How Many Satellites Can Our Atmosphere Handle Before It Breaks?

About a year before the crash, Baumgarten’s colleague Michael Gerding unpacked a device that he had used to measure the atmosphere for his doctoral thesis 30 years ago. “The Lidar was still there, it worked, and it’s wonderfully suitable for quickly measuring something new.” The Lidar device stands on a simple laboratory table. At night, the ceiling above opens, and it sends light particles into the night sky via laser beams. The few photons scattered by tiny atoms and molecules several kilometers above the Earth find their way back to Gerding’s institute, where they are captured by telescopes and counted by extremely sensitive detectors.

The Human Impact on the Mesosphere

Initially, the atmospheric physicist was tracking meteoroids and cosmic dust with this metal resonance measurement. Meteoroids are small objects in orbits around the Sun, while meteorites are the leftover fragments that sometimes impact Earth after a meteoroid has entered the atmosphere. During the journey, most material evaporates, leaving behind calcium ions or traces of iron, sodium, or potassium at altitudes of 80 to 100 kilometers. “These measurements were often a fringe topic for us,” says Gerding. It was not about proving the metals, but rather that one could derive other important data from these measurements, such as the temperature in the upper atmospheric layers.

Impact of Space Debris on the Mesosphere

This changed in 2023 when atmospheric chemist John Plane from the University of Leeds visited Kühlungsborn. He gave a lecture on how evaporating space debris affects the mesosphere. “It hit us like lightning,” recalls Baumgarten. The mesosphere is the frigid part of the atmosphere located between 50 and 80 kilometers in altitude. Meteoroids that fly toward Earth turn into shooting stars in this layer. The mesosphere and the lower thermosphere above it mark the gradual transition to outer space. Below the mesosphere lies the stratosphere, which contains the ozone layer as well as the troposphere, where most weather phenomena occur.

When a man-made object re-enters Earth’s atmosphere from orbit, the atmosphere slows it down at about 100 kilometers high, causing it to heat up. Depending on the angle and speed of entry, it typically bursts apart at altitudes of 75 to 80 kilometers above the ground. Larger pieces begin to melt and evaporate at this stage. Observers on Earth can see trails of glowing dust and gas made up of various aluminum oxides and metals like iron, copper, lithium, or rare earth metals. None of these belong naturally and in large quantities in the mesosphere. Only the larger fragments do not fully evaporate and fall like candle wax into the sea. Rotor wheels or round tanks made of titanium or carbon fibers also sometimes land intact on Earth.

Tracking Individual Atoms at 90 Kilometers

“Within a few months, we modified the Lidar so that we could detect lithium, as it’s relatively easy to measure,” says Gerding. This metal, for example, is part of the aluminum alloy used for rocket upper stages and in lithium-ion batteries. Moreover, lithium is particularly suited for detecting evaporated space debris in the atmosphere, as this metal hardly occurs in meteoroids. It is estimated that about 80 grams of lithium naturally enter the atmosphere from space daily. A Falcon 9 upper stage contains 30 kilograms of this material alone. Lithium evaporates very quickly when the aluminum alloy melts at 660 degrees Celsius.

To track the metal, the researchers directed the laser beams of the makeshift Lidar into the night sky whenever possible. The first measurement dates back to August 2024. They used a dye laser that could be precisely tuned to a specific wavelength by selecting the fluorescence dye. For the lithium atoms, the resonance wavelength is around 671 nanometers. After four to five hours of operation, the dye runs out, and the measurement for the night ends.

“Lidar measurements of lithium have a relatively poor signal-to-noise ratio,” says Gerding, “because there is simply very little lithium in the upper atmosphere.” The researchers send an astonishing number of photons up, but only a few return. “The computer at the detectors records the time of each individual photon that is backscattered from the middle atmosphere.” The actual measurements revealed during the data analysis the next day. “We must integrate the measurement data over an hour to even detect the one, two, or three lithium atoms per cubic centimeter at 90 kilometers high.”

The Significance of Space Debris Research

Initially, the researchers planned to measure the natural input of lithium into the atmosphere for perhaps two or three years. They speculated that the lithium content in the mesosphere would gradually rise, allowing them to document the ‘space age’ of this atmospheric layer. However, everything changed faster than they expected. On February 20, the lithium concentration skyrocketed to ten times higher than normal precisely at an altitude between 94.5 and 96.8 kilometers. “We saw lithium increasing like crazy there,” says Baumgarten. Could this be related to the recently crashed Falcon 9?

The timing was there, but the rest didn’t quite match. The path of the tumbling debris came from the British Isles over East Frisia and on to Berlin – quite a distance south of Kühlungsborn, near Rostock, where the lidar is located. At the time of measurement, the wind was blowing from the north. So, where did the air masses carrying the lithium over the Baltic city come from?

