LONDON, July 7 — Every few months, a constellation of Starlink satellites begins its final, deliberate fall. The process is slow by design: onboard ion thrusters lower the orbit over a period of months, until the spacecraft can no longer maintain altitude. Then the atmosphere does the rest.
The technical reality of reentry is violent. The satellites hit the upper atmosphere at roughly 17,000 miles per hour, or 27,000 kilometres per hour.
Friction heats the structure to over 3,000 degrees Fahrenheit — about 1,650 degrees Celsius. At those temperatures, most components vaporise. The design intent is that no debris reaches the ground, which complies with international space debris mitigation guidelines.
But the detail that matters is what happens to the vapour. Each satellite contains aluminium, silicon, copper, and other metals.
As the craft disintegrates between 50 and 85 kilometres altitude — a region known as the mesosphere — those materials are released as fine particles. A 2023 study led by Dr. Joseph Wang at the University of Southern California modelled the cumulative effect. The finding: reentering satellites could increase aluminium oxide concentrations in the mesosphere by up to 650% compared to natural levels.
That figure is striking. Aluminium oxides are known catalysts for ozone depletion. The chemistry is not new — the same compounds have been studied in the context of solid rocket boosters and volcanic plumes.
What is new is the scale of the source. Where once reentry was rare, it is now becoming routine.
Thousands of satellites will be deorbited each year as the megaconstellations refresh their fleets. The long-term effect on the ozone layer remains under study. That is the honest answer from the scientific community: no one yet knows what a 650% increase in a catalytic compound means when distributed across the mesosphere over a decade or more.
The process is designed to be clean at ground level. Under the hood, the atmospheric chemistry is another matter.
The mechanics of a clean burn
The ion thrusters that drive Starlink satellites are electric propulsion systems, using xenon or krypton gas accelerated by electric fields. They produce low thrust but high efficiency, making them suitable for gradual orbit lowering. The satellite does not plunge; it decays in a controlled spiral, giving engineers time to ensure the final trajectory targets an ocean region.
At the point of reentry, the satellite is moving at orbital velocity. The shockwave ahead of the vehicle compresses the air, generating extreme heat.
Aluminium, which makes up much of the structure, melts at about 1,220 degrees Fahrenheit. It boils at roughly 4,500 degrees. The reentry temperature sits between those points, meaning the aluminium becomes molten and then vaporises, but does not necessarily combust fully.
The resulting particles are fine — on the order of nanometres to micrometres — and they linger in the mesosphere. Silicon from solar panels and electronics follows a similar path.
Copper from wiring behaves differently; some may form larger particles that fall more quickly. The net effect is a steady injection of metal aerosols into a layer of the atmosphere that is normally very clean. The mesosphere is not well understood.
It is too high for balloons and too low for most satellites. Sounding rockets and lidar measurements provide occasional data, but the region is under-studied relative to the stratosphere and troposphere. That makes the impact of metal deposition hard to model with confidence.
A new area of scientific investigation
Dr. Wang’s 2023 study was a modelling exercise, not an observational one. It used known satellite composition data and projected reentry rates to estimate the chemical load. The 650% increase in aluminium oxide is a modelled figure, not a measured one.
But it is based on conservative assumptions about satellite numbers and reentry schedules. The study has drawn attention from atmospheric chemists and space policymakers alike.
The ozone layer in the stratosphere is protected by the Montreal Protocol and has been slowly recovering. The mesosphere is not the stratosphere — it sits above it, between 50 and 85 kilometres. But chemical processes in the mesosphere can influence the stratosphere below, particularly through the formation of polar stratospheric clouds and the transport of aerosols during winter.
Aluminium oxide is a known catalyst for ozone destruction. It does not get consumed in the reaction; it facilitates the breakdown of ozone molecules and remains available to do so again.
Even small concentrations can have outsized effects over time. The question is whether the concentration in the mesosphere will reach levels that matter. As ever, the answer depends on numbers.
If the current rate of satellite launches continues, and if the deorbit timeline remains at a few months per satellite, the cumulative injection of metals will grow. The study’s 650% figure was based on a specific set of assumptions about fleet size and replacement rate. Those assumptions may already be conservative.
Compliance and curiosity
The deorbit process itself is not controversial. It meets international guidelines for space debris mitigation, which require that spacecraft be removed from low Earth orbit within 25 years of mission end. Starlink satellites are designed to reenter within a few months, well under that limit.
The process ensures no debris reaches the ground, which satisfies safety requirements.
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