MONTREAL - Don't panic, says Chris Hadfield.
That was the Canadian astronaut's down-to-earth advice after many people tweeted they saw a blue flash and heard a thundering boom in the Montreal and Ottawa areas Tuesday night.
Two Ontario-based professors said Wednesday the phenomenon was likely a fireball, with one suggesting it was probably between the size of a baseball and a basketball when it passed through the atmosphere.
Hadfield, the former commander of the International Space Station, tried to offer some words of assurance, saying it's just part of normal life in the universe.
"Earth gets hit by 100 tons of meteorites every day," he told The Canadian Press on Wednesday while in Montreal to promote his new book. "Most of them are grains of dust — it's like we're perpetually being sandblasted by the universe.
"Once in a while we get a big rock that comes into the Earth's atmosphere and like in Montreal last night, it actually gets down low enough in the atmosphere to burn up as a blue flame and then make like a pressure wave that we hear as a sonic boom.
"What happened I think was just a part of the universe that arrived here on Earth."
Peter Brown, a physics professor at Western University in London, Ont., said sound usually indicates something has penetrated deeper into the atmosphere.
"It's certainly consistent with the effect we would expect for a relatively good-sized fireball that might produce meteorites," he said in an interview.
Brown said it would be difficult to speculate about the size of the fiery object because of a lack of information.
The university has a network of about a dozen video stations that watch the sky all night looking for bright meteors.
Brown, who researches meteors and comets, said it was cloudy and overcast Tuesday night and that no meteor was spotted in data that was collected.
But he suggested the fireball may have been far enough away in the sky that none of his cameras was able to record the event.
One person in St-Lazare, west of Montreal, said she heard what sounded like a huge garbage truck rumble past her house for a few seconds.
WPTZ, a television station in Plattsburgh, N.Y., said it received numerous phone calls reporting what sounded like an earthquake.
Paul Delaney, an astronomy expert at Toronto's York University, agreed with Brown's assessment that it was a meteor.
"Everything we've got at this point in time certainly suggests that we've had a meteor event in the atmosphere — a fireball of some description," he said from Toronto.
"Whether or not it has resulted in meteorites hitting the ground (is) to be determined."
The York professor said it's conceivable that based on its brightness and its height, the fireball could have measured as much as one metre in diameter.
But ''we're probably talking of tens of centimetres of size," Delaney said. "Baseball, basketball-size.''
Delaney explained that the noise heard by some witnesses was caused by an object travelling at "tens of thousands of kilometres a second."
"When you've got that sort of speed through the atmosphere then almost anything is possible.''
Cathy Woodgold, a seismologist with Earthquake Canada, said nothing was picked up on the federal agency's equipment.
"Usually with a meteor we wouldn't see anything on seismographs," she added. "We might, but likely not, and we didn't."
Delaney said Tuesday night's fireball was nothing like the event that occurred in Russia earlier this year.
The Chelyabinsk meteor, estimated to be about 10 tons, streaked across the sky and exploded over the Ural Mountains on Feb. 15 with the power of an atomic bomb. The sonic blasts from that fireball shattered windows and injured about 1,100 people.
"We're talking about five-car lengths — that's what Chelyabinsk was," Delaney noted.
Hadfield admitted some space rocks can pose a threat to Earth.
"There are millions of rocks big enough to do significant damage like the one in Russia,'' he said.
''I wouldn't go around like Chicken Little thinking the sky is falling but it is one of the natural disasters that has happened repeatedly in the Earth's history.''