Investigating with Weather Models

“The lithium measurement alone wasn’t enough,” Baumgarten explains, “we needed a weather model to track where the material was coming from.” The researchers used local measurements of air circulation in the mesosphere that were captured by radar alongside a model representing the global dynamics in the mesosphere and the lower thermosphere. This UA-ICON model was developed by the Max Planck Institute for Meteorology and the German Weather Service. The Kühlungsborn researchers calculated thousands of possible trajectories for the air masses they measured at the time over their institute – and they kept landing in the same origin area: off the coast of West Ireland.

A sample trajectory shows how the air masses drew in from the north and lost some altitude along the way. They previously moved over northern Denmark and southern Norway; from Scotland, the route went back south to Wales and then straight west to the origin point off the Irish coast: 52.5 degrees North, 12.38 degrees West, and 100.2 kilometers over the Atlantic. Exactly where the 4.7-millimeter thick shell of the rocket upper stage entered the atmosphere 20 hours earlier, starting to melt and evaporate. It is the first time that pollution in the upper atmosphere has been linked to the re-entry of a spacecraft.

What followed was the calm before the storm. “In March, I first presented the results to fellow scientists,” recalls Baumgarten. In the summer, the researchers submitted the related paper to “Nature”. However, the academic journal hesitated. What would SpaceX say about the new findings? The company did not respond to the request for a statement. The paper sat in Nature’s legal department. Then, in December, the final “go” came. “They told us, ‘Hey, we want to make a story out of this. We’ll publish it about a year after the event,'” Baumgarten says. That was in February 2026. After that, no stone was left unturned at the IAP. The institute was inundated with press inquiries from around the world, and the paper was downloaded 25,000 times in a short period. “For our standards, that was through the roof.”

Not Comparable to Cosmic Dust

Manuel Metz at the German Aerospace Center also praised the work. He is responsible for spaceflight management and the handling of space debris, including that which re-enters the atmosphere. “The measurements from IAP are a very important step in researching these effects.” Until now, little has been recorded on what residues space technology leaves in the atmosphere or, when recorded, they weren’t localized in time and space. For instance, NASA detected aluminum particles at 19 kilometers high in 2023 with a high-altitude research aircraft and linked them to re-entering space objects descending from great heights over the polar regions into the stratosphere. “With the methods from IAP, it is now possible to directly measure residues in the mesosphere, the same place where satellites break apart.”

“One ton of pollution at 75 kilometers is as bad as one hundred thousand tons of pollution on the ground.”Gerd Baumgarten, atmospheric physicist

The very flat Starlink satellites, now launched by the thousands, consist of a base plate, batteries, small engines, a laser unit for communication, and sensors. They weigh between 305 and 960 kilograms and are designed to fall into the atmosphere and completely burn up after their operational lifespan of typically five years. This is considered a commendable practice by space experts. This way, retired satellites do not remain in the already overcrowded orbit, and they do not crash into anyone’s house. However, they leave atoms and aerosols in the mesosphere, and the rule of thumb is: the higher, the worse. Unlike the lower atmospheric layers—the stratosphere and troposphere—the mesosphere has no self-cleaning mechanism. What evaporates up there can linger for decades. “One ton of pollution at 75 kilometers is as bad as one hundred thousand tons of pollution on the ground,” says Baumgarten.

However, space travel is not the only source of pollution in the extremely thin air up to 100 kilometers high. Cosmic dust penetrates there daily, summing up to about 12,000 tons of material from space each year, composed of sodium, magnesium, or iron. In contrast, the mass contributed by decaying satellites and rocket stages is still small today. Not even three percent of the material entering the upper atmosphere is human-made. However, the issue is not merely the mass but the types of substances. Satellites and rocket stages introduce aluminum alloys, copper, plastic compounds, rare earths, and many other metals into the atmosphere, which only occur in traces in cosmic material.

Today, the European Space Agency (ESA) observes 22,500 satellites orbiting Earth. Nearly half of these belong to SpaceX’s Starlink constellation. Within the next decade, 65,000 more satellites are expected to be launched into low Earth orbit, including from new players like China’s GuoWang or Amazon. The planned 40,000 Starlink satellites alone would weigh a total of 10,000 tons—material that will eventually return to Earth to clear space in orbit. And to ensure it does not cause harm on Earth, the technology is designed to decompose into tiny particles and vapor as they travel through the atmosphere. Estimates suggest that with the currently planned satellite constellations, the human-made share of substances entering the atmosphere from outside will rise from the current 3% to 13% to 40%.

A study from University College London in May 2026 concludes that descending satellites from mega-constellations have comparatively little impact on the ozone layer, which resides in the stratosphere. Fortunately, the reason for this is that the kerosene used to launch these rockets does not release ozone-depleting chlorine. However, it does leave behind soot particles that absorb sunlight, thereby heating the upper atmosphere and allowing less sunlight to reach Earth, effectively cooling the planet. Additionally, there are also metal oxides, primarily aluminum oxides like Al₂O₃, released during the re-entry of nearly all spacecraft. What this means for the Earth’s radiation budget remains uncertain today; it’s an open-ended experiment.

