Communications companies such as Starlink plan to launch tens of thousands of satellites into orbit around Earth over the next decade or so. The growing swarm is already causing problems for astronomers, but recent research has raised another question: what happens when they start to come 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 metallic particles.
According to a study published last week by a team of American researchers, this satellite rain may dump 360 tonnes of tiny aluminium oxide particles in the atmosphere each year. The aluminium will mostly be injected at altitudes between 50 and 85 kilometres, but it will then drift down to the stratosphere – home to Earth’s protective ozone layer. What does that mean? According to the study, the satellite’s contrail could facilitate ozone-destroying chemical reactions.
That’s not wrong, but as we will see the story is far from simple. 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 admire: NOx, HOx, ClOx and BrOx, as all involve oxygen as well as nitrogen, hydrogen, chlorine and bromine, respectively.) These radicals are created when stable gases are broken up by ultraviolet light, which there is plenty of in the stratosphe.