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Researchers in Germany are melting chunks of satellites in high-temperature plasma wind tunnels to simulate how space debris burns up in the Earth’s atmosphere. These tests are revealing concerning trends about satellite reentry as the population of objects in low Earth orbit swells today.
Only a handful of facilities around the world are capable of conducting reentry simulation tests like the German researchers are performing. In particular, the researchers raise concern that ashes from reentry that remain in the upper atmosphere could increase ozone depletion and alter the atmosphere’s thermal balance in the process.
Three or more large satellites or used rocket stages burn up in Earth’s atmosphere every day, according to the European Space Agency’s Space Environment Report published last year. Every year, hundreds of tons of human-made objects reenter the atmosphere and evaporate, leaving behind clouds of microscopic ash along with some fragments that make their way all the way to the ground.
Researchers say the amount of satellite ash and the number of human-made objects hitting Earth so far is relatively low. However, the rapidly increasing numbers of launches to low Earth orbit now threaten to increase the levels of space junk reentering the atmosphere.
As of June, according to the ESA, some 18,000 operational and defunct satellites orbit the planet. And companies like SpaceX and Blue Origin have ambitions to launch hundreds of thousands of satellites, including giant orbiting data centers, in the next decade. China, too, is targeting up to 140 launches this year, and filing ambitious plans with the International Telecommunication Union for more launches toward its own megaconstellation of satellites and its own orbiting data center fleets.
Since most satellites are currently designed to be replaced every five years with newer technology, tens of thousands of tonnes of old spacecraft could be vaporizing in Earth’s atmosphere into the foreseeable future.
Meanwhile, the amount of space rocks today entering the atmosphere, to be clear, outstrips the amount of reentering space junk. An estimated 44 tonnes of space rock enters the atmosphere per day, most of which burns up. But space rocks and satellites are chemically very different things. While space rock mostly consists of silicon with small amounts of metals such as nickel and iron, space junk is predominantly made of aluminum and titanium.
“When a satellite demises, it does not disappear,” says Fabian Hufgard, a researcher working on his Ph.D. at the Institute of Space Systems at the University of Stuttgart in Germany. “It’s still there in the atmosphere in the form of small particles of aluminum, and we don’t know what these particles actually do in the atmosphere.”
Hufgard is a member of a research group that uses plasma wind tunnels to recreate the end-stage conditions of satellites disintegrating and burning up as they spiral back to Earth. Observations from the ground and from aboard planes show that most of this destruction happens at altitudes between 60 and 80 kilometers—too high for balloons and planes to reach, but too low for satellites to sample. To date, testing space debris chemistry and physics has been extremely difficult.
Researchers are working to close that gap with a combination of computer modeling, remote observations, and experimental research.
“We see this ever increasing number of satellites, and eventually, the number of satellites reentering the atmosphere will get to the point when it might start having consequences,” Hufgard said.
Testing the Atmospheric Chemistry of Space Debris
Atmospheric researchers worry that the aluminum from the incinerated satellites forms aluminum oxide. Also known as alumina, aluminum oxide is known to trigger ozone depletion and reflect sunlight, possibly altering the thermal balance in Earth’s upper atmosphere. Due to the high altitudes where these chemicals get released, the atmosphere has a much harder time getting rid of them than of the air pollution emitted closer to the planet’s surface.
“We don’t normally inject anything into that area of the atmosphere,” says Dominic Kuenstler, a graduate student in the Stuttgart group working on his doctorate. “We produce a lot of emissions from air travel, but that happens at much lower altitudes where we have better air circulation. The upper atmosphere is much more isolated.”
Then there is the danger of satellite fragments surviving the fiery descent through the atmosphere, causing damage to property or even putting human lives at risk.
Clemens Mueller, also a Stuttgart doctoral researcher, says that currently, the risk of a person on the ground getting hit by a piece of space debris is very low. With the growing numbers of satellite re-entries however, the overall risk is set to rise.
