Unusually bright meteors called fireballs have been blasting across the night sky all over Canada in recent weeks. Last week, a fireball thought to have produced a meteorite fall was spotted in southern Ontario. In Nova Scotia, fireballs were spotted two days in a row. The first one was also visible from P.E.I. and parts of Quebec. Just two weeks earlier, a spectacular fireball exploded over Yellowknife.
How strange and unusual is this?
Well, in some ways, it isn't.
Bill Cooke, head of NASA's Meteoroid Environment Centre, describes spring as "fireball season."
"We do tend to see more fireballs just before early spring," he said at a news conference in St. Thomas, Ont., Friday.
Canadian astronomer Ian Holiday first reported the seasonal phenomenon decades ago, he added.
In fact, NASA reports that the nightly rate of fireballs increases 10 to 30 per cent around the spring equinox.
But scientists still don't have a good handle on why that is, Cooke said.
Martin Beech, professor of astronomy at the University of Regina's Campion College, said the explanation may be pure chance, and it may be that "the Earth just happens to encounter more of these objects at this time of year."
Fewer meteors typically spotted in March
The strange thing about that is that, in general, March is the time of year when the number of meteors spotted overall is at its lowest. There are no major meteor showers and rates of "sporadic" meteors unrelated to meteor showers are also at their minimum.
That's because sporadic meteors tend to come from the direction the Earth is travelling, the apex, which is at its highest point in the night sky in the autumn. Consequently, that's the time of year when sporadic meteors peak.
However, Robert Hawkes, a solar system astrophysicist at Mount Allison University in Sackville, N.B., said fireballs may not necessarily have the same distribution as fainter meteors and in fact, some meteor showers are known to have a larger number of brighter meteors than fainter ones.
But are there more fireballs this year than in other years?
No, not according to a network of cameras across North America that track fireballs.
"This year appears to be a pretty normal year," Cooke said Friday.
No relationship between Ont., N.S. fireballs
Nor does he think there was any relationship between last week's Ontario and Maritime fireballs, since they appeared before midnight and before dawn respectively. That suggests that they came from different directions in space relative to the Earth.
Meteor experts have a number of explanations for why there are so many reports of fireballs this year.
Beech said fireballs actually occur fairly regularly.
"Most of the time, they go unnoticed, believe it or not," he said.
He suggested that the location of the fireballs may have made a difference – they happened to be visible in highly populated parts of Canada.
CBC's national science correspondent, Bob McDonald, thinks the ability to quickly share reports with friends also plays a role.
"Everyone's got a webcam now on their phone, so we're seeing them more and their pictures are being posted online."
Timing plays a role
McDonald added that the fireballs tend to show up around 5 a.m. because that's the time of day when we're on the "front" of the Earth as it moves through space: "You're actually underneath the windshield as we run into them."
Hawkes thinks that the timing the fireballs, as well as the weather, probably increased the number of people who saw them.
He noted that the Maritime fireballs were visible during a period of clear skies and they appeared around 5 a.m., when many people are awake.
Hawkes, who used to be in charge of the Canadian fireball reporting network, noted that media reports of fireballs tend to increase the number of subsequent reports.
"It gives people the confidence and the interest to bother reporting," he said.
He added that the massive and spectacular Russian meteor that caused $33 million in damage and injured more than a thousand people in February 2013 likely also "sensitized" people to similar events.
"They're more apt to report bright events than they were three years ago."