When it falls back to Earth and burns up, all of that aluminum doesn't just vanish into the ether; it's burned down into microscopic aerosol particles called alumina. A few alumina particles aren't a big deal, but as more satellites get sent up, higher concentrations of alumina are finding their way into the atmosphere. In 2022, the United States Government Accountability Office estimated that, by 2040, there could be well over 60,000 satellites in low Earth orbit, each with a reduced lifespan of just five years. That's assuming they aren't knocked down prematurely, like Starlink satellites have been due to solar storms. If those satellites all started falling back to Earth at around the same time, we could see burning satellites in the sky on a daily basis, each one depositing alumina into the atmosphere.