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A team of young researchers from Justus Liebig University Giessen and THM University of Applied Sciences (Technischen Hochschule Mittelhessen) is preparing to launch its CLOUDS experiment aboard the BEXUS-38 stratospheric balloon. The team is now at Esrange Space Center in northern Sweden, where on-site testing and experiment integration have been successfully completed.
CLOUDS stands for »Calcium carbonate Light Observation Under stratospheric Dynamics and Scattering«. The experiment investigates how calcium carbonate particles (CaCO₃) change the spectrum of incoming sunlight. A dual-channel microspectrometer from Eureca serves as the central measuring instrument.
Esrange Launch Site: A stratospheric balloon ready for launch—the platform for BEXUS experiments. (©CLOUDS-Team)
e9u-SPMD-350/850-10-Duo dual microspectrometer (350 – 850 nm): Two optical paths for measurement and reference. (©EURECA)
CLOUDS has successfully completed the key preparations for flight. On 18 September, the experiment passed its thermal vacuum test at ZARM in Bremen and, according to the team, was cleared for flight. The Eureca dual-channel microspectrometer was inside the thermal vacuum chamber during the test.
The team is now participating in the launch campaign at Esrange in northern Sweden. On-site testing and experiment integration have been successfully completed. BEXUS-38 is tentatively scheduled to launch on Saturday, 17 October. With the larger balloon now allocated to the flight, the team expects to reach an altitude of around 33 kilometres.
| Date | Milestone | Status |
|---|---|---|
| 16 June 2026 | Integration Progress Review successfully completed | Completed |
| Late July 2026 | Further development of the optics, reference path and test concept; spectrometer undergoing testing | Documented development stage |
| 18 September 2026 | Thermal vacuum test at ZARM in Bremen passed, including the Eureca spectrometer; cleared for flight according to the team | Completed |
| 12 October 2026 | Ground test recording measurement and reference spectra before and after calcium carbonate dosing | Conducted |
| Mid-October 2026 | Testing and integration during the launch campaign at Esrange successfully completed | Completed |
| Tentatively 17 October 2026 | BEXUS-38 launch carrying CLOUDS; expected altitude approximately 33 kilometres | Planned |
The REXUS/BEXUS program is carried out under a bilateral agreement between the German Aerospace Center (DLR) and the Swedish National Space Agency (SNSA).
BEXUS stands for »Balloon Experiments for University Students«. Here, students have the opportunity to conduct their own scientific experiments using stratospheric balloons. BEXUS-38 is scheduled to launch on 17 October 2026; the typical flight duration is approximately 2–5 hours.
»REXUS/BEXUS Student Experiment Programme«; Credit: DLR/SNSA
How does the presence of CaCO₃ particles alter the incident solar spectrum under stratospheric conditions—and what can be inferred from this about the scattering properties of these particles?
Conceptual diagram: SRM reflects a portion of the sun's radiation via aerosols in the stratosphere – CLOUDS measures the spectral effects. (©CLOUDS-Team)
The Earth’s climate depends heavily on how much sunlight the atmosphere scatters back into space and how much radiation ultimately reaches the ground. Aerosols in the stratosphere can influence this radiation balance—but it is not just »how much« light is scattered or absorbed that matters, but also »at which wavelengths.« This is exactly where CLOUDS comes in: Spectral measurements reveal how calcium carbonate particles alter incoming sunlight and how strong the scattering is across different spectral ranges. Such data are crucial for improving physical models of particle-radiation interactions and for estimating effects more realistically. In addition, the experiment makes a practical contribution to atmospheric research because it demonstrates how spectroscopy can be robustly implemented as a measurement method under near-stratospheric conditions (low pressure, low temperatures, vibrations).
The team cites calcium carbonate (CaCO₃) for its high scattering efficiency, low absorption in the thermal infrared, favorable chemical properties, and potentially longer atmospheric lifetime, among other reasons. Furthermore, different wavelengths contribute differently to warming—and CaCO₃ can behave differently depending on the wavelength. This is precisely why spectral measurements are so fascinating: they show how light changes as it passes through the particles and how strong the scattering is.
CLOUDS uses an Archimedean spiral to release and dose the calcium carbonate. The mechanism rubs the material through a fine mesh, combining particle release and dosing in a single process. According to the team's experience so far, it works reliably for this application.
The CLOUDS setup uses a dual-channel microspectrometer from Eureca with a spectral range of 350 – 850 nm and a spectral resolution of approximately 10 nm. Two optical paths record measurement and reference signals. Comparing these signals supports the analysis of spectral changes and helps account for fluctuations in incoming light.
The spectrometer has already been used in ground tests and was also inside the chamber during the experiment’s successful thermal vacuum test.
Block diagram of the experiment: chamber containing CaCO₃ aerosol, sensor array, and light-tight optics leading to the dual microspectrometer (350 – 850 nm).
During a ground test on 12 October 2026, the Eureca dual-channel microspectrometer recorded measurement and reference spectra before and after calcium carbonate dosing. The team has provided an initial preview of these data. The recordings document the use of both optical paths in the assembled experiment and provide a basis for further analysis.
BEXUS-38 is currently scheduled to launch on Saturday, 17 October 2026, subject to confirmation. We will update this page once the launch details are confirmed and a livestream link is available. After the flight, we will report on the mission and further results from the CLOUDS experiment.
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Last update: 2026-10-16
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