Since its discovery by Russian astronomers in Sept. 2012, comet ISON has become the subject of much speculation and excitement. ISON is a sun-grazing comet, which means its orbit will take it extremely close to the sun. If ISON survives its perilous journey through the inhospitable solar atmosphere in late Nov. 2013, it is anticipated that it will put on a rare and spectacular show.
It is thought that comet ISON began its journey from the oort cloud, a vast sphere of icy bodies encompassing the outer reaches of the Milky Way, around one million years ago. The most probable scenario is that ISON is part of a much larger comet which broke up at some point in the distant past, each fragment now following an almost identical trajectory to the parent comet. It is also thought that ISON is making its first circuit around the sun. In light of this theory, many astronomers believe that ISON may be related to the sun-grazing comet of 1680 informally known as “Kirch’s Comet.” The “Great Comet of 1680″ not only put on a magnificent show in the night sky, but was also visible during daylight hours.
The Hubble Space Telescope produced the first image of comet ISON in April 2013. At roughly 400 million miles from Earth, as it passed through Jupiter’s orbit, the sun’s effects on its icy surface were already becoming apparent as a hazy coma (head) of gas. As Hubble continues to track the comet, new images plot its progress, noting changes in its brightness, fragmentation and gas production rates. The data gathered by Hubble aids in the difficult task of predicting the details of ISON’s future development.
Due to a notable outburst, or increase in brightness, during November, Comet ISON became visible to the naked eye in the Northern hemisphere’s night sky. Veteran comet observer, John Bortle, stated that as the outburst is only in its early stages the comet may well brighten further. On Nov, 28, 2013 providing it doesn’t break up beforehand, comet ISON will reach perihelion, it’s closest point to the sun. While the comet won’t be visible for a few days as it completes its passage of the sun, the following days and possibly weeks are the most anticipated.
At any stage of its journey from here on, there is a possibility that ISON will burn out due to solar radiation, losing its nucleus, then quickly disappear from view. On the other hand, should it survive this most dangerous leg of its journey it may reappear from within the sun’s glare, quickly developing a bright, long tail that will be visible for many weeks.
The Northern hemisphere will provide the best view of the comet. Assuming the most favourable scenario, during early December it will be visible in the Eastern sky, just before sunrise and again after sunset. In mid December as ISON moves further away from the sun and closer to Earth, it will become visible throughout the night, passing closest to Earth on Dec. 26 and 27.
Viewing times in the Southern hemisphere are restricted to pre-perihelion, low in the East just before sunrise, unless the comet brightens sufficiently to be seen during daylight hours.
For those interested in observing comet ISON, a free app called Comet Watch is available which gives precise directions and updates every minute.