The comet was discovered by chance only on September 20th this year by two Russian astronomers, Vitali Nevski and Artyom Novichonok who had booked time on the International Scientific Optical Network (ISON) at the Kislovodsk Observatory (on the northern slopes of the Caucuses). Having been clouded out for most of the night, they saw a brief opening in the cloud and snapped some survey images in Gemini and Cancer immediately before dawn.
Following analysis of the images, the unexpected object was quickly identified by computer which then permitted a reverse look through older images of the same region taken around the world. It was from these images that it was determined that the comet had first been photographed, but not noticed in December 2011.
Credit: Remanzacco Observatory
The multiple images also permitted a rough orbit to be calculated. And it was at that point that the collective jaws hit the floor.
The comet is on a near-parabolic path, suggesting it has made the long journey in from the Oort Cloud, a ring of rocky material left over from the formation of the Solar System around one light year from the Sun (that's about a quarter of the way to the nearest star). This also suggests that it is on it's first journey.
The pity is that clear viewing will largely be confined to the Northern Hemisphere as for them, it will rise before the Sun and set after the Sun sets, but in the Southern Hemisphere it will rise and set more closely aligned with the Sun. This isn't a conspiracy, it is purely related to the curvature of the Earth and how objects are geometrically aligned. We saw a similar effect with the recent Transit of Venus, where observers on various parts of the Earth saw the planet take somewhat different paths across the face of the Sun.
Orbital details are available at the NASA / JPL web site, but you'll need to type Ison into the search field to access the data.
Credit: NASA / JPL
Comet Ison will reach its closed to the Sun on November 28th and will be good for viewing (particularly in the Northern Hemisphere) until late January. Those with a telescope may wish to see the comet pass close by Mars in October 2013, where there is speculation that the Mast Cameras on the Curiosity Rover may be able to photograph it.
…and if you can't wait for the end of next year, there's a reasonable chance that Comet Pan-STARRS will put on a good show in March. Details to follow.