|image (c) David Murr, used with permission|
On June 23 at 01:51:58 UT, ESA launched a Vega rocket with the Sentinel-2A satellite from Kourou, French Guyana.
About two hours later, US astrophotographer David Murr was watching the sky as part of an aurora watch in Louisa, Virginia. Just after he had set up, he saw and imaged a "cloudy blob" moving from SE to NW through Ursa major (just above the trees in the image):
@Astroguyz Saw a cloudy blob flying through the northwestern sky while out aurora-watching. Launch-related? No idea. pic.twitter.com/QgFEFPeWSf— David Murr (@davidmurr) 23 juni 2015
After Murr's tweet and image were brough to my attention through a retweet by David Dickinson, I could quickly confirm this was related to the Sentinel-2A launch two hours earlier.
What is visible is probably fuel venting [but: see update below: it could actually be gasses from the actual last engine burn] by AVUM, the Attitude and Vernier Upper Module that assists in putting the payload into its intended orbit after separation from the third stage. The AVUM has liquid fuel engines (the Zefiro third stage has solid fuel engines) and did a final burn, aiming at
The map above shows the predicted track for Murr's Virginia location for Sentinel-2A and the Sentinel Zefiro 3rd stage : it matches the sky track and direction of movement of the "fuzzy blob" on his image extremely well. There is a small time difference of only a few minutes, but all taken it is very clear this object was connected to the Sentinel-2A launch (the plot is based on orbital elements for the Vega 3rd stage and Sentinel-2A from some hours later: there are no orbital elements for the AVUM).
Murr was not the only one spotting and photographing the object: more photographs by several other US observers are here.
UPDATE 20:35 UT: Jim Cook from Germantown, Maryland, describes what appears to be the actual last burn of AVUM in a post on the Seesat list. Putting all information together, I have come to the conclusion that the fuzzy cloud is not so much a fuel vent, but gasses from the actual AVUM burn.
Also: the press kit talks about a "de-orbit burn" for AVUM, but as AVUM has been catalogued as still in orbit (as in previous cases), it is a burn to lower the orbit rather than a true de-orbit burn.