Update on an Experimental Approach to Assess Particle Formation from Re-entering Spacecraft

There is a significant lack of knowledge about the impact of the ever-increasing number of satellites in the Low Earth Orbit (LEO) that are supposed to demise during re-entry into the upper atmosphere. Aluminum is injected into the upper atmosphere as a rather new element, because it is a major constituent of satellites, while being only a minor constituent of micrometeorites [1]. The impact of this new trace element on the atmospheric behavior is hardly investigated so far.

Current research assumes the immediate oxidation of molten or evaporated aluminum due to the high abundance of reactive atomic oxygen in the upper atmosphere. The reaction leads to either gaseous aluminum monoxide (AlO), to aluminum hydroxides (Al(OH)_x), or solid aluminum oxide (Al₂O₃) particles are formed. During the re-entry airborne observation campaign of the CYGNUS-OA6 re-entry in 2016, we detected spectral signatures of AlO at an altitude of approximately 70km [2]. The formation of (Al(OH)_x) [3], as well as the formation of solid aluminum oxide (Al₂O₃) particles is discussed in literature [4] [5]. However, few experimental data sets are available of these processes. In our group, we are trying to experimentally evaporate aluminum and detect the paths toward aluminum containing products by suitable diagnostic means.

These experimental simulations are performed in the plasma wind tunnels at the Institute of Space Systems (IRS) at the University of Stuttgart. We observed the evaporation of aluminum in a series of experiments using different experimental setups. The sole injection of solid aluminum only led to larger molten droplets released form the solid. In a second setup, a sample of aluminum powder cured in epoxy resin was placed in the plasma flow. The sample ablated, which lead to the evaporation of aluminum powder. A formation of AlO was observed by acquiring spectral signatures of known AlO bands. In a new approach, aluminum powder was ejected against the plasma flow direction through a water-cooled brass probe. This injection method allows for a higher entrainment time and the evaporation of aluminum. Again, the formation of AlO was observed through spectral signatures.

In this presentation, we will give a detailed insight in the experimental work developing an experimental setup to study the processes after the demise of re-entering satellites. Also, we will provide an outlook in the development of experimental setups for the detection of eventually formed solid particles. These experimental studies are of high interest to gain an understanding of the environmental impact of the rising number of re-entering satellites.

[1] Schulz and Glassmeier, Advances in Space Research, 2021.

[2] S. Loehle et al., Meteoritics and Planetary Science, 2021.

[3] Plane et al., JGR Space Physics, 2021

[4] Maloney et al., JGR Atmospheres, 2025.

[5] Park and Leyland, Acta Astronautica, 2021.