ANCHORAGE, Alaska (AP) — Viewers of the northern lights got a surprise mixed in with the green bands of light dancing in the Alaska skies: A blue spiral appeared amid the aurora borealis for a few minutes.

The cause of Saturday morning’s display was excess fuel that had been released from a SpaceX rocket that launched from California about three hours before the spiral appeared.

Sometimes rockets have fuel that needs to be jettisoned, said space physicist Don Hampton, a research associate professor at the University of Alaska Fairbanks Geophysical Institute.

“When they do that at high altitudes, that fuel turns into ice,” he said. “And if it happens to be in the sunlight, when you’re in the darkness on the ground, you can see it as a sort of big cloud, and sometimes it’s swirly.”

While it’s not a common sight, Hampton said he’s seen such occurrences about three times.

The appearance of the swirl was caught in time-lapse on the Geophysical Institute’s all-sky camera and shared widely. “It created a bit of an internet storm with that spiral,” Hampton said.

The rocket took off from Vandenberg Space Force Base in California Friday night with about 25 satellites as its payload. It was a polar launch, which made it visible over a large swath of Alaska.

The timing of the fuel dump made for optimal visibility over Alaska.

In January, a similar spiral was seen over Hawaii’s Big Island. A camera at the summit of Mauna Kea, outside the National Astronomical Observatory of Japan’s Subaru telescope, captured a spiral swirling through the night sky.

Researchers have said it was from the launch of a military GPS satellite that lifted off earlier on a SpaceX rocket in Florida.