On the night of Saturday, May 17, skywatchers in parts of the U.S. were treated to a dazzling display of auroras from a surprise geomagnetic storm. But then observers spotted something very weird. A mysterious white streak cut clean across the night sky, leaving many wondering: what the heck was that?

On Sunday evening, the skies over Colorado and states farther south lit up with bands of green, blue, and red auroras. The storm was a result of solar flares and a coronal mass ejection (CME)—a massive bubble of coronal plasma and magnetic field ejected from the Sun—that occurred about four days earlier. The CME was initially expected to miss Earth, but a portion of it dealt a glancing blow to our planet’s magnetic field, triggering a geomagnetic storm.

But the real weirdness started at around 11:30 p.m. MT on May 17, when a bright white streak lit up the sky. Naturally, people took to social media for answers. Many observers initially thought it might be a strange form of STEVE, a unique band of purple and green light that often appears near auroras. STEVE is also caused by space weather events.

Photographer Mike Lewinski captured long-exposure shots of the whole thing unfolding over the skies of Crestone, Colorado.

“The aurora was rippling low on the northern horizon when suddenly a bright streak of light, reminiscent of a rocket re-entry, appeared high in the sky and flowed down to the horizon,” Lewinski told Spaceweather.com.​​

The strange event was not STEVE after all, but a recent rocket launch. Space.com reported that roughly an hour before the mysterious white streak appeared across the southwestern U.S., the Chinese company Landscape launched its Zhuque-2E methane-fueled rocket from the Jiuquan Satellite Launch Center in China. The rocket was shuttling six satellites into orbit, and the white streak may have occurred as the rocket prepared for reentry.

On X, astronomer Jonathan McDowell wrote that the odd event was likely the result of the rocket’s upper stage passing over the U.S. He elaborated that the white streak may have been caused by a fuel dump at an altitude of 155 miles (250 kilometers) as the rocket passed over the Four Corners region.

It’s not the first time rocket launches have left skywatchers puzzled. SpaceX’s Falcon 9 rockets have created dizzying spiral patterns across the night sky. So we’re likely in for more mysterious sky phenomena.