In January, about 120 SpaceX Starlink satellites burnt up in Earth’s atmosphere. Reports say there were three to four re-entries each day which led to the creation of artificial meteor showers, and were visible to many around the world.
While these spectacular showers are seemingly harmless or even eye-pleasing, scientists have raised alarms about their serious threat to the environment. Scientists have raised concerns about the satellite re-entries in the upper atmosphere or mesosphere which then settle in the stratosphere that houses the Earth’s protective ozone layer. The major concern is the release of aluminium oxide particles that could, in the long run, damage the ozone layer.
When satellites re-enter and burn up, many of the metals on the satellites get oxidised, including aluminium. Small Low Earth Orbit (LEO) satellites like Starlink have aluminium in abundance, and they have a lifetime of about five years, according to reports. There is a constellation of such satellites that enable Starlink to operate its SpaceX satellites. Since the first batch of 60 satellites took place in May 2019, many of them have been coming down regularly.
According to the European Space Agency (ESA), there are over 28,000 objects in space and most of them are in low Earth orbit. In the last few years, nearly 8,000 Starlink Satellites have been launched. The increasing demand for internet coverage globally is accelerating the rapid launch of small communication satellite constellations. As of now, SpaceX is the front-runner with permission to launch another 12,000 Starlink satellites and as many as 42,000 planned. Meanwhile, Amazon and other companies around the world are also planning satellite constellations ranging between 3,000 and 13,000 satellites.
What happens during the re-entry of satellites?
January witnessed the maximum re-entries. Upon reentry, the aluminium in these satellites creates aluminium oxide, which is a threat to the ozone layer. The LEO satellites usually orbit between 550 and 1,200 km above the Earth. Once their operational period comes to an end, they are decommissioned and allowed to fall back on Earth. This mechanism has been designed to prevent space debris from accumulating. This is so far seen as a responsible approach to space sustainability.
During their re-entry into the atmosphere, satellites travel at about a speed of 27,000 km per hour. At this high speed, the collision of the satellite with the dense atmosphere generates extreme heat through aerodynamic friction. Following this, the satellite almost instantly disintegrates, and most of its components vaporise. Most satellites are designed to burn up entirely before reaching Earth’s surface to avert any danger to people or property.
According to scientists, this burning-up process is not ‘environmentally neutral.’ During the process, the metals in the satellite undergo chemical transformations, especially concerning aluminium, which usually constitutes about 40 per cent of a satellite’s mass. Research shows that a typical Starlink satellite weighs around 250 kg and produces about 30 kg of aluminium oxide particles upon re-entry to the atmosphere. They are not huge debris but microscopic nanoparticles that stay suspended in the upper atmosphere.
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Why is aluminium oxide a concern?
The re-entries usually happen in the mesosphere, which is around 50 to 80 km above Earth’s surface. Reportedly, the aluminium oxide nanoparticles emitted during the burn-up stay afloat in this region for long periods before they descend into lower altitudes. The scientific concern here is about what happens when these particles eventually reach the stratosphere, which is home to the ozone layer that protects all life from harmful ultraviolet radiation.
According to researchers from the University of Southern California’s Department of Astronautical Engineering, aluminium oxide can act as a catalyst for chemical reactions that involve chlorine, much like the process that led to ozone depletion from chlorofluorocarbons (CFCs) in the past. CBFs are capable of destroying ozone molecules.
Unlike the CFCs, which were banned under the 1987 Montreal Protocol, the aluminium oxide particles do not consume ozone directly. However, as per the research, they act as catalysts or substances that can enable chemical reactions without being consumed themselves. Reportedly, one aluminium oxide particle could potentially contribute to the destruction of thousands of ozone molecules over decades.
Satellites falling is a norm
LEO satellites such as those in the Starlink constellation have been designed to fall or what is known as end-of-life disposal strategies to prevent the accumulation of space debris. 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.
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This deorbiting process is a standard practice to ensure the sustainability of space. In case an LEO satellite malfunctions and goes out of control, the atmospheric drag will eventually slow it down, causing it to re-enter the atmosphere and burn up over time. According to experts, this passive mechanism is a default safety feature in LEO satellites. It should be noted that these satellites do not fall prematurely during their operational life; however, they are intentionally deorbited at the end of their missions.
What do scientists say?
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 at about 60,000 feet. 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.
According to researchers, the rate of increase is more concerning. 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 (tiny space rocks leading to 16.6 metric tonnes of aluminium oxide in the mesosphere). Researchers say if the pace of current satellite deployment persists, aluminium oxide releases could reach 360 metric tonnes annually — a 646 per cent increase over natural atmospheric levels.
When it comes to impact, there is a time delay involved, making the situation particularly grave. Based on molecular dynamic simulations, the particles created in the mesosphere may take around 20 to 30 years to descend into the ozone layer, meaning 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 that will likely continue to affect ozone chemistry for years to come, till some regulatory changes are implemented. These modelling studies also suggested that in the extreme case, these particles could contribute to an additional 0.05 per cent ozone loss over Antarctica each year. Although this percentage seems small, it could likely delay or reverse the ozone layer’s expected recovery.
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Challenges and potential solutions
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. Reports say 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 the 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 say 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.