The phenomenon prompted much social media speculation, including on the various "expat" groups connected to Estonia, but experts believe an expiring rocket booster, probably one used by Elon Musk's SpaceX project, was the culprit.

One observer was Riigikogu MP Mario Kadastik (Reform), who is also a physicist.

Kadastik posted on his social media page that the origin of the glow in the sky: "Based on its movement, appears to be a (sub)orbital rocket component."

"The spiraling shape with its bright core seems to indicate a slowly rotating rocket stage discharging gases," he added.

Former head of TalTech's satellite project, Rauno Gordon meanwhile said: "This is usually the phenomenon observed when a rocket is secured at the end of its mission. The tanks get emptied during this rotation, so the escaping gas will not alter the orbit trajectory. Once the batteries are drained, it remains in its final orbit until it re-enters the atmosphere and burns up."

Meteorologist Kairo Kiitsak concurred, posting on his social media that a rocket's fuel tanks were the origin, and more specifically a SpaceX rocket.

"While many of us were excitedly awaiting the aurora, spiraling glows could be seen moving across the sky all of a sudden. These were fuel tanks from a SpaceX rocket launched into space," he wrote.

"In the rocket's second stage, the expelled fuel instantly freezes into ice crystals due to the low temperatures. Being at such a high altitude, it gets illuminated by sunlight, which causes the mysterious glow against the darkening sky," Kitsak noted.

Rauno Gordon said no one should be concerned about debris from the booster falling into their backyard, an extremely unlikely event nowadays.

He told ERR: "Indeed, various stages of older rockets did occasionally fall back to Earth, but SpaceX deploys the latest technology. They have all these aspects very thoroughly modeled."

That this occurred over Estonia or at least near to its airspace could, Gordon said, be due to the fact the rocket was ferrying satellites to be set to orbit around the poles.

The object captured on video was expelling fuel, possibly to reduce its weight, he added.

SpaceX did launch a Falcon 9 rocket just after 9.30 p.m. Estonian time Thursday, from the Vandenberg Air Force Base in California, carrying two satellites into orbit.

Both satellites belong to Maxar Space Systems and are intended for Earth observation.

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