Scientists have solved the mystery of a spinning orb of bluish light that slowly streaked across the sky above Alaska last month, stealing the show from the famous northern lights: The unusual ball was most likely debris from a Chinese rocket passing overhead.
Eyewitnesses across the state spotted the strange phenomenon March 29 at around 5 a.m. local time. "It seemed like it had something that was spinning inside it," Leslie Smallwood, a Fairbanks resident who witnessed the event, told local news station KUAC. The orb appeared much larger than a full moon and moved from the northeast to the southwest, he added.
An automatic camera trap captured images of the orb streaking in front of the northern lights (also called the aurora borealis). The camera trap, operated by The Aurora Chasers Ronn Murray and Marketa Murray, a husband and wife duo in Fairbanks who run northern lights photography tours, takes regular photos of the sky every 45 seconds so people can experience the northern lights in close to real time. The camera took six photos of the orb, which suggests that it was visible for at least four and a half minutes.
"It's not like it shot across the sky," Smallwood told KUAC. "It was like, taking its time."
The orb came and went without any real explanation. However, after analyzing the photos, scientists determined that the big blue ball was likely the result of a photobombing Chinese rocket.
"I am very confident that what people saw was the dumping of fuel from a Chinese rocket stage," Jonathan McDowell, an astronomer at the Harvard Smithsonian Center for Astrophysics in Massachusetts, told KUAC. The orb corresponded with the flight path of a Chinese rocket that was delivering two satellites into orbit, he added. The rocket was a two-stage Long March 6 carrier rocket that launched from Taiwan, according to a tweet by McDowell.
The rocket likely released leftover fuel into space, where the fuel froze and spread out into a large ball that was illuminated by sunlight, McDowell told KUAC. "This cloud is probably hundreds of miles across; that's why it looks so big," he added.
Other scientists agree with McDowell's explanation. "A glowing cloud of gas that was sunlit would look like that," Mark Conde, a physicist at the University of Alaska Fairbanks, told KUAC.
The orb seemed to be spinning because, when rockets dump their fuel, they enter a controlled tumble to maintain the rocket's orbit. The rocket would have been rotating "end over end while spewing out this fuel like a garden hose," McDowell said.
This is not the first time this phenomenon has happened. In October 2017, an even larger blue orb was seen in the sky above Siberia, according to Science Alert. On that occasion, the frozen fuel was left by Russian military rocket tests in the area.
Originally published on Live Science.