Aluminium oxide threat: Satellite debris and the ozone layer recovery

According to the World Economic Forum, over 6,000 satellites are orbiting the Earth, but only 60 per cent are still in use. However, the space industry is accelerating, so we should expect a significant increase in the number of satellites launched into space each year in the coming years. This includes the Starlink satellite constellation operated by SpaceX, which aims to provide broadband internet worldwide. Thousands of satellites have already been placed into orbit as part of the project, and many more are expected to be launched in the coming years. Each of them is designed to last on orbit for five years. After that, they are deorbited.

When a satellite is deorbited, it re-enters the Earth's atmosphere. Smaller components usually melt and eventually evaporate. However, the aluminium used in the construction, when burning in the atmosphere, forms aluminium oxide, which remains in the upper layers of the atmosphere for years. This topic is little studied. We do not fully know the impact, but scientists indicate that aluminium oxide particles destroy the Earth's ozone layer.

New research published in the journal "Geophysical Research Letters" has shown that the concentration of aluminium oxides increased eightfold between 2016-2022 and will continue to rise as the number of satellites in low Earth orbit rapidly increases.

The depletion of the ozone layer is linked to humans' emission of freons. Freons (CFC, short for Chlorofluorocarbons – a group of chloro and fluoro derivatives of aliphatic hydrocarbons) are long-lived chemical compounds that rise to the stratosphere, where they are dispersed by ultraviolet radiation, releasing chlorine atoms that then destroy ozone molecules.

Freons were widely used in the industry until 1987 when an international agreement to counteract the ozone hole – the Montreal Protocol – was signed. This led to the ban on the use of many chemicals in industry that harm the ozone layer.

The ozone hole over the South Pole appears in September and October when spring begins in the southern hemisphere. The first rays of the sun after the polar winter release numerous chlorine atoms, which destroy ozone and deplete the entire layer. These reactions occur on the surface of clouds formed in the cold stratospheric layers, ultimately leading to uncontrolled reactions that destroy ozone molecules.

The Montreal Protocol has successfully reduced the ozone hole over Antarctica. UN experts estimated that the ozone layer would regenerate by around 2060. However, an unexpected increase in aluminium oxides in the stratosphere may halt the recovery of the ozone layer.

The demand for internet access is causing a rapid increase in the number of communication satellites. SpaceX is a leader in this endeavour. It has permission to launch 12,000 Starlink satellites into orbit, but there are plans for many more. Other companies also plan to build similar constellations, which will also number thousands of satellites.

Internet satellites in low Earth orbit have an expected operational life of up to five years. After this period, the satellite enters the atmosphere, and another one replaces it in orbit.

The aluminium oxide produced during deorbitation provokes chemical reactions that destroy the stratospheric ozone, which protects the Earth from harmful UV radiation. Aluminium oxides do not chemically react with ozone molecules but trigger destructive reactions between ozone and chlorine, which destroy the ozone layer. Since aluminium oxides are not destroyed in these processes, they can remain in the stratosphere for decades.

Little attention has been paid to the pollution problem caused by satellites burning in the atmosphere. Previous research focused on the consequences of launching rocket mechanisms into space, such as the release of rocket fuel. A new study by a research team from the University of Southern California is the first realistic estimate of the size of this long-term pollution in the upper layers of the atmosphere.

“Only in recent years have people started to think that this might become a problem,” said Joseph Wang of USC, co-author of the new study. “We were one of the first teams to look at what the effects might be,” he added.

Earlier studies used analyses of micrometeoroids generated to estimate the pollution caused by satellite deorbitation because collecting data from a burning spacecraft is impossible. However, micrometeoroids contain very little aluminium, which makes up between 15 percent and 40 percent of the mass of most satellites.

To obtain more accurate data, scientists created a model based on the chemical composition of the materials from which satellites are made and how they interact at the molecular and atomic levels. The results allowed scientists to understand how these materials change with different energy inputs.

Thanks to the new model, scientists discovered that in 2022, satellite deorbitation increased the amount of aluminium in the atmosphere by 29.5 per cent compared to the natural level. Modelling also showed that a typical satellite weighing 250 kilograms, with 30 per cent of its mass being aluminium, will generate about 30 kilograms of aluminium oxide particles (sized 1-100 nanometres) when re-entering the atmosphere. Most of these particles are formed in the mesosphere, 50-85 kilometres above the Earth's surface. Given their size, scientists estimated it would take aluminium oxide up to 30 years to descend to the stratospheric altitudes, where 90 per cent of Earth's ozone is located.

Scientists also estimated that by the time the currently planned satellite constellations are completed, 1,000 tonnes of aluminium will fall to Earth each year. This will release approximately 450 tonnes of aluminium oxides into the atmosphere annually, an increase of 646 per cent compared to the normal level.