You know it can't be a UFO because this picture is actually in focus and doesn't look like it was taken from the back of a moving truck.
Lauren Hitchings at New Scientist has the answer:
According to Jonathan McDowell at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, this is just one of many spectacular formations this kind of rocket can make as the core-stage rocket plume balloons out in the thin air high in the atmosphere. The cross at the top of the formation was made by the Soyuz's four boosters.
"It looks so interesting because we're not used to seeing gas dynamics in a near-vacuum. At that altitude there's not really any weather to interfere with the gases, so the gases form very regular formations and fill up a lot of space," says McDowell.
(h/t Doubtful News)