Communications companies like Starlink plan to launch tens of thousands of satellites into Earth orbit over the next decade. The growing swarm is already causing problems for astronomers, but recent research has raised another question: what happens when they start falling?
When these satellites reach the end of their life, they fall into the Earth’s atmosphere and burn up. They leave a trail of tiny metal particles along the way.
According to a study published last week by a team of American scientists, this satellite rain can spew 360 tons of tiny aluminum oxide particles into the atmosphere every year.
The aluminum will mostly be injected at altitudes between 50 and 85 kilometers, but will then drift down into the stratosphere – home of Earth’s protective ozone layer.
What does it mean? The satellite’s condensation path could facilitate ozone-destroying chemical reactions, according to the study. That’s not bad, but as we’ll see, the story is far from simple.
How is ozone destroyed?
Ozone loss in the stratosphere is caused by “free radicals” – atoms or molecules with a free electron. When radicals are produced, they start cycles that destroy many ozone molecules. (These cycles have names Dr Seuss would have admired: NOx, HOx, ClOx, and BrOx, because they all involve oxygen, as well as nitrogen, hydrogen, chlorine, and bromine.)
These radicals are created when stable gases are broken down by ultraviolet light, which is abundant in the stratosphere.
Nitrogen oxides (NOx) start with nitrous oxide. It is a greenhouse gas produced naturally by microbes, but human fertilizer production and agriculture have increased its amount in the air.
The HOx cycle involves hydrogen radicals from water vapor. Not much water vapor enters the stratosphere, although events such as the 2022 eruption of the Hunga Tonga–Hunga Ha’apai submarine volcano can sometimes inject large amounts.
Water in the stratosphere creates numerous small aerosol particles that create a large surface area for chemical reactions and also scatter more light to create beautiful sunsets. (I will return to both of these points later.)
How CFCs Created the ‘Ozone Hole’
ClOx and BrOx are the cycles responsible for the most well-known damage to the ozone layer: the “ozone hole” caused by chlorofluorocarbons (CFCs) and halons. These chemicals, now banned, were commonly used in refrigerators and fire extinguishers and introduced chlorine and bromine into the stratosphere.
CFCs rapidly release chlorine radicals in the stratosphere. However, this reactive chlorine is quickly neutralized and locked in molecules with nitrogen and water radicals.
What happens next depends on aerosols in the stratosphere and near the poles also on clouds.
Aerosols speed up chemical reactions by providing a surface on which they can take place. As a result, aerosols in the stratosphere release reactive chlorine (and bromine). Polar stratospheric clouds also remove water and nitrogen oxides from the air.
So in general, when there are more stratospheric aerosols around, we are likely to see more ozone loss.
Increasingly metal stratosphere
The details of the specific injection of aluminum oxides by falling satellites would be quite complex. This is not the first study to highlight the increasing pollution of the stratosphere from space debris re-entry.
In 2023, researchers studying aerosol particles in the stratosphere detected traces of metals after re-entering the spacecraft. They found that 10 percent of stratospheric aerosols already contain aluminum and predicted that this would increase to 50 percent over the next 10–30 years. (About 50 percent of stratospheric aerosol particles already contain metals from meteorites.)
We don’t know what effect it will have. One likely result would be that aluminum particles stimulate the growth of ice-containing particles. This means there would be more smaller, cold, reflective particles with more surface area on which chemistry can occur.
We also don’t know how aluminum particles will interact with sulfuric acid, nitric acid, and water in the stratosphere. As a result, we cannot really say what the consequences will be for ozone loss.
Learning from volcanoes
To really understand what these aluminum oxides mean for ozone loss, we need laboratory studies, more detailed modeling of the chemistry, as well as a look at how the particles would move through the atmosphere.
For example, after the Hunga Tonga–Hunga Ha’apai eruption, stratospheric water vapor mixed rapidly around the Southern Hemisphere and then moved poleward. At first this extra water caused intense sunsets, but a year later these water aerosols are well diluted throughout the southern hemisphere and we no longer see them.
A global current called the Brewer-Dobson circulation moves air up into the stratosphere near the equator and back down at the poles. As a result, aerosols and gases can remain in the stratosphere for a maximum of six years. (Climate change is speeding up this circulation, meaning the time aerosols and gases are in the stratosphere is shorter.)
The famous eruption of Mt Pinatubo in 1991 also created beautiful sunsets. It injected more than 15 million tons of sulfur dioxide into the stratosphere, cooling the Earth’s surface by just over half a degree Celsius for about three years. This event is the inspiration for geoengineering proposals to slow climate change by deliberately releasing sulfate aerosols into the stratosphere.
Many questions remain
Compared to Pinatuba’s 15 million tons, 360 tons of alumina seem like small potatoes.
However, we do not know how aluminum oxides will behave physically in stratospheric conditions. Will they create aerosols that are smaller and more reflective—thus cooling the surface, similar to geoengineering scenarios of stratospheric aerosol injection?
We also don’t know how aluminum will behave chemically. Will they form ice cores? How will it interact with nitric and sulfuric acid? Will it release trapped chlorine more efficiently than current stratospheric aerosols and facilitate ozone depletion?
And of course, aluminum aerosols don’t stay in the stratosphere forever. When they eventually fall to earth, what will this metal contamination do to our polar regions?
All these issues need to be addressed. According to some estimates, more than 50,000 satellites may be launched between now and 2030, so we had better address them quickly.
Robyn Schofield, Associate Professor and Associate Dean (Environment and Sustainability), The University of Melbourne
This article is republished from The Conversation under a Creative Commons license. Read the original article.