In January, some 120 SpaceX Starlink satellites broke apart in Earth's atmosphere, producing what most called artificial meteor showers. Three to four satellites were reported to be re-entering daily, leaving brilliant streaks visible to people worldwide. Though the spectacle appeared harmless, even beautiful to spectators, scientists are growing increasingly uneasy about their environmental cost.

Experts warn that satellite re-entries release aluminum oxide particles into the atmosphere, which could harm the ozone layer over time. When satellites disintegrate upon re-entry, their materials, particularly aluminum, undergo oxidation. These particles then settle in the stratosphere, the layer of the atmosphere that houses the Earth’s protective ozone shield.

Low Earth Orbit (LEO) satellites, like Starlink's constellation, circle the Earth between 550 and 1,200 km. At the end of their useful life—typically after five years—they are intentionally destroyed to avoid creating space debris. During re-entry, they move at approximately 27,000 km per hour. The extremely high friction with the Earth's atmosphere creates intense heat, and satellites burn up practically instantly. Most are designed to burn up completely before reaching the ground, ensuring safety.

However, scientists emphasize that this process is not environmentally neutral.

A study indicates that aluminum oxide debris from burning satellites might serve as catalysts for chemical reactions that involve chlorine, similar to the formerly banned chlorofluorocarbons (CFCs) responsible for ozone depletion. Although aluminum oxide does not destroy ozone itself, researchers estimate it may contribute to reactions breaking up ozone molecules with long-term implications for the environment.

A University of Southern California Department of Astronautical Engineering study determined that a single aluminum oxide particle might destroy thousands of ozone molecules over many decades. 

Unlike CFCs, which were outlawed by the 1987 Montreal Protocol, aluminum oxide emissions are unregulated.

Since May 2019, SpaceX has launched almost 8,000 Starlink satellites, with thousands more on the horizon. The European Space Agency (ESA) puts the number of objects in orbit around Earth at more than 28,000, the majority in LEO. As the need for worldwide internet coverage increases, satellite constellations are being launched at record levels. SpaceX alone has authorization to launch another 12,000 satellites, with a long-term plan to launch as many as 42,000.

Other entities, such as Amazon, also intend to launch thousands of satellites in the next few years.

Aluminum oxide is growing in the air at an alarming rate, say scientists. In 2023, NASA identified metals like aluminum in samples of high-flying aerosols from above Alaska. Researchers suggest the concentration of aluminum oxides in the atmosphere increased eightfold from 2016 to 2022 and parallels the growth spurt in satellite constellations.

Re-entries deposited an estimated 41.7 metric tonnes of aluminum into the air in 2022 alone, 30% higher than natural micrometeoroid input. At the pace of satellite deployment, if repeated, aluminum oxide emissions would be 360 metric tonnes a year, 646% higher than natural atmospheric concentration. Scientists worry that by the time ozone depletion is quantifiable, the air might already contain aluminum oxide particles to the point of saturation, with long-term impacts.

In spite of increasing concerns, there are no international regulations on the environmental effects of satellite re-entries. The US Federal Communications Commission (FCC) authorizes satellite mega-constellations but does not yet take atmospheric pollution into account in its evaluations. Commercial satellites are also not included under the National Environmental Policy Act (NEPA).

The UN Committee on the Peaceful Uses of Outer Space (COPUOS) has initiated consideration of space sustainability, but efforts have been glacial. The experts say that immediate, collective action is necessary from governments, space agencies, and private industry.

One solution is to make satellites out of different materials that emit fewer toxic emissions when they re-enter the atmosphere. Another solution might be to put decommissioned satellites into "graveyard orbits" instead of letting them burn up in the atmosphere. But this would take more fuel, so it would be a short-term solution, not a long-term one.

As thousands of satellites are going to re-enter Earth's atmosphere over the next few years, researchers emphasize urgent action to protect the ozone layer from any damage that might occur. 

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