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Comet C/2012 S1 (ISON) was discovered on September 21, 2012 by astronomers Artyom Novichonok and Vitali Nevski, using a 16-inch telescope that is part of the International Scientific Optical Network (“ISON”), and after which the comet was named.
Immediately you may ask why the comet was given the name “ISON” and not “Novichonok-Nevski”, and that's a very valid question. Comets discovered as part of so-called sky surveys (or discovered by satellites, like SOHO) are named after the survey program or satellite, and not the individuals who identified the comet in the images taken as part of that survey. (There is a gray area here, in my opinion, but this is not the place to comment on that.) Within a couple of days of Novichonok and Nevski reporting their find, other observatories had successfully located the comet, including in some images that actually predated the discovery date by over nine months. (The latter, known as precovery, is quite common -- faint comets can be hard to spot unless you know exactly where in your data to look.) By September 24, 2012, Comet ISON had received an official designation of C/2012 S1 (ISON), and a preliminary orbit determination that brought it rather close to the Sun in November of 2013.A SOHO/LASCO C2 image. The black circle represents a distance of 3 Solar Radii from the Sun (the white circle).
At time of discovery, Comet ISON was very faint with a brightness of only magnitude 18 or so. The faintest star you can see with your eye, assuming very clear skies, is about magnitude 7; Comet ISON was approximately twenty-five thousand times fainter than this, testing the limits of even professional telescopes in very clear locations. Not only was ISON very faint, it was also very far away: over 6 AU from the Sun (1 AU = One Astronomical Unit, or the mean distance between the Sun and the Earth) and outside of the orbit of Jupiter. However, given the available precovery data, officials at the Minor Planet Center in Cambridge, Massachusetts, were confident of their orbit for Comet ISON that brings it to perihelion (its closest point to the Sun) on 28th November, 2013 at a distance of 0.012AU -- only around 3 solar radii, which is extremely close to the Sun, and small enough of a distance to qualify it as a Sungrazing Comet. According to its calculated orbit, in December 2013 Comet ISON is set pass approximately 0.42AU from Earth. That's close enough to (potentially) put on a good show, but absolutely nowhere near close enough to be concerned for our safety!
Where did it come from?
Before we get into current and future prospects for ISON, we want to take a moment to mention something of the far more distant history of this comet. It was discovered in September 2012, of course, but is that the first time we have seen it? Probably, but maybe not. The current orbit for this comet places it on a trajectory that, assuming it survives passage by the Sun, ejects it from the solar system never to return. So for this and several other reasons, it seems probable that this may be ISON’s first and only visit to our solar system. But perhaps not...
In November of 1680, German astronomer Gottfried Kirch made history by making the first ever comet discovery by telescope. The comet he found, formally known as C/1680 V1, but informally as “Kirch’s Comet” or “The Great Comet of 1680” became more-or-less as the latter suggests: a great comet! In late November of 1680 it blazed past Earth at just 0.42AU (sound familiar...?), and grazed the Sun at 0.0062AU in December of that year, allegedly visible in daylight at the time. Paintings from that era show a brilliant comet with a long, thin tail arcing across the European night skies.
The time of year (November/December) is just coincidence, but Comet ISON does have orbital properties that are remarkably similar to that of the 1680 comet. They are certainly not the same object as it would be obvious to us if either object had a ~300yr orbit, but is it possible that they are at least related? Well, it is conceivable that they once were part of a single, larger sungrazing comet that broke apart many millennia ago, and due to gravitational interactions and long periods of time, one piece just ended up well ahead of the other and on a slightly different trajectory. After all, this is exactly what we see with the vast majority of the Sungrazing and near-Sun comets that we discover as part of the Sungrazer project: tens or hundreds of tiny fragments resulting from the break-up of larger parent comets near the Sun.
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