The launch happened around 5:30 p.m. in Southern California at Vandenberg Air Force Base.
The sky wasn't the only thing to light up after the launch, however. Twitter and Facebook were ablaze with users posting about the strange-looking light overhead.
People from downtown Los Angeles to Orange County to the Inland Empire reported seeing the rocket and its trail in the darkened sky.
The Ventura County Sheriff's Department even tweeted to let residents know there was no reason to be alarmed.
SpaceX rocket launch from Vandenberg was the flash you saw in the sky! No cause for concern.— Ventura Co. Sheriff (@VENTURASHERIFF) December 23, 2017
This mission sends 10 satellites into orbit, using another used Falcon 9 rocket for the flight.
SpaceX says this is the fourth flight for long-time customer Iridium.
Both companies are under contract to launch a total of 75 satellites to provide communications services for regions all over the world.
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