From Component to Atom in Seconds

What exactly occurs during the re-entry of space technology into the atmosphere is investigated by Stefan Löhle from the Institute for Space Systems at the University of Stuttgart. Originally, he was not focused on technology that disintegrates. Instead, he was exploring the high-temperature chemistry of heat shields designed to protect space capsules during their return to Earth after being exposed to temperatures of up to 3000 degrees. But with the rise of commercial space travel and the massive amounts of material being sent into orbit, his focus is shifting. “I’m interested in what happens between re-entry and the disintegration into individual particles.”

On multiple missions, he has tracked space debris falling through the atmosphere from an aircraft, carrying a spectrometer to measure ISS transporters or other spacecraft rushing towards Earth at seven kilometers per second. The most recent mission occurred in the summer of 2024, when a satellite from the Cluster mission was deliberately crashed. “The ESA wanted to understand how they burn up,” he explains. But his primary workplace is in the lab. When a car-sized and 2.6-ton block of retired batteries was set to re-enter in March 2024, he closely examined the materials in the battery. “Our experiments showed that this battery block would never burn up. It would actually fall somewhere on Earth”—which is precisely what happened, including a 700-gram fragment crashing through a house roof in Florida.

Löhle and his team work with a specialized wind tunnel that realistically simulates the extremely high temperatures and aerodynamics conditions during re-entry. “When an object enters the atmosphere, the air in front of it compresses significantly,” Löhle states. This creates a plasma state, as the air gets extremely hot and disintegrates. “We artificially produce this extremely hot air generated during the compression shock using an electric arc.”

Löhle shows an image of a plasma beam hitting a plastic-coated aluminum beam. Even before it melts and falls, the plasma around the beam turns blue-green. “These are aluminum oxides, just like those produced during the evaporation of rocket stages.” In an extended test, researchers inject aluminum powder directly into this hot flow. Using optical emission spectroscopy, they characterize the resulting gases and derive important insights that support the work of atmospheric physicists like Gerd Baumgarten or Michael Gerding.

“With satellite constellations, an enormous amount of aluminum is introduced into the atmosphere, and the question is what happens next.”Stefan Löhle, expert in high-energy flows

Aluminum oxides constitute the majority of what space technology leaves in the mesosphere. Meanwhile, lithium is relatively simple to detect from Earth but constitutes less than two percent of the aluminum alloys used. “With satellite constellations, an incredible amount of aluminum is brought into the atmosphere, and the question is, what happens next?” says Löhle. However, it is challenging to pinpoint whether the aluminum or another metal is more harmful to the atmosphere. It could just as well be measured in copper, titanium, or silver—the latter also entering the atmosphere unnaturally due to space activities. However, Löhle does not yet consider this a looming danger. “We should take a scientific approach and state: We do not know.”

The Danger of Not Knowing

Baumgarten strikes a similar note: “The fact that we don’t know what happens—that’s the real danger.” The very thin air in the mesosphere protects us from the harsh and energetic UV radiation of the sun. The mesosphere is the middle of the five atmospheric layers, resting at an altitude of about 50 to 80 kilometers. Above it are the lower and upper thermospheres, marking the transition to outer space. Unlike the lower stratosphere with its ozone layer, many people are unfamiliar with the mesosphere. Only in recent years have researchers intensified their efforts to fill these knowledge gaps. Through simulations, ground-based measurement devices, and dedicated satellites, they investigate the delicate interplay of winds, temperature, chemistry, and external influences in the mesosphere. “I like to call it the ‘ignorosphere.’ We ignore the mesosphere because we don’t use it,” says Baumgarten. “But we should care for it because it protects us from harsh solar radiation.”

His “favorite scenario” is that rockets may no longer be able to fly through this layer, or that radio communication may not function normally, as the propagation of radio waves also depends on the composition of atmospheric layers. “That would motivate the industry to take action rather than waiting until politics and citizens push them to do something.”

But what scenario is realistic, Baumgarten cannot say. It might still be that we could counteract climate change. “That would be the absolute stroke of luck—although I’d find it annoying if we later had to pay SpaceX to do that for us.” However, it could also be that we destroy the ozone layer. Or we provide condensation nuclei for clouds and change the hydrological cycle. “Everything is possible; we cannot currently rule out any of it. That is the risk in my view.”

Perhaps nothing will happen. “However, in the past, people often dumped a ton of oil into a river, trusting that the much larger volumes of rain would compensate for it. That’s a normal human behaviour.” But now, an understanding must be built to determine whether we are conjuring a problem in the thin air of the mesosphere and, if so, how significant it is.

However, Baumgarten does not confront space travel. On the contrary, he wants to provide solutions. He reports that in discussion rounds, he is often the only atmospheric physicist among numerous space technicians, and his research sparks significant interest there. “None of them want to destroy the atmosphere.”

And Baumgarten remains optimistic. If a real problem arises in the mesosphere, it might simply be solved. “Maybe rockets just need to be designed to burn up at a different height. Or maybe we need to construct them so they disintegrate entirely into millimeter particles that do not evaporate—and then the problem will be gone.”