*By Eric Roston*

The "law of conservation of mass" is a concept from high school science that I never quite grasped, but it is essential for understanding today's environmental challenges, including a significant issue developing in low-Earth orbit.
This law states that the fundamental materials that make up everything—rocks, Pez dispensers, kites, fish, you, and me—cannot be destroyed under ordinary conditions. Instead, atoms and molecules simply rearrange themselves into different forms. Many gradual environmental catastrophes stem from our tendency to overlook the law of conservation of mass. When we discard something, it doesn't cease to exist; it merely transforms into something else.

A classic illustration of this is the carbon dioxide released into the atmosphere from burning hydrocarbons. However, a new type of mass that we are unintentionally conserving is increasing at an alarming rate: the aluminum, copper, lithium, and carbon associated with rockets, fuel, and satellites, posing risks to the ozone layer. A SpaceX Falcon 9 rocket carrying 20 Starlink satellites over California. We are currently experiencing a satellite boom. Elon Musk's SpaceX has launched over 7,000 satellites to build its Starlink internet constellation, with plans to position up to 42,000 in orbit for complete coverage. Recently, Amazon's Project Kuiper deployed its first 27 production satellites, as Executive Chairman Jeff Bezos aims to create a competing internet constellation. Industry leaders and regulators are eager to prevent overcrowding in low-Earth orbit, which could lead to collisions. To counter this, new satellites are designed to descend into the atmosphere and disintegrate at the end of their approximately five-year lifespan. This is where the conservation of mass comes into play. Historically, the rate at which satellites have de-orbited and burned up has posed little threat to the atmosphere.

However, this situation is changing, with the European Space Agency forecasting that 100,000 satellites will orbit the Earth by 2030—a tenfold increase from current levels. In recent years, scientists have started assessing how satellite re-entries affect the atmosphere and modeling potential future scenarios. Currently, 10% of aerosols in the stratosphere contain aluminum or other particles from burnt satellites. Black carbon, or soot, produced at high altitudes during launches can directly heat the stratosphere and contribute to ozone depletion, the atmospheric layer that protects life from ultraviolet rays.

Additionally, aluminum oxides created during satellite re-entry may raise concerns regarding ozone chemistry. Major satellite companies are increasingly aware of the research highlighting the risks associated with satellite re-entries, but it remains uncertain how regulations will adapt in response, or how such changes might affect the industry and launch costs. While satellites likely pose a smaller risk to the ozone layer compared to the refrigerants and aerosol sprays of the 1980s, they are "going to become more of a problem," according to Kostas Tsigaridis, a research scientist at Columbia University's Center for Climate Systems Research. "We should not ignore this issue just because it seems small right now." My colleague Sana Pashankar and I spoke with leaders in the satellite industry and scientists monitoring these increasing risks, and we've compiled an in-depth exploration of what could evolve into a significant environmental concern. We invite you to read the full story, and we promise there won't be a pop quiz afterward!