Plasma Tunnels Reveal How Dying Satellites Really Fall to Earth

German researchers simulate satellite reentry in 8,000°C plasma wind tunnels, uncovering alarming atmospheric chemistry as mega-constellations multiply.

When a satellite dies, it doesn't just vanish. Researchers at the University of Stuttgart are melting chunks of spacecraft in plasma wind tunnels to find out what actually happens — and the chemistry is unsettling.

The setup is extreme. The team pumps 6 megawatts of electrical power — 2,000 amperes — through a tungsten cathode to generate plasma at 5,000 to 8,000°C. Water-cooled tunnel walls prevent the equipment itself from melting. A 100-gram cylinder of aluminum alloy 7075, the same material used in satellite bodies, liquefies in about six and a half minutes.

Why it matters. Three or more large satellites or used rocket stages burn up in Earth's atmosphere every day, according to the European Space Agency's 2025 Space Environment Report. As of June, some 18,000 operational and defunct satellites orbit the planet. SpaceX, Blue Origin, and China all have ambitions to launch hundreds of thousands more — including orbital data centers. Since most satellites are designed for five-year replacement cycles, tens of thousands of tonnes of old spacecraft could vaporize in the atmosphere within a decade.

The chemistry is the problem. Unlike the 44 tonnes of natural space rock that enters the atmosphere daily — mostly silicon with traces of nickel and iron — satellites are made of aluminum and titanium. When aluminum burns up at altitudes between 60 and 80 kilometers, it can form aluminum oxide (alumina), a compound known to deplete ozone and reflect sunlight, potentially altering the atmosphere's thermal balance. The upper atmosphere is isolated with poor circulation, meaning pollutants linger far longer than those released closer to the surface.

It's not theoretical. In March 2024, a battery pallet jettisoned from the International Space Station — made of a nickel-based alloy called Inconel — pierced the roof of a Florida house. NASA had claimed it would burn up completely. When the Stuttgart team tested a similar Inconel cylinder in the wind tunnel, it refused to melt. "No matter on which trajectory this part was falling down, there was no chance that it would demise," said doctoral researcher Clemens Müller.

The research is just beginning. The team is now working to simulate the later stages of reentry, where aluminum reacts with atmospheric oxygen to produce either ozone-depleting aluminum oxide or less harmful aluminum hydroxide. "We don't know what these particles actually do in the atmosphere," said Fabian Hufgard, a Ph.D. researcher on the project. "This is only the first step."

Commenters on Hacker News were characteristically blunt: "There will be zero stomach for limiting launches for the ozone layer. I'm calling it now."

Sources: IEEE Spectrum, ESA Space Environment Report 2025