The huge, glowing UFO spotted hovering above Shanghai, Beijing and other regions in China on Saturday evening has led to a debate among astronomers who are trying to explain the phenomenon.

Mystery of glowing white ball in the sky.[Photo by an internet user in Beijing]

The "white ball" was spotted about 9pm above Shanghai by pilots who reported the sightings to the East China Air Traffic Control Bureau. At almost the same time, residents in Beijing, Shanxi and Anhui provinces and Inner Mongolia Autonomous Region also witnessed and photographed the similar, round bubble-like object rapidly expanding in the sky.

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.

Other astronomers, however, argued that the object was an aircraft beyond human technology after they studied the pictures taken by witnesses and drew its flight 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.

Wang said he calculated that the object was 300 kilometers above Inner Monglia, moving at low speed when it was spotted. He said he located it by observing the position of stars behind it from pictures taken from different places.