Communications companies such as Starlink plan to launch tens of thousands of satellites into orbit around Earth over the next decade. The growing swarm is already posing problems for astronomers, but recent research has raised another question: What happens when they start to descend?
When these satellites reach the end of their useful life, they will fall into the Earth’s atmosphere and burn up. Along the way, they will leave a trail of tiny metal particles.
According to a study published last week by a team of American researchers, this shower of satellites could release 360 tons of tiny aluminum oxide particles into the atmosphere each year.
The aluminum will primarily be injected at altitudes between 50 and 85 kilometers, but it will then drift toward the stratosphere, home to Earth’s protective ozone layer.
What does that mean? According to the study, the satellite’s contrail could facilitate chemical reactions that destroy the ozone layer. This is not false, but as we will 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 one free electron. When radicals are produced, they trigger cycles that destroy many ozone molecules. (These cycles have names that Dr. Seuss would admire: NOx, HOx, ClOx, and BrOx, because all involve oxygen as well as nitrogen, hydrogen, chlorine, and bromine, respectively.)
These radicals are created when stable gases are destroyed 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 manufacturing and agriculture have increased the amount in the air.
The HOx cycle involves hydrogen radicals from water vapor. Little water vapor enters the stratosphere, although events like the Hunga Tonga-Hunga Ha’apai submarine volcanic eruption in 2022 can sometimes inject large amounts.
Water in the stratosphere creates many small aerosol particles, which create a large surface area for chemical reactions and also scatter more light to create beautiful sunsets. (I will return to these two 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 into the stratosphere. However, this reactive chlorine is quickly neutralized and locked into molecules containing nitrogen and water radicals.
What happens next depends on the aerosols in the stratosphere, and near the poles it also depends on the clouds.
Aerosols speed up chemical reactions by providing a surface on which they 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, we are likely to see greater ozone loss.
An increasingly metallic 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 increasing stratospheric pollution due to the re-entry of space debris.
In 2023, researchers studying aerosol particles in the stratosphere detected traces of metals from spacecraft re-entry. They found that 10 percent of stratospheric aerosols already contain aluminum and predicted that this proportion would increase to 50 percent over the next 10 to 30 years. (About 50 percent of stratospheric aerosol particles already contain metals from meteorites.)
We don’t know what effect this will have. A likely outcome would be that the aluminum particles cause the growth of ice-containing particles. This means there would be more smaller, cold, reflective particles, with more surface area on which chemistry could 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 can’t really say what the implications of ozone loss will be.
Learn about volcanoes
To truly understand the impact of these aluminum oxides on ozone loss, we need laboratory studies, to model the chemistry in more detail and also examine how the particles would move through the atmosphere.
For example, after the Hunga Tonga-Hunga Ha’apai eruption, water vapor from the stratosphere quickly mixed 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 are no longer seen.
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 stay in the stratosphere for a maximum of six years. (Climate change is accelerating this circulation, meaning the length of time aerosols and gases remain in the stratosphere is shorter.)
The famous eruption of Mount Pinatubo in 1991 also created magnificent sunsets. It injected more than 15 million tons of sulfur dioxide into the stratosphere, which cooled the Earth’s surface by just over half a degree Celsius for about three years. This event sparked geoengineering proposals to slow climate change by deliberately releasing sulfate aerosols into the stratosphere.
Many questions remain
Compared to Pinatubo’s 15 million tons, 360 tons of aluminum oxide seems like small potatoes.
However, we do not know how aluminum oxides will behave physically in stratospheric conditions. Will this produce smaller, more reflective aerosols – thereby cooling the surface, much like stratospheric aerosol injection geoengineering scenarios?
We also don’t know how aluminum will behave chemically. Will this create ice cores? How will it interact with nitric and sulfuric acid? Will it release trapped chlorine more effectively than current stratospheric aerosols, making ozone destruction easier?
And of course, aluminum aerosols won’t stay in the stratosphere forever. When they eventually fall to the ground, what will this metal contamination do to our polar regions?
All of these questions need to be addressed. According to some estimates, more than 50,000 satellites could be launched by 2030; It is therefore best to remedy this quickly.
Robyn Schofield, Associate Professor and Associate Dean (Environment and Sustainability), University of Melbourne
This article is republished from The Conversation under a Creative Commons license. Read the original article.