If you're bracing for little men in flying saucers, you can probably relax.

Astronomers say the colourful, bright object that captured attention as it burned across the night sky Sunday was more than likely a very rare meteor event.

Another expert speculated it could have been a piece of a satellite, so-called space junk, knocked back to Earth by a passing meteor.

People across southern Ontario and the northern United States reported sightings.

In Wisconsin, television stations interrupted their news broadcasts to assure viewers they were not being visited by a UFO.

In Michigan, some people reported that the thing had landed in their backyards -- claims authorities quickly discounted. In Grand Rapids, Mich., some said they saw a meteor about the size of a softball or a basketball travelling as close as 80 kilometres from the ground.

Michael Burns, past president and member of the Kitchener-Waterloo Centre of the Royal Astronomical Society of Canada, said a minor meteor shower known as the Gamma-Normid likely sent tiny space particles shooting through the Earth's atmosphere, creating the appearance of "fireballs streaking across the sky."

The Gamma-Normid event hasn't been visible in the past few years but occurs annually and peaks around March 14 -- when it's most likely to be visible, said Burns, who teaches mathematics and physics at Waterloo Collegiate Institute.

Waterloo's Anna Townshend, a graduate of the University of Western Ontario's astrophysics program, said the object she saw -- a bright, glowing disc -- came from space, based on the angle of descent. She speculated it might have been space junk burning up as it fell to Earth.

"Different satellites and space stations quite commonly get hit by meteors," Townshend said. "It's pretty common to have little pieces of metal flying into the atmosphere."

But despite assertions from experts, some witnesses say the object had a rectangular shape and didn't look like a meteor. Some told reporters they initially thought it was a nuclear warhead. Others feared a crashing commercial airplane.

"At first, it looked like an airplane, but it was on a rapid, horribly wrong descent," said John Sweeney, who watched the object burn out of the night sky as he drove west on Highway 7/8. "Then poof. It was gone."

He breathed a sigh of relief that his 10-year-old son in the passenger seat wouldn't have to witness a plane crash.

Kitchener's Michael Clarke swears he saw an F117 Stealth Fighter, an American spy plane, travelling above University Avenue on Sunday night.

"After I spotted it, I ran inside for my binoculars and made a positive ID when I returned outside," Clarke wrote in an e-mail. "There did not appear to be any escort aircraft and, indeed, it was almost silent."

The people charged with defending our skies, North American Aerospace Defense Command, or Norad, reported nothing unusual.

Some witnesses said the object seemed on a collision course with Ayr, west of Kitchener. A Toronto-area radio station suggested the flaming ball touched down in tiny Nobleton, near Newmarket, although the local fire department found no suspicious debris.

Paul Wiegert of the University of Western Ontario's astronomy department said if any parts of it came to Earth, it would likely have landed somewhere in Michigan.

Waterloo regional police believe the sighting was a meteorite, based on information they received from other agencies, Insp. Bryan Larkin said.

A meteor shower occurs when the Earth, which rotates around the sun, passes through debris in space. As it does, stones and tiny sand particles enter the Earth's atmosphere and burn up, which makes them look like fireballs, said Burns, the local astronomer.

They burn up because they enter the Earth travelling at about 108,000 kilometres per hour and can be seen over large distances, he said.

"For two seconds or three seconds, you can see it covers about 90 kilometres," he said. "That's why people from different cities can see them."

The fiery streaks in the sky could appear orange, red or blue, depending on the angle at which they're travelling, how fast they're moving and the material they're made of, Burns said.

Even tiny grains of sand from space will appear much larger because the high speed at which they're travelling makes them look brighter, he said.

The higher they are in the sky, the farther away they can be seen. However, the fiery particles can suddenly disappear into the dark sky if they slow down or burn up completely, Burns said.

If people did witness a Gamma-Normid event Sunday, it would have lasted only a couple of seconds, Burns said. Depending on the angle, the particles were travelling, however, it may have lasted longer, he said.

The space debris involved may have come from a comet, but it's hard to say for sure, Burns said.

"It's such a minor event that nobody's done a lot of research on what causes the debris that was left over."

Burns said it's tough to predict whether more Gamma-Normid events will be seen in the region over the next few days.

"The odds are very, very low, but you know, if we've seen something one night, there's no guarantee that we're not going to see something another night."