Photos and videos of the mysterious phenomenon quickly spread on social networks, and users put forward a variety of versions, in particular about the presence of aliens, reports BBC.
However, the true explanation of the phenomenon is much simpler, although no less impressive.
On March 24 at 21:48 Kiev time (18:48 GMT), a rocket was launched from the Cape Canaveral Space Centre in Florida as part of a secret NROL-69 mission for the US National Office of Military Space Intelligence.
At around 23:00 Kiev time (20:00 GMT), a mysterious blue spiral appeared over Europe and remained visible for several minutes. The Shanghai Daily published in social networks timelapse video, which shows how the spiral is gradually forming and expanding in the sky over Croatia.
How the “SpaceX spirals” are formed
These mysterious light phenomena, known as “SpaceX spirals,” occur after the second stage of a rocket separates from the main carrier. During this process, the first stage returns to Earth while the second stage continues its journey in space.
When the upper stage’s mission is complete, it jettisons the remaining fuel, which at high altitude instantly freezes to form a swirling pattern. The light reflected from these crystallised particles creates the illusion of a bright spiral structure resembling a miniature galaxy.