“In Europe, if you want to launch a satellite, you have to show that the probability of a piece of it surviving reentry is no more than one in 10,000,” Mueller says. “But obviously, if you have 10,000 satellites, then you would have a very high probability that one of them survives. So the rules need to be stricter.”
SpaceX, for example, claims that its Starlink satellites burn up completely during the reentry. Some researchers, however, question that claim. A portion of a satellite’s components, including tanks and reaction wheels, are made of durable metals such as titanium, which, unlike aluminum, are unlikely to melt completely during reentry.
The deficit of knowledge today about atmospheric reentry was revealed in March 2024 when a piece of a battery pallet, which’d been jettisoned from the International Space Station, pierced through the roof of a Florida house. Prior to the incident, NASA claimed the battery pallet, made from a nickel-based alloy called inconel, would completely burn up.
Mueller said that in the aftermath of that incident, the Stuttgart team took a similar inconel cylinder and put it through a wind tunnel test simulating the reentry conditions. It did not melt.
“We applied a very, very high heat flux on it, and it was almost impossible to melt,” Mueller said. “So, basically, no matter on which trajectory, this part was falling down, there was no chance that it would demise.”
Turning Up the Plasma Wind Tunnel Heat on Space Debris
No existing plasma wind tunnel today is big enough to melt an entire satellite, so the Stuttgart researchers burn individual satellite components or pieces of standard satellite materials, to understand what happens during reentry.
Reentering satellites zoom through the thickening upper atmosphere at velocities of around 8 km per second. The friction between the satellite’s body and the surrounding air creates heat, which can reach temperatures of more than 1,600 degrees Celsius.
Muller says that to recreate such plasma speeds in wind tunnels for timespans long enough to melt satellite fragments is currently impossible. The team therefore uses slower flow speeds, of up to 3 km per second, and adds extra heat using an electric arc generated between a tungsten cathode and a copper anode.
“We can go up to 6 megawatts of electrical power,” said Mueller. “2000 Amperes of electrical current. The plasma in the tunnel can get up to 5000 to 8000 degrees Celsius so we have to constantly pump water through the tunnel walls otherwise the components would instantly melt.”
During the experiments witnessed by IEEE Spectrum, the team melted a 100-gram cylinder of the aluminum alloy 7075, a material frequently used to make satellite bodies, which contains traces of zinc, chromium, and magnesium.
The chunk melted away in large drops after about six and a half minutes in the glowing plasma flow, after the temperature climbed to 600 ºC.
“This is the closest you can get to replicating what happens to a solid chunk of aluminum in the atmosphere,” said Hufgard.
He adds that simulating the entire satellite burn-up process all the way to ash is currently impossible. Once the aluminum drops fall off in the wind tunnel’s cylinder, they land on its water-cooled bed and, naturally, cool down. In a real reentry, the molten aluminum would continue its flight through the thickening air, further reacting with the surrounding gases, generating heat, until nothing but gas and metallic dust would remain.
Kuenstler said the team is developing ways to simulate the later stages of the reentry process, which leave behind potentially dangerous pollutants that can accumulate in the upper atmosphere. Studying previous reentry observations, the researchers know what chemical compounds should arise during each reentry stage. The team then attempts to recreate the same kind of emission spectra in the tunnel using powdered aluminum alloys suspended in the plasma flow.
“In the airborne observations, we discovered emissions spectra from atomic aluminum and aluminum monoxide, so we are trying to find an experimental setup to acquire a similar signal,” Kuenstler said. “We want to bring this aluminum to the point of evaporation and then see how it reacts with oxygen.”
Researchers are still assessing the specific effects that the growing numbers of incinerated satellites will have on the planet. Some models show that ozone-depleting aluminum oxide is likely produced in large quantities. However, other models indicate that more possibly less harmful aluminum hydroxide might be produced instead.
“It would be very important to find out which types of particles arise from the aluminum,” said Kuenstler. “This is only the first step.”
