ENTRE-DEUX (974) 21.08.2015

At 02:00 on 21 August 2015 on Réunion, a witness photographed an intriguing phenomenon: a bluish luminous cone surrounded by a halo. Only 1 account was collected. GEIPAN describes the sighting of this large, translucent luminous phenomenon moving slowly across the sky as exceptional, and says the high-quality photographs allowed it to be characterised precisely. Its resemblance to similar phenomena, including one discussed in an article about a luminous halo over the Indian Ocean on 29 September 2013, quickly directed the investigation towards recent satellite launches. The object's position was found to match the trajectory of the cryogenic upper stage of Ariane flight VA225, launched from Kourou on 20 August at 20:16 UTC. This stage, catalogued as 40876, began passivation, or tank emptying, at 21:12 and passed over Indonesia at more than 8,000 km altitude; see the attached map. At about 21:30 UTC, given as the photograph time, it was visible from Réunion. That time corresponds to about 01:30 local time, whereas the opening account gives 02:00; the source does not explain the difference. GEIPAN explains that because the geostationary satellite launch used a highly elliptical transfer orbit of about 300–36,000 km, the launcher was already very high and visible over a wide area. The gas cloud followed a trajectory close to the launcher's, modified by the gas ejection velocity, which is difficult to model. The passivation cloud could persist until it dispersed into space. The halo was unused gas vented after the propulsion phase, illuminated by sunlight and visible to observers on the ground in darkness. GEIPAN classified the case A as conclusively identified: a gas cloud from passivation of the cryogenic upper stage (ESC-A) of Ariane VA225.