One Twitter user, sharing a picture of the phenomenon, questioned: “The end of the world!?”
Shamika B tweeted: “NASA what’s that flying over California – UFO?”
Meanwhile Eric de Franco tweeted: “What is this?? UFO in California or SpaceX”
Residents from across California can also be heard marvelling at sight on videos uploaded on to social media.
But a tweet from the SpaceX Twitter account posted at the time of the alleged UFO sighting at 1.28am GMT reveals all.
The SpaceX account, sharing a link to a webcast, said: “Liftoff!”
What is SpaceX’s Falcon 9 rocket?
The Falcon 9 is a family of two-stage-to-orbit medium lift launch vehicles designed and manufactured by SpaceX.
Both are powered by rocket engines utilising liquid oxygen and rocket-grade kerosene propellants.
So far there have been five versions of the rocket including Falcon 9 v1.0 , Falcon 9 v1.1, Fallcon 9 Full Thrust, Falcon 9 Block 4 and Falcon 9 Block 5.
The first stage engines are arranged in a structural form SpaceX calls “Octaweb”.
Falcon 9 v1.0 and Falcon 9 v1.1, have been retired, meanwhile the current Falcon 9 Full Thrust, is the world’s first partially reusable launch system.
Including Saturday morning’s launch there have been 18 Falcon 9 launches by SpaceX this year.
The rockets conduct missions for NASA and other space agencies as well as for governments and private companies.