The spectacle, which was a “bright flash with a green tail,” lit up the evening sky around Lake Baikal – the deepest and oldest in the world – in the early evening on Tuesday.
Social media buzzed with claims of visitors from outer space.
Some witnesses posted: “Maybe aliens?” and “soon there will be reports about mutants”.
But local experts disagreed sharply over the unidentified flying object (UFO) seen low in the sky.
One eyewitnesses posted online that “a comet fell from the sky – it was glowing bright green and quite large.”
Another said: “At 7.30pm in the sky a comet or a meteor was flying. There was a powerful flash. Who else saw it?”
A third said: “A strange flash in the sky – looked like a comet with a green tail.”
Some believed it may have exploded over or close to Lake Baikal, a version backed by the head of a major nature reserve Mikhail Ovdin.
He said: “In the area of Goryachinsk we saw a sharp light of bright green colour.
“And then we saw the object flying, leaving behind a trail.
“It flew within five seconds, and disappeared over Lake Baikal. I think it burned down in the atmosphere.”
The head of the Astronomical Observatory at Buryat State University in Ulan-Ude, Liliya Mironova, insists the object was manmade.
She said: “I myself did not witness the incident, but I saw the videos.
“I can say with certainty that it was not a meteorite, nor a meteor or a comet.
“It is clear that we are talking about some sort of man-made object.
“Firstly, if it reached the ground, there would be a shock wave, and everybody would have felt it.
“But it did not reach the ground.”
In contrast to other experts, she said it was travelling “very slow” in the sky.
She also disputed the object was burning out in the sky.
The director of the Observatory of Irkutsk State University, Sergei Yazev, said: “Without doubt, we are talking about a space bolide.”
A bolide is a meteor that explodes in the atmosphere rather than falling to the ground as a meteorite.
The images of the object were “typical” of a bolide, he said.
He added: “It was burning in the atmosphere, the green glow was because of the high temperature.”
He also ruled out other manmade objects like a satellite falling to Earth from space orbit.
He said: “We can assume that the size of the meteor was about 50 centimetres.
“It may well be that it was not all burned out and some small stone debris could fall to the ground.”
But the director of Irkutsk planetarium Pavel Nikiforov said there had been no reports of a “celestial object” falling to earth and he believed this sighting was manmade.
He believe the cause was a falling part of a missile.
Initial speculation was that it was a missile launched by Vladimir Putin’s military machine in Orenburg region which “successfully” hit its target on the Kamchatka peninsula close on Russia’s pacific coast.
But the timing appears wrong, as the ICBM RS-18 intercontinental ballistic missile would have crossed this part of Siberia several hours earlier, it was claimed.