Defunct satellites and spent rocket stages are generally expected to break apart and burn up as they descend through the atmosphere. Most previous work has focused on the risk that surviving fragments may pose at the surface, or on the frequency and location of re-entry events, rather than on chemical contamination in the thin air of the upper atmosphere. As a result, the impact of such re-entries on the mesosphere, roughly 50 to 85 kilometers above sea level, and the lower thermosphere, roughly 85 to 120 kilometers above sea level, has remained poorly characterized.

In the new study, Robin Wing and colleagues used a lidar system in northern Germany to measure lithium atom concentrations in the lower thermosphere. Lidar, a laser-based remote sensing technique, allows scientists to probe the composition and structure of atmospheric layers during clear-sky conditions. Lithium is particularly useful as a tracer because it is widely used in spacecraft components yet occurs only in very small natural background amounts at these altitudes.

Shortly after 00:20 UTC on 20 February 2025, the team observed a sudden and pronounced rise in lithium atom concentration, reaching about ten times the baseline value recorded earlier that night. The enhanced lithium layer extended from about 97 kilometers down to 94 kilometers above sea level. This lithium-rich plume persisted in the instrument's field of view for approximately 27 minutes, until the scheduled end of data acquisition.

To investigate the origin of this plume, the researchers combined their lidar measurements with atmospheric wind and transport models. By tracing how air masses moved at the observed altitudes and times, they were able to backtrack the likely source region of the lithium-containing air. The reconstruction pointed to an area along the recent path of a Falcon 9 upper stage that had re-entered the atmosphere in an uncontrolled manner over the Atlantic Ocean, west of Ireland, roughly 20 hours before the plume was detected over Germany.

The team then evaluated whether natural processes could plausibly explain the lithium enhancement. They considered meteoric input and other known high-altitude sources, and used additional calculations to test how such sources would evolve under the prevailing atmospheric conditions. Their analysis indicated that natural mechanisms were highly unlikely to produce the magnitude and vertical extent of the observed lithium plume at the specific time and location of the measurements.

Wing and colleagues present this event as a detailed case study of pollution from a single piece of space debris as it re-enters and breaks apart. At the same time, they demonstrate that ground-based lidar can serve as a sensitive tool to detect and characterize at least some of the chemical species released during re-entry. However, they note that not all materials can be observed in this way because some species undergo rapid chemical transformation or become incorporated into particles during descent, making them invisible to their measurement technique.

The authors argue that a combination of further targeted observations and advanced atmospheric chemistry modeling will be essential to assess the broader impact of re-entry pollution. They emphasize that the number of objects in orbit and the frequency of launches have risen substantially over the last decade, which is expected to drive a corresponding increase in the total mass of material re-entering the atmosphere. This trend suggests that anthropogenic inputs to the upper atmosphere from space activities will likely grow, raising questions about possible long-term effects on atmospheric chemistry and circulation.

Research Report:Measurement of a lithium plume from the uncontrolled re-entry of a Falcon 9 rocket

Related Links
Leibniz Institute of Atmospheric Physics
Rocket Science News at Space-Travel.Com