When ESA's groundbreaking Aeolus satellite began experiencing a mysterious signal loss in 2019, help came from an unexpected source: the Pierre Auger Observatory had detected the spacecraft's UV laser beam. This chance discovery sparked a collaboration between two completely different scientific communities – one studying Earth's winds from space, the other hunting ultra-high-energy particles from the cosmos – that would prove invaluable to both.
Artist’s impression of the Aeolus laser beam being detected at the Pierre Auger Observatory. The Observatory has four fluorescence detector sites positioned around its 3,000-square-kilometre array. Each site uses large telescopes to observe ultraviolet light produced when cosmic rays enter Earth's atmosphere. These same detectors serendipitously detected the UV laser pulses from Aeolus as the satellite passed overhead. © Oliver Lux and Steven Saffi / Pierre Auger Observatory, CC BY-SA 2.0
A mission in trouble
Aeolus, launched in August 2018, was the first satellite to measure global wind profiles from space. Its ultraviolet lidar system – ALADIN – was designed with two redundant lasers to fire pulses through the atmosphere, detecting how the light shifted frequency as it bounced off moving air molecules and particles.
The first laser, Flight Model A (FM-A), began experiencing problems within months of launch. A thermal drift caused the laser to become misaligned, and by mid-2019, ESA decided to switch to the backup Flight Model B (FM-B) laser. Initially, this was a success – the second laser delivered higher, more stable performance and better wind measurements.
However, within the first year of FM-B operations, the atmospheric backscatter signal began to decline dramatically. By 2021, the signal had dropped by more than 70%, degrading the precision of Aeolus's wind observations...
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