Many filmed the event reportedly seen from Las Vegas, California, into neighbouring Nevada.
US defence officials have since confirmed it was space junk – a Russian SL4 rocket body which re-entered the atmosphere above Arizona before burning up.
But many who took to social media were not convinced, declaring it was a crashing alien craft, or missiles being fired.
On clips of the incident on YouTube, those recording it could not contain there shock, with cries of “What the f••• is that going up in the sky” and “There’s f***ing alien in f***ing Cali.”
Others joked it was Father Christmas on an early jet-powered delivery run.
California has been UFO hotspot in recent weeks, with a bizarre “alien corpse” found outside someone’s home, the day after another mass “blue UFO” sighting, which was later explained by the authorities as a Trident missile test.
Pastor Paul Begley, an evangelical Christian who records videos which endlessly prophecies the world is about to end, was not convinced by the official explanation or that aliens were on a collision course.
He speculated it was a missile being fired due to tensions between the US and Russia and China.
He said in a video: “What are the odds of it coming over the same area.
“The blue rocket (six weeks ago), they tried to tell us it was a certain type of trident missile, but we know they do not operate like that.
“I know it was not a meteor, it was way too slow. It was a rocket of some sort, I don’t know if it was from the US, Russia or China, but there is a lot of tension.”
