 1987 Montreal Protocol successfully regulated ozone-damaging CFCs to protect the ozone layer, shrinking the ozone hole over Antarctica and expecting recovery within fifty years. But the unexpected growth of aluminum oxides could interrupt the ozone layer’s success story in the coming decades.

Of the 8,100 objects in low Earth orbit, 6,000 are Starlink satellites launched in recent years. The demand for global Internet coverage is driving a rapid increase in the launches of small swarms of communications satellites. SpaceX is leading the charge in this endeavor, with approval to launch another 12,000 Starlink satellites and as many as 42,000 planned. Amazon and other companies around the world are also planning constellations ranging from 3,000 to 13,000 satellites, the study authors said.

Internet satellites in low Earth orbit have a short lifespan: about five years. Companies must then launch replacement satellites to maintain Internet service, perpetuating a cycle of planned obsolescence and unplanned pollution.

Aluminum oxides cause chemical reactions that destroy the ozone in the stratosphere, which protects the Earth from harmful UV radiation. The oxides do not react chemically with ozone molecules, but instead cause destructive reactions between ozone and chlorine that deplete the ozone layer. Because aluminum oxides are not consumed by these chemical reactions, they can continue to destroy molecule after molecule of ozone for decades as they drift through the stratosphere.

Yet little attention has been paid to pollutants created when satellites fall into the upper atmosphere and burn up. Previous studies of satellite pollution have largely focused on the effects of propelling a launch vehicle into space, such as the release of rocket fuel. The new study, conducted by a research team from the University of Southern California’s Viterbi School of Engineering, is the first realistic estimate of the extent of this long-lived pollution in the upper atmosphere, the authors said.

“It’s only in recent years that people have started to think this could be a problem,” says Joseph Wang, an aerospace researcher at the University of Southern California and corresponding author of the new study. “We were one of the first teams to investigate what the implications of these facts might be.”

Dormant threat

Because it is effectively impossible to collect data from a spacecraft on fire, previous studies used micrometeoroid analysis to estimate potential pollution. But micrometeoroids contain very little aluminum, the metal that makes up 15% to 40% of the mass of most satellites, so these estimates did not apply well to new “swarm” satellites.

To get a more accurate picture of the pollution from satellite re-entry, the researchers modeled the chemical composition of and bonds in satellites’ materials as they interact at the molecular and atomic levels. The results gave the researchers insight into how the material changes with different energy inputs.

In 2022, satellite re-entry increased aluminum levels in the atmosphere by 29.5% above natural levels, the researchers found. The modeling showed that a typical 250-kilogram (550-pound) satellite with 30% of its mass composed of aluminum will generate approximately 30 kilograms (66 pounds) of alumina nanoparticles (1-100 nanometers in size) during its return to Earth . 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 the aluminum oxides to drift to stratospheric altitudes, where 90% of Earth’s ozone is located.

The researchers estimate that by the time the currently planned satellite constellations are completed, 912 tons of aluminum (1,005 U.S. tons) will fall to Earth each year. That will release about 360 tonnes (397 US tons) of aluminum oxides into the atmosphere per year, a 646% increase over natural levels.