Astronomers in Beijing can’t quite put their finger on the composition or origin of a giant, expanding bubble that appeared Saturday in thee night sky at around 9 p.m. China time.

You can see the bubble here:

The glowing cloud was seen in several Chinese cities and officials believe it was the same thing. But what that “thing” is, no one quite knows.

Eastday, a news portal site in Shanghai, has some local theories as to the source of the white ball:

Yu Jun, a former editor of a scientific magazine and amateur astronomer who witnessed the object, published an analysis saying the expanding bubble likely was a phenomenon caused by the “fuel of a thruster that diffused in outer space.”

Yu compared the bubble to an incident in Hawaii on June 22, when a cloud-like object rose from the horizon and expanded into a huge bubble and disappeared. Astronomers linked the bubble to the launch of an intercontinental missile from California four minutes before the object appeared.

The report quoted Phil Plait, an American astronomer, who thought that unused fuel from the missile was left in outer space and coalesced into an expanding spherical shell, which people observed as a glowing ball as it reflected sunlight.

Still, others went the UFO route:

“The object should be a special spacecraft which cannot be explained so far with our current aircraft technology,” Wang Sichao, a researcher with the Purple Mountain Observatory under the Chinese Academy of Sciences in Nanjing, told Chinanews.com.

Wang said he had studied UFOs for 40 years and called the latest incident one of the 20 most significant UFO events he had ever come across.

ITN from the United Kingdom reported an observer from the Beijing Planetarium as saying: ”At first, it’s relatively small and bright, the upper part is something like a semi-circle, a spherical ring of light, it’s obviously becoming bigger and bigger then.”

The Blaze contacted NASA to see if it could offer an explanation, but officials were unavailable for comment.

Earlier this month a “UFO” sighting diverted planes from a Chinese airport for several hours. The sighting was later identified as an unusual cloud.

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