A star appeared on the seventh of the Calends of May, on Tuesday after Little Easter, than whose light the brilliance or light of the moon was not greater; and it was visible to all in this manner till the end of four nights afterwards.
– The Annals of the Four Masters, 1066
ALMOST A THOUSAND years ago, Irish annalists recorded the appearance of a comet, which would, hundreds of years later, be discovered by Edmond Halley and named after him. We know it was Halley’s Comet that appeared for four nights over Ireland because we know that it takes 75-76 years for it to orbit the Sun, and so we’re able to trace the appearances of it back through time.
Humans have been observing the movements of stars and planets for thousands of years, and now we have enough knowledge of their motions that we can predict with great accuracy when and where known objects will appear, or have appeared, in the sky for millennia into the future and past.
There are other comets, however, that do not orbit the Sun periodically like Halley’s Comet. It’s believed that most comets originate from a vast cloud, known as the Oort Cloud, containing billions of comets that surrounds our Solar System.
This cloud is huge, and while the exact size is disputed, it probably has a diameter of about two light-years (in kilometres, that’s just under two with 13 zeros after it!), with some estimates putting it closer to Alpha Centauri, one of the closest stars to us, than to the Sun itself.
From the Oort Cloud, comets are nudged towards the inner solar system by colliding with other objects, or by getting a ‘push’ from a passing star. Some of these ‘non-orbiting’ comets then spend thousands of years slowly travelling through space towards the Sun, before swinging around it and being ejected off into space, never to be seen again.
One of these comets was discovered in September 2012 by Vitali Nevski and Artyom Novichonok, and is commonly known as Comet ISON.
It probably has never passed through the inner solar system before, but it certainly won’t do so ever again. It has received a lot of attention so far, with some reports having predicted it will be brighter than the Full Moon.
Comet ISON is currently making its way towards the Sun, and very recently passed the orbit of Earth. It’s very faint at the moment, but will it flare up to become bright like Halley’s Comet in 1066, or like the more recent Comet Hale-Bopp in 1997?
The short answer is: maybe. For the long answer, we have to consider a few things about Comet ISON.
First of all, what is a comet made of? The nucleus of a comet is often described as a ‘dirty snowball’, but is in fact made of bits of rock, ice, dust, and various frozen gases. When the nucleus heats up, the ice starts to melt, and a debris and vapours are ejected out into space, giving it its characteristic comet-like appearance.
While the nucleus may only be a few tens of kilometres across, the tail can extend for tens or hundreds of millions of kilometres, and the coma of material coming off the nucleus can sometimes be bigger than the Sun itself.
Because comets are largely frozen objects, we also need to consider how close to the Sun Comet ISON will get. After all, if it gets too close, it will just break up, evaporate away, and disappear. Too far, and it won’t eject enough material to become very bright. But what’s the ‘just right’ distance?
Unfortunately, that’s very hard to tell. It all depends on how much ice and rocky materials are actually in the comet, as well as its internal structure. Assuming it survives the trip, Comet ISON will pass the Sun at a distance of 1.86 million km on November 28th. That’s so close that the comet is known as a ‘sungrazer’. Most sungrazers simply plunge into the Sun or disintegrate.
So will we see it? If Comet ISON survives its close pass to the Sun and doesn’t disintegrate, then yes, we should be able to see it. How bright will it be? Only time will tell.
On November 28th when it is closest to the Sun, it will be too close to it to see properly from Earth as the Sun’s glare will be too bright.
As it moves away from the Sun in December, however, we should be able to see something. Initial light curve observations suggested that Comet ISON may become even brighter than the Full Moon, but comets are quite unpredictable, and this increase in brightness has slowed down, meaning it may become as bright as the planet Venus (which is still quite bright and spectacular).
It’s very hard to predict what a comet will do. If one day a comet looks like it’s going to be very faint, the next day it could turn out that it’ll be very bright, and vice-versa. A general rule of thumb astronomers use when it comes to comets is very simply “wait and see”.
As an optimist, though, I’m very hopeful that Comet ISON will give us some stunning views in the night sky during the run up to Christmas. So keep your fingers crossed for clear skies and keep looking up!
Conor Farrell is an avid science enthusiast and studied physics with astronomy at Dublin City University. He now works with Astronomy Ireland to promote all things space-related to a wider audience. In his spare time he writes about science and current affairs, and can be followed on Twitter at @conorsthoughts.
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