Ever since Comet ISON (C/2012 S1) became breaking news as the ‘comet of the century’ nearly a year ago, there has been plenty of speculation by the media on the current state of the comet.
Comet ISON passed nearly 0.0724 AU (6,730,000 mi) from Mars on October 1, about six times closer than it will ever come to Earth. The Mars flyby came at a key time in the comet's journey. It had crossed the frost line, a place just outside the orbit of Mars where solar heating is intense enough to vaporize frozen water. The volatiles in a comet are 80% to 90% water ice. In early September almost all the water was still frozen, and the outgassing we saw in ISON was driven by carbon dioxide and other lesser constituents. But when ISON crossed the frost line, the whole comet erupted in geysers of gas.
I encourage everyone to check out this recent image. This link is provided by NASA Comet ISON Observing Campaign (CIOC) and is filled with good information. The latest data strongly indicates the comet should gradually increase brightness and be a binocular comet by the middle of November.
Comet ISON will come to perihelion (closest approach to the Sun) on November 28, 2013 at a distance of 0.012 AU or 1,100,000 miles from the center point of the Sun. It will pass approximately 680,000 miles above the Sun's surface. At this point, the comet will either break apart by the sun’s intense energy or survive the journey.
Predicting the brightness of a comet is difficult, especially one that will pass so close to the Sun and be affected by the forward scattering of light. Originally researchers speculated ISON might become brighter than the full Moon. Based on more recent observations it is now only expected to reach around apparent magnitude −3 to −5, about the same brightness as Venus. ISON is expected to be brightest around the time it is closest to the Sun; however, it may be less than 1° from the Sun at its closest, making it difficult to see against the Sun's glare.
In December, Comet ISON will be growing more dim, but assuming that it remains intact, the comet will be visible from both hemispheres of Earth, possibly with a long tail. ISON will pass about 0.429 AU (39,900,000 mi) from Earth on 26 December 2013. It will be well placed for observers in the northern hemisphere during mid to late December 2013. After perihelion, it will move north on the celestial sphere, passing within two degrees of Polaris on January 8, 2014.
Note that if the comet turns out to be bright in middle of November, OMSI will attempt an impromptu Comet ISON Viewing Party on Water Ave.Stay tuned for further information!