A recent aircraft chase of a falling spacecraft has shed light on the complex and fiery processes that occur during the atmospheric demise of retired satellites. This unique observation offers valuable data that could help scientists understand the impact of satellite air pollution on Earth's atmosphere. In early September of last year, a dedicated team of European scientists embarked on a mission aboard a rented business jet from Easter Island to track the atmospheric reentry of Salsa, one of the European Space Agency's (ESA) four identical Cluster satellites.
The aircraft used for this mission was equipped with 26 cameras, designed to capture the brief yet critical events during reentry across different wavelengths of light. Initial results from this groundbreaking observation campaign were unveiled in early April at the European Conference on Space Debris held in Bonn, Germany. The satellite's dramatic burn-up occurred above the Pacific Ocean just before noon local time on September 8, 2024, lasting less than 50 seconds—akin to a meteor event.
Despite the bright daylight complicating the observations and limiting the use of more powerful instruments, which could have provided more detailed insights, the research team managed to collect significant data regarding satellite incineration, a phenomenon that remains poorly understood and difficult to study. According to Stefan Löhle, a researcher at the Institute of Space Systems at the University of Stuttgart, "The event was rather faint, fainter than we expected. We think that the breakup of the satellite produced fragments that were much slower than the main object and produced less radiation."
After the initial breakup at an altitude of approximately 50 miles (80 kilometers), the researchers successfully recorded the fragmentation for about 25 seconds before losing track of the fading streak of debris at around 25 miles (40 km). Utilizing filters of various colors, the team was able to detect the release of numerous chemical compounds during the burn-up, providing important clues about the nature of the air pollution produced during satellite incineration. "We detected lithium, potassium, and aluminum," stated Löhle. However, he noted, "At this stage, we don't know how much of it ends up in the atmosphere as long-term air pollution and how much falls to Earth in tiny droplets."
The increasing frequency of satellite reentries has raised significant concerns within the global atmospheric science community. Satellites are primarily constructed from aluminum, which, upon incineration, produces aluminum oxide, also referred to as alumina. Scientists are aware that alumina can contribute to ozone depletion and influence Earth's ability to reflect sunlight, potentially disrupting the thermal balance of the atmosphere. With the number of satellite launches on the rise, the byproducts generated during atmospheric burn-ups are expected to accumulate in the upper atmosphere over the coming years. However, the effects of this satellite air pollution remain largely unexplored.
The altitudes at which satellites disintegrate present a unique challenge; they are too high for standard meteorological balloons to reach and too low for satellites to sample effectively. Aircraft chases, like the one that tracked the Cluster Salsa reentry, provide a rare opportunity to collect accurate data on the chemical processes that occur during these events. Yet, these observation campaigns are expensive and complex to execute. To date, only five spacecraft reentries have been tracked from the air, including an Ariane rocket stage and three resupply missions to the International Space Station.
Researchers currently modeling these events admit that they lack comprehensive knowledge about the processes during satellite fragmentation. "That's the first thing we need to answer," Löhle explained. "We want to ensure that nothing falls on people's heads and then determine how harmful this material is for Earth's atmosphere." Early data suggests that the titanium fuel tanks from the 1,200-pound (550 kilograms) Cluster Salsa may have survived reentry and likely splashed down into the Pacific Ocean, which is an important finding given the increasing volume of satellites reentering Earth's atmosphere.
According to a report released by ESA last month, an average of three satellites returns to Earth every day, with the majority belonging to SpaceX's Starlink megaconstellation. The first generation of Starlink satellites weighed approximately 570 pounds (260 kg) each, while the current V2 mini variant has a mass of around 1,760 pounds (800 kg). The planned V2 version will be even larger, tipping the scales at 2,750 pounds (1,250 kg). While SpaceX asserts that their satellites are designed to incinerate completely upon reentry, the company has acknowledged that some remnants may occasionally reach the Earth's surface.
The European research team continues to analyze the data collected during the Cluster Salsa reentry and aims to align their observations with existing computer models. This alignment could yield deeper insights into the events that transpire during satellite fragmentation and subsequent incineration. "We are comparing what we have seen with models of satellite fragmentation to understand how much mass is being lost at what stage," explained Jiří Šilha, CEO of Slovakia-based Astros Solutions, which coordinated the observation campaign.
Löhle elaborated that researchers currently have insufficient understanding of the incineration process to accurately estimate the impact of satellite reentries on the atmosphere. As the aluminum body of a reentering satellite disintegrates, it melts and forms large droplets of molten metal. Some of these droplets vaporize into aluminum oxide aerosol, while others cool down and drift to the ground as nano- and micrometer-sized particles. The aerosolized aluminum is particularly concerning, as it has been linked to ozone depletion and various climate effects. "We don't have the data yet to quantify how much of it turns into aerosol," Löhle noted. "We hope that, at some point, we will be able to reconstruct a fragmentation sequence and determine how much aluminum each of the subsequent explosions released into the upper atmosphere."
Researchers are optimistic about gathering more data during the reentries of Cluster Salsa's three sister satellites—Rumba, Tango, and Samba—scheduled for 2025 and 2026. These satellites have been orbiting Earth since 2000, measuring the planet's magnetic field and its interactions with solar wind. However, all upcoming reentries will occur during daylight hours, which could hinder the researchers' ability to capture detailed spectroscopy measurements that could further illuminate the chemical processes occurring in the fragmentation cloud. Spectroscopy is an observational technique that dissects incoming light into individual wavelengths, but the signal from a reentering spacecraft is often too weak and obscured by bright solar light.
