"If you want to see Ison with your own eyes, do it now," said a blog post on the website of NASA's Comet Ison observing campaign Friday. "We can not and do not guarantee that it will survive the next few weeks and become naked-eye visible in our night skies."
This week, the comet suddenly had an "outburst" — an event where it jumped dramatically in brightness — so that it may now already be visible with the naked eye in some places.
"It's naked eye if you're deep, deep in the country, the sky is pristine and clear and your eyes are fantastically good," said Robyn Foret, chair of the education and public outreach committee for the Royal Astronomical Society of Canada, in an interview from Calgary Friday.
"That said, with a good pair of binoculars on your deck, if you know where to look in the sky, you will see it."
Right now, astronomers recommend looking for the comet just above the star Spica in the constellation Virgo in the southeast sky, just before dawn.
Comets are often described as "cosmic snowballs," as they are made of frozen gas, rock and dust. When their orbits bring them close to a star such as the sun, they heat up, causing some of the gas and dust to spew from the surface, forming a "coma," or atmosphere, that makes them look like fuzzy balls in the sky.
As they get closer to a star and heat up even more, water ice and other heavier gases start to evaporate, making them look bigger and brighter. Sometimes, the coma may expand into a long, beautiful tail pointing away from the sun.
Not as bright as expected
Comet Ison is currently closer to the sun than the Earth is and is getting nearer the sun each day. The "sungrazer" — the name given to comets that pass very close to the sun, becoming very bright -— is expected to brush past our nearest star a distance of 1.6 million kilometres on Nov. 28. That distance is just a little bit bigger than the sun's own diameter.
When Ison was first detected near the orbit of Jupiter last year, astronomers predicted it might become one of the brightest comet ever seen. However, so far it has not brightened as much as expected over the course of its sunward journey.
As the comet approaches the sun, it is expected to keep brightening. But that doesn't mean it will be easier to see.
"Because it's moving closer to the sun, it moves closer and closer to where the sun is in the sky," Foret said.
While it is now visible a fair distance above the horizon before dawn, it will get lower and lower in the sky each day, until it is too close to the sun to view safely.
"If you accidentally look at the sun through a pair of binoculars, it can have lifelong negative effects on your vision," Foret warned.
'Handful more days'
NASA's Comet Ison Observing Campaign says you have "maybe a handful more days" to see the comet before it gets lost in the sun's glare.
If the comet survives its close encounter with the sun on Nov. 28, it will be at its brightest in the month of December, as it moves away from the sun and may be "spectacular" at that point, Foret predicts.
But comets sometimes break up into pieces as they get close to the sun, and Foret said it wouldn’t be surprising if Ison does just that.
In fact, NASA suggests it's possible that the comet's recent brightening may have been caused by the comet's solid core fragmenting.
"Given that ISON's nucleus is shrouded in such a tremendous volume of light-scattering dust and gas right now, it will be almost impossible to determine this for at least a few day," said the NASA blog post.
If the comet's core is already breaking up, it is unlikely that any piece will survive the comet's close approach to the sun, since a chunk would have to be at least 200 metres wide to survive, NASA says. It estimates that as of this week, the comet was estimated to be 500 metres to two kilometres in diameter.
That means those who want to see Comet Ison shouldn't wait.
Foret said comets are always worth looking for, even if you've seen a comet before, because they all look different, especially their tails.
"Sometimes, comets appear to have two tails; sometimes, they'll have what looks like a tail in front of comet."
He himself hasn't had a chance to see Ison yet.
"I'm in Calgary, it's that time of year," he said. "It's very nice and cloudy in the east the past few days. But we're hopeful we'll get a break."