Communications companies such as Starlink plan to launch tens of thousands of satellites into orbit over the next decade. The growing shower is already causing problems for astronomers, but recent research has raised another question: What happens when they come down?
When these satellites reach the end of their lifespan, they will fall into the Earth’s atmosphere and burn up. Along the way they leave a trail of small metal particles.
According to a study published last week by a team of American researchers, this satellite rain could dump 360 tons of tiny aluminum oxide particles into the atmosphere every year.
The aluminum will usually be injected at an altitude between 50 and 85 kilometers, but will then drift into the stratosphere, home to Earth’s protective ozone layer.
What does that mean? According to the study, the satellite’s contrail could promote ozone-destroying chemical reactions. That is not wrong, 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 a free electron. When radicals are produced, they start cycles that destroy many ozone molecules. (These cycles have names that Dr. Seuss would admire: NOx, HOx, ClOx, and BrOx, because they all include oxygen, nitrogen, hydrogen, chlorine, and bromine, respectively.)
These radicals are formed when stable gases are broken down by ultraviolet light, many of which are present in the stratosphere.
Nitrogen oxides (NOx) start with nitrous oxide. This is a greenhouse gas produced naturally by microbes, but human fertilizer production and agriculture have increased the 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 Hunga Tonga-Hunga Ha’apai underwater volcanic eruption can sometimes inject large amounts.
Water in the stratosphere creates countless tiny aerosol particles, which create a large surface area for chemical reactions and also scatter more light to make beautiful sunsets. (I will return to both points later.)
How CFCs created the ‘ozone hole’
ClOx and BrOx are the cycles responsible for the best-known damage to the ozone layer: the ‘ozone hole’ caused by chlorofluorocarbons (CFCs) and halons. These chemicals, now banned, were often used in refrigerators and fire extinguishers, releasing chlorine and bromine into the stratosphere.
CFCs quickly release chlorine radicals into the stratosphere. However, this reactive chlorine is quickly neutralized and trapped in molecules with nitrogen and water radicals.
What happens next depends on aerosols in the stratosphere, and near the poles it also depends on clouds.
Aerosols speed up chemical reactions by providing a surface for them to 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 overall, we’re likely to see more ozone loss when more stratospheric aerosols are present.
An increasingly metallic stratosphere
The details of the specific injection of aluminum oxides by falling satellites would be quite complex. This isn’t the first study to highlight growing stratospheric pollution from the re-entry of space debris.
In 2023, researchers studying aerosol particles in the stratosphere discovered traces of metals from spacecraft reentry. They found that 10 percent of stratospheric aerosols already contain aluminum, and predicted this will increase to 50 percent over the next 10 to 30 years. (About 50 percent of the aerosol particles in the stratosphere already contain metals from meteorites.)
We don’t know what effect this will have. One 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 a larger surface area for chemistry to take place.
We also don’t know how aluminum particles will interact with the sulfuric acid, nitric acid and water in the stratosphere. As a result, we can’t really say what the impact will be on ozone loss.
Learning from volcanoes
To really understand what these aluminum oxides mean for ozone loss, we need laboratory studies to model the chemistry in more detail, as well as how the particles would move in the atmosphere.
For example, after the Hunga Tonga-Hunga Ha’apai eruption, water vapor in the stratosphere mixed rapidly around the Southern Hemisphere and then moved poleward. Initially, this extra water caused intense sunsets, but a year later these water aerosols are well diluted over the entire 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 again at the poles. As a result, aerosols and gases can remain in the stratosphere for up to six years. (Climate change is accelerating this circulation, meaning the time aerosols and gases spend in the stratosphere is shorter.)
The famous eruption of Mt Pinatubo in 1991 also provided 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 Pinatubo’s 15 million tons, 360 tons of alumina seems like small potatoes.
However, we do not know how aluminas will physically behave under stratospheric conditions. Will it create aerosols that are smaller and more reflective – cooling the surface, much like geoengineering scenarios for stratospheric aerosol injection?
We also don’t know how aluminum will behave chemically. Will it create ice cores? How will it interact with nitric and sulfuric acids? Will the trapped chlorine release more effectively than current stratospheric aerosols, facilitating ozone destruction?
And of course, the aluminum aerosols won’t stay in the stratosphere forever. What will this metal pollution do to our polar regions when it eventually falls to the ground?
All these questions need to be answered. By some estimates, more than 50,000 satellites could be launched between now and 2030, so we better get to them quickly.
Robyn Schofield, Associate Professor and Associate Professor (Environment and Sustainability), University of Melbourne
This article is republished from The Conversation under a Creative Commons license. Read the original article.