EUMETSAT lightning data detectives make spooky discovery

“We immediately knew this couldn’t have come from lightning activity, because it was a straight line of detections crossing our view of the Earth disc in only 20 seconds,” said Fabian Müller, an optical instrument system engineer at EUMETSAT, whose work focuses on the Lightning Imager and Infrared Sounder instruments on board MTG satellites.

“The Lightning Imager detects lightning candidate events when transient peaks in energy are measured against background images monitored by detectors on the instrument. We wondered at first whether it could have been a bright meteoroid that had whizzed past, because it was the peak of the Perseids meteor shower.”

However, when analysing the data, specialists saw that the object was in fact dark, shading the Earth’s radiance from reaching the cameras on its path. This raised the bewitching question of how a dark object could have caused the instrument to record energy peaks?

“The Lightning Imager was designed to detect even faint sudden differences in the light coming from Earth,” Müller explains. “It does this every millisecond, so even an object this fast poses no problem for the instrument.

“In this particular and rare case of a dark object moving across the field of view, the cameras go straight from looking at something dark, to a much brighter Earth background. Those pixels in the path of the object triggered the instrument to record a detection.”

The team also wanted to know what the spooky object was in the first place.

“The answer came when we turned to EUMETSAT’s Flight Dynamics Team, who keep a constant watch over orbiting satellites and objects in their vicinities to ensure they avoid collisions with unexpected objects,” Müller said. “They identified the mysterious object as a piece of debris from a Proton K rocket launched in 1991 by the then USSR as part of a military satellite mission.”

The closest the debris came to MTG-I1 was estimated at 36km which, while relatively close, did not require a manoeuvre to move the satellite out of harm’s way - in accordance with EUMETSAT's monitoring and risk mitigation approach for objects in the proximity of Meteosat satellites.

"Data provided by the Lightning Imager will soon support specialists in spotting the tell-tale signs of storms across Europe and Africa," Müller added. "Solving even spooky mysteries such as this can help specialists maximise the potential of precious data when the instrument becomes operational early next year."

This is preliminary commissioning data, and is not for operational use.