The comet ISON has been on stargazers' radar since late last year when it was seen hurtling towards the sun and showing every sign of passing very close to the centre of the solar system.
Those predictions will come to pass today at around 1 p.m. or 2 p.m. eastern time, when ISON is expected to be just 1.2 million kilometres from the sun.
Astronomers say the intense heat from its brush with solar energy may cause ISON to disintegrate in mid course. If it survives intact and begins its trajectory away from the sun, however, experts said sky watchers may have a lot to look forward to in the coming weeks.
Paul Delaney, professor of physics and astronomy at Toronto's York University, said ISON's size and proximity to the sun could usher in a spectacular light show for those willing to wake up early and examine the pre-dawn skies.
Comets are little more than vast chunks of rock that fall towards earth from the outer solar system. Delaney said that within the centre of those rock masses is a nucleus of volatile material that reacts to heat.
While several dozen comets travel near Earth every year, Delaney said many of them don't come within close range of the sun. Those that do, he added, are often too small to become visible to the naked eye.
ISON appears to be a different beast, he said, explaining that a comet becomes visible when the material contained in its nucleus becomes heated and begins emitting gas.
"The comet becomes surrounded by this sort of gas cloud, and this gas cloud can end up being literally tens of thousands of kilometres in diameter," Delaney said in a telephone interview.
"It can trail material behind it for literally millions of kilometres, meaning that we've now got this object which has got a very large surface area to reflect light. And that's what gives us a really terrific show here on Earth."
Comets that pass close to the sun — often known as sun-grazing comets — are often too small to have much impact for stargazers, he said.
But ISON itself has a nucleus of approximately two kilometres wide, making it comparatively large for one with such close proximity to the sun.
ISON's relative size means it's likely to produce a similarly large gas cloud, Delaney said.
"As it rounds the sun and gets heated to nearly 2,700 degrees Celsius — it will really be roasted — all of the volatile materials trapped within have the potential of outgasing," he said. "And if that happens the comet gets bigger, the tail gets longer, and the comet gets that much brighter."
Delaney said ISON's light show won't begin for several days after it's brush with the sun — assuming it hasn't been incinerated altogether.
Canadians hoping for a glimpse of ISON can start looking for it on Saturday or Sunday morning if they're willing to train their eyes on the sky about half an hour before sunrise, he said.
ISON is likely to appear in the east as a particularly bright point of light with a trailing tail, he said, adding its visibility will improve throughout next week.
"Over the ensuing few days, the comet will climb higher and further away from the sun and into darker, background skies. And then, if ISON is going to live up to expectations, you'll see a really good show."