Some scientists agreed, saying you don't need a rocket or a missile to leave a trail like that. All you need is a jetliner flying in the right direction to trick your brain into seeing an optical illusion.
The vapor trail made headlines across the country.
Former Deputy Secretary of Defense Robert Ellsworth said he thought it could be a test firing of an intercontinental ballistic missile from a submarine. But the FAA's radar replays didn't detect anything.
NORAD did not monitor any missile activity, and nearby Vandenberg Air Force Base had no planned rocket launches.
Then a physics professor said it was probably a plane.
"It seemed to change directions. Ballistic missiles don't do that. It doesn't accelerate. Ballistic missiles accelerate," said Dr. Michio Kaku, professor of physics at City University of New York.
Kaku also said the mysterious object was going in the wrong direction for it to be a ballistic missile.
"Ballistic missiles go east to west. This one seems to be going north and south," he said.
Could the vapor trail have been an optical illusion?
If a plane leaves a contrail while climbing into the sky, its trajectory would appear angled, but if you see the contrail from another angle, with it traveling away from you, it could look as though the object is going straight up -- like a rocket.
"If you figure that it would go up into the atmosphere, and other nations would pick it up on their radar, but you see no indication of an accelerating object in the upper atmosphere," Kaku said. "Therefore, it's probably not a ballistic missile. I'm beginning to lean more toward the airplane theory."
Some people had speculated that it was a secret launch by the U.S. military, but experts point out that if the government wanted to keep it secret, it wouldn't launch a rocket or missile so close to Los Angeles and its millions of residents.