Comet ISON (C/2012 S1) was discovered in September 2012 by Vitali Nevski and Artyom Novichonok of the International Scientific Optical Network observatory near Kislovodsk in the North Caucasus region of Russia. It is currently on a parabolic trajectory that will bring it to perihelion (the closest point of its trajectory to the Sun) within 2.7 Solar Radii of the Sun on 28 November 2013, before departing the inner Solar System again. It is not thought that this trajectory is, strictly speaking an orbit; it appears to have been disrupted from an orbit in the outermost part of the Oort Cloud, around seven million times as far from the Sun as the Earth, by an encounter with some object, and to be entering the inner Solar System for the first time, and it is by no means clear that it will survive its close encounter with the Sun, or that, if it does, it will remain in orbit about the Sun rather than being thrown completely out of the Solar System. Weather-or-not ISON (C/2012 S1) will survive perihelion is of great interest to astronomers, since objects from the outer Oort Cloud are seldom available for close observation, and if it does survive then its outward path should be easily observable from a number of Earth and space-based telescopes.
The calculated orbit of ISON (C/2012 S1). JPL Small Body Database Browser.
In a paper published on the arXiv online database at Cornell University Library on 9 September 2013 and submitted to the Astrophysical Journal Letters, Matthew Knight of the Lowell Observatory in Flagstaff, Arizona and the Applied Physics Laboratory at Johns Hopkins University and Kevin Walsh of the Southwest Research Institute attempt to calculate the likely fate of ISON (C/2012 S1) building a model based upon observations of the Kreutz Family of sungrazing comets.
The Kreutz Family of comets are thought to have a common origin, all following similar pathways, with orbital periods of 500-1000 years, thought to have diverged when a parent body broke up at some point in the remote past. All members of the family are thought to have made several passes through the inner Solar System, and as such they are likely to have rather different physical and chemical properties to ISON (C/2012 S1). Their paths also take them somewhat closer to the Sun than ISON (C/2012 S1). However they are the best available model for the possible fate of ISON (C/2012 S1).
Kreutz Family comets smaller than 200 m in diameter typically sublimate away (turn directly from a solid to a gas - liquids do not form in a vacuum) on their passage past the Sun. Comets larger than 200 m in diameter but less than 1 km do not simply sublimate away, but may break up into a number of smaller pieces as they pass the Sun, and these pieces commonly do sublimate away. Comets larger than 1 km usually survive perihelion, and even if they do break up, typically do so into pieces large enough to survive as viable comets in themselves.
Comet ISON (C/2012 S1) is though to be between 400 m and 2 km in diameter. This suggests that it will not simply sublimate away as it passes the Sun, but that there is a distinct chance that it may break up at perihelion. Knight and Walsh calculate that the comet will probably break up if its rotation is prograde (in the same direction as the body which it is orbiting, in this case the Sun) and its orbital period less than 9 hours, but that if it is rotation is retrograde (in the opposite direction to the Sun) or its rotation is prograde with an period of more than 9 hours then the comet will probably survive. They further estimate that there is a 30% chance that ISON (C/2012 S1) has a prograde rotation with a period of less than 9 hours.
