via PC Mag:
A comet recently discovered by two Russian astronomers in CCD images has the potential to become very bright late next year, by virtue of an extremely close passage by the Sun followed by a somewhat close approach to the Earth.
Unlike the two most recent bright comets, last year’s Comet Lovejoy and 2007’s Comet McNaught, prospects for the new comet—officially known as C/2012 S1 (ISON)—strongly favor observers in the northern hemisphere. Predicting a comet’s behavior, though, can be tricky, and highly touted comets have been known to fizzle. As noted comet discoverer David Levy once quipped, “Comets are like cats. They both have tails—and they do exactly what they want.”
Artyom Novichonok and Vitaly Nevski found the comet in CCD images taken on Sept. 21 with the 0.4-meter telescope of the International Scientific Optical Network (ISON) in Kislovodsk, Russia. The comet—barely distinguishable from a star and more than 10,000 times fainter than the faintest star visible to the unaided eye—is still far from the Sun, but is likely to brighten dramatically as it approaches our star.
The initial orbit announced by the Minor Planet Center on Monday has the comet passing just 1.16 million miles from the Sun next Nov. 28. Around that time, if the more optimistic predictions are accurate, it could become a very brilliant object, perhaps even visible in daylight. After making a hairpin turn around the Sun, the comet will head north, becoming visible in the morning and evening sky and likely growing a long tail. It will pass within about 40 million miles of Earth, and could remain visible to the unaided eye through January.
Not all comets live up to their rosiest projections—although Comets McNaught and Lovejoy exceeded expectations, others—such as 1973’s Comet Kohoutek—fell victim to hype. Kohoutek put on a respectable showing, shining as brightly as Jupiter when seen by Skylab astronauts at its closest point to the Sun, and then emerging into the evening sky where, although faded, it was visible to the naked eye. (I remember it fondly, as it was the first comet I ever observed.) However, it couldn’t hope to live up to the overly optimistic predictions made by some astronomers and inflated in the press, where it was widely heralded as “Comet of the Century.” Last year’s Comet Elenin, which was predicted to become easily visible to the naked eye, instead broke apart and disappeared. There are already headlines touting Comet ISON’s (still hypothetical) extreme brilliance – one has it “brighter than the Full Moon” – but it’s far too early to know how it will behave.
The main uncertainty in Comet ISON’s prospects is whether this is a new comet, making its first appearance from the Oort cloud—a spherical shell of comets that surrounds the Sun at a great distance—or a comet that regularly—if infrequently—returns to the inner solar system (with an orbital period of perhaps millennia). Dynamically new comets tend to be brighter when farther from the Sun, as volatile ices in their surface layers may rapidly evaporate, and the brightening may slow or even cease when they are burned off. A comet that’s already made close passages to the Sun is likely to brighten at a steadier and more consistent rate as it approaches the Sun. If the latter is the case for Comet ISON, an intriguing possibility has been suggested.
Comet researcher John Bortle has pointed out similarities between the orbits of this comet and the Great Comet of 1680, a brilliant, long-tailed object that graced northern skies in the winter of that year. The frequency of its passages through the inner solar system is not known accurately, but recent estimates suggest that its orbital period may be as great as 10,000 years. If that renowned object and Comet ISON are related—if they were both once one object that fragmented sometime in the distant past—it bodes well for the latter’s prospects. The 1680 comet passed closer to the Sun than Comet ISON will, and Comet Lovejoy—which few astronomers expected would even survive its solar encounter— even closer. The intense solar heat could spur the comet into extreme activity and brilliance and release material to form an immense tail. It could, however, also vaporize the comet completely, putting an abrupt end to the show. Whichever the case, 2013 should be an exciting year for skywatchers.