When a satellite disintegrates in the upper atmosphere, it does not just disappear. Rather, it deposits a record of aluminum oxide nanoparticles small, lasting particles that are now causing concern among atmospheric scientists. According to a new study released in Geophysical Research Letters and sponsored by NASA, the levels of these particles have risen eight times from 2016 to 2022, and the projection is even more dismal: with the intended growth of mega-constellations such as Starlink, yearly aluminum oxide emissions will skyrocket to 640% more than natural levels in the years ahead.
The mechanics behind this process stem from the chemistry of satellite reentry. Satellites, which are usually built out of aluminum alloys, dive into the atmosphere at the end of their lifespan and subject themselves to severe ablation. This procedure converts much of their aluminum into aluminum oxide clusters, which are injected into the mesosphere 30 to 50 miles above Earth’s surface. These particles are not removed rapidly; it may take 30 years for them to fall to the stratosphere, where the Earth’s ozone layer is located.
It is its involvement in stratospheric ozone chemistry that makes aluminum oxide especially problematic. Unlike the older ozone-depleting compounds, including chlorofluorocarbons, aluminum oxide is not expended by the reaction. Rather, it serves as a catalyst to facilitate the activation of chlorine species that destroy ozone molecules and then remain in the atmosphere to do the same again. According to Joseph Wang, a professor of Astronautics and Aerospace and Mechanical Engineering at USC, who explained it all to Space.com, “This study used atomic scale molecular dynamics simulation to quantify the amount of aluminum oxide generated for a model satellite reentry, and then used the number of reentering satellites planned for satellite megaconstellations to predict the amount of aluminum oxide that will be generated in the future.”
The size of the problem is unparalleled. In 2022 alone, approximately 332 metric tons of satellite material incinerated in the atmosphere, producing an estimated 17 metric tons of aluminum oxide. With forecasts for mega-constellations going up to 42,000 satellites for Starlink alone, scientists expect as much as 3,200 metric tons of satellite bodies to be incinerating every year by the 2030s and releasing up to 630 metric tons of aluminum oxides every year.
The health and environmental consequences are extreme. The ozone layer, dense in the stratosphere between nine and 28 miles high over Earth, acts as a protective barrier against damaging ultraviolet (UV) radiation. Disruption of this layer enhances the risk of skin cancer, cataracts, compromised immune systems, and disrupts crop yields and marine life.
Ignoring the severity of these discoveries, the area remains shrouded in uncertainty. As lead study author José Pedro Ferreira pointed out to Space.com, The chemistry and physics of these reentry byproducts as they cool down and settle in the atmosphere, including chemical reactions with ozone, are not the subject of this study and are not completely understood by the community. Such a lack of knowledge highlights the importance of further research, particularly given the high rate at which the number of satellites in orbit is increasing.
Mitigation measures are starting to appear in engineering communities. International guidelines currently suggest that satellites deorbiting within 25 years of mission completion, whereas the FCC has a more stringent five-year policy for U.S.-licensed satellites. Engineers also are experimenting with different satellite materials and guided reentry trajectories like aiming at distant oceanic areas to reduce atmospheric contamination.
As mega-constellations redefine the size of human presence in low Earth orbit, the intersection of space technology and atmospheric science enters a critical period. It has never been more pressing to require strong forecasting models, creative engineering solutions, and international coordination.
