Sighting in Entre-Deux, August 21, 2015

At around 2 a.m., a witness photographed something in the sky that puzzled them. They described a bluish, luminous cone-shaped form surrounded by a halo. Only one account was collected. It was a large, translucent, luminous phenomenon moving slowly across the sky, an exceptional sighting, and the quality of the photographs made it possible to characterise it very well.

The phenomenon resembled other cases, in particular a luminous halo seen in the Indian Ocean sky in late September 2013. The investigation therefore quickly turned to recent satellite launches. The object's position matched the trajectory of the cryogenic upper stage of the Ariane VA225 launcher, which had lifted off from Kourou the previous day at 20:16 UTC. This stage began its passivation, meaning the emptying of its tank, at 21:12 UTC, while passing over Indonesia at an altitude of more than 8,000 km. Around 21:30 UTC, when the photos were taken, it was in view of Réunion. The launch was of a geostationary satellite into a highly elliptical transfer orbit (about 300 km by 36,000 km), so the launcher was already very high up and could be seen from a very wide area.

The cloud followed a path close to that of the launcher, but altered by the speed at which the gas was ejected, which is hard to model. It may have persisted until it dispersed into space. The halo was the unused gas released by the launcher after the propulsion phase; lit by the sun, it became visible to observers on the ground in darkness. GEIPAN classified the case A, a fully identified phenomenon: a gas cloud from the passivation of the cryogenic upper stage (ESC-A) of Ariane VA225.