Imagine looking up at the night sky and seeing a spectacular light show. That's what happened in January when about 120 Starlink satellites from SpaceX burned up in Earth's atmosphere. This created artificial meteor showers that were visible to many people around the world. While these shows might seem harmless or even beautiful, scientists are worried about the impact on our environment.

        The main concern is the release of aluminium oxide particles. When satellites re-enter the atmosphere, they burn up and release these particles. These particles can stay in the upper atmosphere for a long time before they reach the stratosphere, where the Earth's protective ozone layer is located. The ozone layer is crucial because it protects all life on Earth from harmful ultraviolet radiation.

        The satellites in question are small Low Earth Orbit (LEO) satellites, like those used by Starlink. These satellites have a lifespan of about five years and are made mostly of aluminium. Since the first batch of 60 satellites was launched in May 2019, many of them have been coming down regularly. This is part of a larger trend of increasing satellite launches to provide global internet coverage.

        The European Space Agency (ESA) reports that there are over 28, 000 objects in space, with most of them in low Earth orbit. In recent years, nearly 8, 000 Starlink satellites have been launched. SpaceX has permission to launch another 12, 000 satellites and plans for as many as 42, 000 in total. Other companies, like Amazon, are also planning their own satellite constellations, ranging from 3, 000 to 13, 000 satellites.

        So, what happens when these satellites fall back to Earth? During re-entry, the aluminium in the satellites creates aluminium oxide. This process is not environmentally neutral. The metals in the satellite, especially aluminium, undergo chemical changes. A typical Starlink satellite weighs around 250 kg and produces about 30 kg of aluminium oxide particles upon re-entry. These are not large pieces of debris but microscopic nanoparticles that stay suspended in the upper atmosphere.

        The re-entries usually happen in the mesosphere, which is around 50 to 80 km above Earth's surface. The aluminium oxide nanoparticles emitted during the burn-up stay afloat in this region for long periods before they descend into lower altitudes. The concern is what happens when these particles eventually reach the stratosphere, where the ozone layer is located.

        Researchers from the University of Southern California’s Department of Astronautical Engineering have found that aluminium oxide can act as a catalyst for chemical reactions involving chlorine. This is similar to the process that led to ozone depletion from chlorofluorocarbons (CFCs) in the past. Unlike CFCs, which were banned under the 1987 Montreal Protocol, aluminium oxide particles do not consume ozone directly. However, they can enable chemical reactions that destroy ozone molecules.

        LEO satellites, like those in the Starlink constellation, are designed to fall or be deorbited at the end of their missions. This is a standard practice to ensure the sustainability of space. These satellites are equipped with propulsion systems that allow them to perform controlled deorbit movements. This mechanism ensures that the satellite re-enters the atmosphere and disintegrates after mission completion.

        Several recent studies have suggested a significant increase in aluminium oxide in the atmosphere related to the re-entry of satellites. In February 2023, NASA conducted high-altitude test flights over Alaska. Closer examination of the aerosols collected revealed the presence of 10 per cent of stratospheric sulphuric acid particles, which were larger than 120 nanometres in diameter, containing aluminium and other metals emitted from satellite and rocket re-entries. These tests confirmed that space hardware was leaving what scientists call a detectable chemical signature in the atmosphere.

        Researchers from the University of Southern California Department of Astronautical Engineering suggested that aluminium oxides in the atmosphere increased eightfold between 2016 and 2022. This coincides with the rapid proliferation of satellite constellations during this period. In 2022 alone, re-entries released an estimated 41. 7 metric tonnes of aluminium into the atmosphere, which is about 30 per cent more than natural inputs from micrometeoroids.

        The impact of these particles on the ozone layer is a concern. Based on molecular dynamic simulations, the particles created in the mesosphere may take around 20 to 30 years to descend into the ozone layer. This means the environmental impact of today’s satellite re-entries will not be apparent for decades. Scientists claim that by the time measurable ozone depletion is detected, the mesosphere could already be overflowing with aluminium oxide particles.

        Even though the concerns are valid, researchers also point to the absence of a comprehensive regulatory framework that addresses the atmospheric impact of re-entries. The US Federal Communications Commission (FCC) provides licenses to satellite mega-constellations, but it does not consider re-entry debris or ozone depletion in its assessments. Also, commercial satellites have been excluded from environmental review under the National Environmental Policy Act (NEPA).

        From a global perspective, while the UN Committee on the Peaceful Uses of Outer Space (COPUOS) has begun discussions around guidelines for space sustainability, the progress has been slow. There is also no binding international agreement regarding pollution from satellite re-entries.

        Experts say coordinated action from various stakeholders will help address the challenge. They suggest that satellite manufacturers could come up with alternatives to aluminium or design spacecraft that can be boosted into higher graveyard orbits rather than allowed to re-enter. A graveyard orbit is an orbit where decommissioned satellites are placed to reduce the risk of collisions between operational satellites and space debris. This, however, may require additional onboard propellant and may only delay the problem for some more years.

        The ESA was in discussions with SpaceX in October 2024 to join an international effort towards reducing space debris, according to reports. As part of ESA’s Zero Debris initiative, it aims to prevent the generation of new orbital debris by 2030.

Satellites Falling: Is Our Ozone Layer Safe?

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