New Zealand stargazers were baffled by strange, billowing light formations in the night sky on Sunday night.

At about 7.25 p.m., Alasdair Burns, Stewart Island/Rakiura’s star guide, received a text message from a friend: Go outside and look at the sky. “Once we actually got out, it was very clear what he was referring to,” Burns said.

He saw a huge blue vortex of light amidst the darkness. “It looked like a gigantic spiral galaxy, hanging there in the sky, slowly drifting through it,” Burns said. “A very strange feeling.”

Burns took some pictures of the lights on the long exposure, and picked up the coil from his phone. “We quickly knocked on all our neighbors’ doors to get them out too. And so there were about five of us, all on our common porch looking up and kind of, well, a little terrified.”

The country’s star-gathering and amateur social media groups were lit up with people who posted photos and questions about the phenomenon, which was visible from most of the South Island. Theories abounded – from UFOs to foreign missiles to commercial light screens.

“Obsessed by our orbiting black hole,” said one stargazer. Another commented: “Aliens at it again.”

The reality was likely somewhat more realistic, said Professor Richard Easter, a physicist at the University of Auckland, describing the phenomenon as “strange but easy to explain”.

Clouds of this type sometimes occur when a rocket carries a satellite into orbit, he said.

“When the propellant is ejected from the back, you have what is basically water and carbon dioxide – which briefly forms a cloud in space that is illuminated by the sun,” Easter said. “The geometry of the satellite’s orbit as well as the way we sit in relation to the sun — this combination of things was just right to produce these totally strange clouds that were visible from the South Island.”

Easter said the missile in question is likely a Globalstar launch from SpaceXwhich the company sent into low Earth orbit off Cape Canaveral, Florida, on Sunday.

Burns had guessed that the coil was likely a rocket, after reading about a similar phenomenon in 2009, when a Russian missile launch caused huge blue swirls over Norway. Even knowing the likely source, he said, it was a confrontation scene. “None of us had ever seen anything like this. It was amazing.”