The Rise of Satellite Mega-Constellations
The number of satellites orbiting the Earth is rapidly increasing as companies build out massive constellations of small satellites in low Earth orbit (LEO). SpaceX’s Starlink constellation already has over 3,000 satellites in orbit, with plans for tens of thousands more. Other companies like OneWeb and Amazon have similar ambitions to deploy mega-constellations to provide global broadband internet coverage.
While these constellations promise to revolutionize telecommunications, there are growing concerns about their potential environmental impacts. One key issue is what happens to the satellites at the end of their operational lives. Most LEO satellites are not retrieved but instead are disposed of through uncontrolled atmospheric reentry. The satellites burn up as they plummet through the atmosphere, with any surviving debris landing in the oceans.
Aluminum Oxides Generated During Reentry
A typical 250 kg satellite is estimated to be composed of about 30% aluminum by mass. When a satellite reenters the atmosphere at the end of its life, much of this aluminum burns up, generating aluminum oxide particles and vapor.
Recent atomic-scale molecular dynamics simulations have provided new insights into this oxidation process that occurs during reentry. The studies found that the demise of a single 250 kg satellite with 30% aluminum mass fraction can generate around 30 kg of aluminum oxide nanoparticles. These nanoparticles may persist in the atmosphere for years or even decades before settling out.
The amount of aluminum oxides being deposited in the upper atmosphere from satellite reentries is rapidly increasing as more mega-constellations come online. Reentry scenarios involving the full deployment of currently planned mega-constellations could result in over 360 metric tons of aluminum oxide particles being injected into the atmosphere annually. This is a dramatic increase compared to the estimated 17 metric tons generated by all satellite reentries in 2022.
Potential for Increased Ozone Depletion
The surge in aluminum oxides from satellite reentries is concerning because these particles are known to contribute to ozone depletion in the stratosphere. Aluminum oxides provide a surface that catalyzes chemical reactions involving chlorine that break down ozone molecules. Even a small amount of aluminum oxides can have an outsized impact – studies have shown these particles have an ozone destruction efficiency up to 10,000 times greater than sulfuric acid aerosols.
Ozone in the stratosphere acts as a protective shield, absorbing harmful ultraviolet radiation from the sun. Thinning of the ozone layer increases UV exposure at the surface, which can lead to higher rates of skin cancer, eye cataracts, and other health issues in humans. It also negatively impacts plant growth and marine ecosystems.
Significant damage to the ozone layer was first noticed in the 1980s, largely due to emissions of chlorofluorocarbons (CFCs). The Montreal Protocol, signed in 1987, successfully phased out the use of CFCs and halted the ozone loss. However, the ozone layer remains vulnerable and is still in the process of slowly healing. A resurgence in ozone-depleting chemicals, even from an unexpected source like satellite reentries, could derail this recovery.
Modeling studies suggest that under a worst-case mega-constellation scenario, the increase in aluminum oxides could result in an additional 0.05% loss of ozone over the Antarctic region each year. While this may sound small, it is comparable in magnitude to the annual ozone loss attributed to past CFC emissions. The unique atmospheric conditions over the poles make the ozone layer especially susceptible to damage in those regions.
Climatic Impacts of Reentry Byproducts
In addition to harming the ozone layer, the particles generated from satellite reentries could have other negative environmental consequences. Aluminum oxides are highly reflective and could alter Earth’s albedo, or reflectivity. Injecting large quantities of these particles into the upper atmosphere may produce an unintentional geoengineering effect, scattering more sunlight back to space.
The atmospheric processes triggered by aluminum oxides from reentries are still poorly understood and difficult to model. However, even small changes to Earth’s delicate radiative balance could have far-reaching climatic repercussions. Some scientists have described the uncontrolled release of material from satellite reentries as a reckless geoengineering experiment with unknown consequences.
Reentry byproducts could also impact the formation of high-altitude clouds. Noctilucent or “night-shining” clouds are sometimes observed in the mesosphere near the poles. These clouds form when water vapor condenses on meteoritic dust particles. There is a risk that aluminum oxide particles from satellite reentries could provide additional nucleation sites, making these clouds more frequent and widespread. Changes in noctilucent cloud cover could further perturb the atmosphere’s thermal equilibrium.
Risks of Debris Surviving Reentry
Not all material from a satellite will completely burn up during reentry. Denser components made of heat-resistant materials like titanium or carbon-carbon composites can survive to reach the surface. While most reentries occur over the oceans, there is always a risk that debris could strike land, posing a hazard to people and property.
As the number of reentries grows with the proliferation of mega-constellations, the chance of a serious debris impact event also increases. A 2020 study estimated that with 60,000 satellites in orbit, there would be a 10% chance of a casualty from reentry debris occurring within a decade.
There are also concerns about the toxicity of reentry debris. Many satellites contain hazardous materials like beryllium, cadmium, and mercury. While most of these materials will burn up, even trace amounts could contaminate soil and water if dispersed in the environment. The long-term ecological impacts are unknown.
The Need for Improved Governance
Despite the rapid growth of commercial satellite constellations, there is currently little international oversight of their environmental impacts. National regulators like the U.S. Federal Communications Commission (FCC) are responsible for licensing mega-constellations, but they have not traditionally considered issues like reentry debris and ozone depletion in their assessments.
In the U.S., the National Environmental Policy Act (NEPA) requires federal agencies to evaluate the environmental effects of their actions. However, the FCC has categorically excluded commercial satellites from NEPA review. In 2020, a legal paper argued that this exclusion is unlawful given the potential for large constellations to cause significant environmental harms. The U.S. Government Accountability Office is currently investigating whether the FCC should close this regulatory loophole.
Other spacefaring nations are also grappling with how to responsibly govern the expansion of mega-constellations. There have been calls for a new international treaty to ensure that the space industry develops in an environmentally sustainable manner. The United Nations Committee on the Peaceful Uses of Outer Space has begun discussing voluntary guidelines for the long-term sustainability of space activities, but progress has been slow.
Some have proposed a moratorium on mega-constellation launches until the full scope of their environmental impacts can be properly assessed. However, with the commercial space race already well underway, many see this as unlikely. Instead, a gradual ratcheting up of regulatory scrutiny and environmental safeguards may be more pragmatic.
Mitigating Strategies and Future Outlook
Satellite operators and manufacturers can take steps to reduce the ozone-depleting potential of their constellations. Using materials other than aluminum in satellite construction would prevent the formation of aluminum oxides during reentry. Developing methods to boost old satellites into “graveyard” orbits rather than letting them reenter could also help, though this requires expending precious onboard propellant.
Ultimately, the most effective solution may be active debris removal – sending up servicing spacecraft to capture defunct satellites and bring them down in a controlled manner over uninhabited regions. Several companies are developing these capabilities, but the technological and economic hurdles remain substantial, especially for removing small debris. Preventing debris from being created in the first place through better satellite design and end-of-life disposal is generally seen as more feasible.
As the number of objects in LEO continues to grow, it will be critical to improve tracking and space situational awareness capabilities. Knowing precisely when and where reentries will occur can help minimize risks to people on the ground. More research is also needed to understand the complex atmospheric chemistry of reentry byproducts and their environmental effects.
The rise of satellite mega-constellations promises to bring transformative benefits to society, but it also creates new challenges and risks that must be carefully managed. Mitigating the threat of ozone depletion from aluminum oxide particles will require a concerted effort from satellite operators, regulatory agencies, and the scientific community. How this issue is addressed in the coming years may set an important precedent as humanity continues to expand its presence in Earth orbit.
