Two aging European satellites made their final descents into Earth’s atmosphere a day apart in late summer, and instead of vanishing unobserved like most reentering spacecraft, they were tracked in real time by a team of researchers flying alongside them in a private jet. The goal was not spectacle but data: almost nothing is known about the chemistry produced when a satellite disintegrates in the upper atmosphere, even as companies plan to launch spacecraft by the tens of thousands over the coming decade.
The satellites were the European Space Agency’s Tango and Samba spacecraft, two of the four probes that made up the Cluster mission, alongside the already-deorbited Rumba and Salsa. For 25 years the quartet studied how Earth’s magnetic field interacts with the solar wind. Rather than let Tango and Samba burn up unmonitored, ESA timed their controlled reentries so scientists could use the moment for atmospheric science.
Chasing Tango and Samba over the South Pacific
The reentries happened on Aug. 31 and Sept. 1, one satellite each day, over the southern Pacific Ocean a few hundred miles from the Polynesian archipelago of Tonga, as part of a campaign ESA describes as an encore for reentry science. An international team of researchers chased both events aboard a small commercial business jet, working from precise trajectory calculations supplied by ESA controllers, who stayed in contact with the spacecraft until moments before atmospheric entry. Jiří Šilha, chief executive of the Slovak company Astros Solutions and one of the researchers on board, said the reentries were visible even to the naked eye despite occurring in daylight, producing fireballs that lasted tens of seconds. “Bolides and meteors only last a few seconds or less,” Šilha told Space.com. “But satellites are slower. They enter the atmosphere at a shallower angle, and the atmosphere slows them down further.”
Thirty cameras, eight scientists, six windows
The aircraft carried 30 cameras and spectrometers fitted with specialized filters, packed in to capture both how the satellites broke apart and the spectral signatures of the chemical elements released as they burned. Eight scientists operated the instruments through the plane’s six windows in a darkened cabin as pilots navigated toward the spot where ESA analysts predicted each breakup would occur. Flying above the clouds also let the team avoid the thickest part of the atmosphere, which would otherwise distort the measurements. The satellites hit the upper atmosphere at roughly 22,370 mph, and the jet closed to within about 75 miles of the breakup point. “We begin observing it when it’s at about 90 kilometers [56 miles] and it stops being visible at about 65 to 70 kilometers [40 to 44 miles],” Šilha said, describing a window of roughly 30 to 40 seconds after the initial explosion during which fragments could still be seen ablating in the air.
Why researchers want to know what a satellite is made of when it burns
The campaign’s central question is what happens chemically as satellite hardware disintegrates, and how much of it survives long enough to reach the ground. That question has taken on new urgency as SpaceX, Amazon and Blue Origin push to put huge numbers of spacecraft into orbit; SpaceX alone has outlined plans for around 100,000 of its Starlink V3 satellites. Researchers still do not know precisely which spacecraft burn up completely during reentry and which shed fragments large enough to pose a risk on the ground, nor how the debris changes the chemistry of the upper atmosphere.
Tracing aluminum oxide and its ozone risk
The team focused on seven chemical compounds released during the breakups, including titanium, sodium and potassium, but paid particular attention to aluminum, which makes up most of a satellite’s structural mass. Aluminum does not occur naturally in Earth’s atmosphere, and researchers suspect it mostly converts into aluminum oxide during incineration. That matters because aluminum oxide has been flagged as a potential contributor to ozone depletion in the stratosphere if it builds up in high enough concentrations. “We tried to measure the aluminum at different altitudes, using specialized filters, which could tell us when this aluminum interacts with the atmosphere,” Šilha said. “That’s something that has never been observed before. We don’t know exactly how and in what quantities the aluminum oxide might be forming.”
Building on a 2024 dress rehearsal
This was not the team’s first attempt. The same researchers chased the reentry of Salsa, the first of the four Cluster satellites to come down, in September 2024. Lessons from that earlier campaign helped the team position the jet closer to Tango and Samba and collect sharper data this time. Šilha said the cameras captured tens of individual fragments in a level of detail that had not been observed before, data the team will now analyze to reconstruct the chemical reactions occurring second by second during breakup.
Feeding models built for a much busier orbital future
The measurements are meant to feed directly into atmospheric physics and chemistry models, helping researchers validate whether existing predictions of reentry pollution match what actually happens in the sky. “The data will help us validate whether the models agree with reality,” Šilha said. With satellite constellations expected to grow sharply over the next decade, that validation is aimed at giving scientists a clearer picture of what a much higher rate of reentries could mean for the atmosphere and climate long before the numbers climb that high.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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