Communications companies like Starlink plan to launch 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 will happen when they start coming down?
When these satellites reach the end of their useful life, they will fall into Earth's atmosphere and burn up. Along the way, they will leave a trail of tiny metal particles.
This satellite rain could put 360 tonnes of tiny aluminium oxide particles into the atmosphere each year, according to a study published last week by a team of US researchers. The aluminium would mostly be injected at altitudes of 50 to 85 kilometres, but it would then move into the stratosphere – home to Earth's protective ozone layer.
What does this mean? According to the study, satellite contrails could be promoting ozone-destroying chemical reactions. That's not wrong, but as we'll see the story is far from simple.
How is ozone destroyed?
The loss of ozone in the stratosphere is caused by “free radicals” — atoms or molecules with free electrons. When radicals are produced, they start cycles that destroy many ozone molecules. (These cycles have names Dr. Seuss would love: NOx, HOx, ClOx and BrOx, because all involve oxygen as well as nitrogen, hydrogen, chlorine and bromine, respectively.)
These radicals are formed when stable gases are broken down by ultraviolet light, which is abundant in the stratosphere.
Nitrogen oxides (NOx) begin with nitrous oxide. It is a greenhouse gas produced naturally by microorganisms, but human fertilizer manufacturing and agriculture have increased its amount in the air.
The HOx cycle involves hydrogen radicals from water vapor. Not much water vapor reaches the stratosphere, although events such as the Hunga Tonga-Hunga Ha'apai underwater volcanic eruption in 2022 can occasionally inject large amounts of water vapor. Water in the stratosphere creates many small aerosol particles, which create a larger surface area for chemical reactions and also scatter more light to create beautiful sunsets. (I'll come back to both of these points later.)
How did CFCs create the 'ozone hole'?
The most damaging cycles of the ozone layer are the cloaks and brokes: the “ozone hole” caused by chlorofluorocarbons (CFCs) and halons. These now-banned chemicals were commonly used in refrigerators and fire extinguishers and introduced chlorine and bromine into the stratosphere.
CFCs rapidly release chlorine radicals into the stratosphere. However, this reactive chlorine is quickly neutralized and locked into molecules along with nitrogen and water radicals.
What happens next depends on aerosols in the stratosphere, and also on clouds near the poles.
Aerosols speed up chemical reactions by providing a surface for them to occur. 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 see more ozone loss.
Rapidly growing metallic stratosphere
The details of the specific injection of aluminum oxide by falling satellites will be quite complex. This is not the first study to highlight increasing stratospheric pollution from re-entering space debris.
In 2023, researchers studying aerosol particles in the stratosphere found traces of metals from spacecraft re-entry. They found that 10% of the stratosphere's aerosols already contain aluminum, and they predicted that this will increase to 50% in the next 10-30 years. (About 50% of the stratosphere's aerosol particles already contain metals from meteoroids.)
NASA/Scott Kelly
We don't know what effect this will have. One possible result is that the aluminum particles will promote the growth of ice-rich particles. This means that there will be more small, cold, reflective particles with more surface area on which chemistry can occur.
We also don't know how the aluminum particles will react with the sulfuric acid, nitric acid, and water found in the stratosphere. As a result, we can't really say what effect this will have on ozone loss.
Learn from volcanoes
To really understand what impact aluminium oxide has on ozone layer damage, we need laboratory studies to model the chemistry in more detail, and to see how the particles move through the atmosphere.
For example, after the Hunga Tonga-Hunga Ha'apai eruption, water vapor in the stratosphere quickly mixed around the Southern Hemisphere, and then moved toward the pole. At first, this extra water caused intense sunsets, but after a year, these water aerosols were well dispersed throughout the Southern Hemisphere and we no longer see them.
NASA
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 only remain in the stratosphere for a maximum of six years. (Climate change is speeding up this circulation, meaning that aerosols and gases have less time to stay in the stratosphere.)
The famous eruption of Mount Pinatubo in 1991 also produced beautiful sunsets. It injected more than 15 million tons of sulfur dioxide into the stratosphere, which cooled the Earth's surface by a little more than half a degree Celsius for about three years. The event is the inspiration for geo-engineering proposals to slow climate change by deliberately introducing sulfate aerosols into the stratosphere.
many questions remain
360 tons of aluminum oxide seems like a small thing compared to Pinatubo's 15 million tons.
However, we don’t know how aluminum oxide would behave physically under stratospheric conditions. Would it create aerosols that would be smaller and more reflective – thereby cooling the surface, just as it would in stratospheric aerosol injection geoengineering scenarios?
We also don't know how aluminum will behave chemically. Will it form ice nuclei? How will it react with nitric and sulfuric acid? Will it release locked-up chlorine more effectively than current stratospheric aerosols, thereby destroying ozone?
And of course, aluminum aerosols won't stay in the stratosphere forever. When they finally fall to the ground, what will this metal contamination do to our polar regions?
All these questions must be addressed. According to some estimates, more than 50,000 satellites could be launched between now and 2030, so we must address these questions as soon as possible.