The Los Angeles "UFO" of December 22, 2017 (SpaceX Falcon 9 Iridium-4 launch)

Friday, December 22, 2017. A SpaceX Falcon 9 stood on Space Launch Complex 4E at Vandenberg Air Force Base in Santa Barbara County. It was carrying Iridium-4, the fourth set of 10 satellites for Iridium's next generation global constellation. The launch window was instantaneous: 5:27 p.m. PST. SpaceX had announced the launch in advance, with a press kit and a live webcast. In Los Angeles, about 150 miles away, most people had no idea.

The sun had just set on Southern California and it was rush hour. Then something appeared in the sky. It was a bright object, moving rapidly, leaving behind a vast teardrop-shaped plume that expanded until it swelled to huge proportions. Bright lights blipped behind it, leaving wispy vapors that expanded and disappeared. The Associated Press called it an iridescent streak. The Washington Post described a shining, billowing streak, jellyfish-like.

Dozens of cars pulled to the side of the 405 Freeway. Traffic stopped on freeways across Los Angeles. Reports came in from El Segundo, Santa Monica, Burbank, Pasadena, Long Beach, Castaic and Carpinteria. TV stations in San Diego, more than 200 miles south of the launch site, took calls. In Arizona, people watched from Phoenix and from Yuma, roughly 400 miles away.

Jimmy Golen was touring the Warner Bros. studio in Burbank. "People were wondering if it had something to do with movies, or TV or a UFO," he said. Some residents feared a nuclear bomb attack. In Phoenix, former Arizona governor Jan Brewer tweeted to ask what the lights were, and tagged President Trump.

The answer came quickly. The Los Angeles Fire Department issued a statement. The mysterious light was "reported to be the result of" the base "launching rocket to put satellite into space."

Elon Musk played along. The SpaceX CEO posted launch video with the caption "Nuclear alien UFO from North Korea." A user said their family was arguing about it, and he replied, "It was definitely aliens." Both were jokes, and they didn't help matters. Even after SpaceX confirmed the launch, some Twitter users kept alleging a government cover-up. Guesses online ranged from aliens to fairy godparents to Santa.

Phil Plait, writing in his Bad Astronomy column, explained the physics. Low in the atmosphere, air pressure holds the first stage's exhaust in a tight, narrow plume. Higher up "there is very little atmosphere," so the second stage's exhaust spreads out sideways, "creating that ethereal teardrop shape." The launch happened during twilight. People on the ground were in darkness, but the rocket had climbed into sunlight, and the sun lit up its exhaust.

This is called the twilight phenomenon. Exhaust particles and water in the plume condensed, froze and then expanded in the thin upper air, scattering the sunlight. Most witnesses never saw the liftoff, so the glowing plume seemed to come from nowhere.

John Greenewald Jr., founder of The Black Vault, watched it himself from Castaic. He filed the case as solved and noted that rocket launches "have been the culprit in many UFO cases over the years." In Yuma, Jesse Watson had started a timelapse about 45 minutes before the launch, and Gizmodo later featured it. There was a close precedent: the November 2015 Trident missile test, seen across Southern California. Since then, later Vandenberg launches have lit up the Los Angeles sky in the same way.