Mysterious whitish-blue spirals of light on Saturday morning, moving quickly were seen on the sky of the northern horizon of Alaska amazed people, reported Anchorage Daily News.
The eye witness Todd Salat said, as he got nearer, "It got bigger and bigger." He had no idea what it was. After about five minutes, it was almost overhead, he said.
"It was a beautiful piece of art in the sky," he said. "I would say this was maybe the most bizarre thing I've ever seen in my life."
Salat, who is a photographer and specialises in the northern lights, said he spent two hours capturing photos of the moving auroras — earlier believed to be "spiral".
Don Hampton, a research associate professor at the University of Alaska Fairbanks Geophysical Institute, said that the spiral "appears to be rocket engine exhaust from a SpaceX Transporter-7 mission that launched on the Falcon 9 about three hours earlier in California."
Hampton wrote: "Water vapour in the exhaust from the second stage engine freezes and catches high-altitude sunlight, effectively glowing and creating this spiral galaxy of a display.”
"As the rocket gained altitude, it did this pass-by over Alaska, stunning many night-watchers,” he said.
There were also other eyewitnesses seeing the lightning sky hundreds of miles away.
Elizabeth Withnall in the Northwest Arctic, who was also waiting to catch a glimpse of the northern lights, said: “We get a lot of very unusual phenomenon in the sky in the far north,” she said.
"I've seen fog bows and rainbows around the moon. So I just thought, 'this is some weird thing in the sky, and I don’t know what it is, but it’s pretty cool'."
Salat, while exploring what caused the auroras, said apparently solving the mystery was not quite as satisfying as seeing the strange phenomenon in the first place.
"The spiral, it was so perfect. It was beautiful. It was kind of a shame to think of it as exhaust, I have to admit," he said. "I did enjoy that mystery, and the unknown because after I found out what it was, I noticed that the wonder of it all kind of faded a little bit."