Sky and Telescope – , the geometry could make C/2012 S1 a “dream comet,” as one eager skywatcher has commented, because it will swing just 40 million miles (0.4 astronomical unit) from Earth a few weeks after perihelion, when it will be high in moonless, northern skies after sunset. Initial predictions by the IAU’s Minor Planet Center suggest that Comet ISON could peak at magnitude –10 or brighter at perihelion (when it will be just 1° from the Sun), and that it could remain visible to the unaided eye from early November to the first weeks of 2014.
Brighter than the Full Moon but far smaller
Astronomy Now -With a perihelion passage of less than two million kilometres from the Sun on 28 November 2013, current predictions are of an object that will dazzle the eye at up to magnitude —16. That’s far brighter than the full Moon. If predictions hold true then C/2012 S1 will certainly be one of the greatest comets in human history, far outshining the memorable Comet Hale-Bopp of 1997 and very likely to outdo the long-awaited Comet Pan-STARRS (C/2011 L4) which is set to stun in March 2013.
Skirting our star means that, to viewers on Earth, the comet will appear close to the horizon and to the sun’s glare, making it difficult to see at first. ISON will fade but become easier to spot as it heads back towards the outer solar system. By 9 December it should be about as bright as Polaris, the North Star, according to Remanzacco Observatory astronomers. ISON should continue to be visible to the unaided eye until mid-January 2014.
As plotted by the Jet Propulsion Laboratory’s “Horizons” system, Comet ISON (C/2012 S1) will pass very close to the Sun — but not crash into it — in late November 2013. At that time it might be at least magnitude –10 — bright enough to be spotted despite very strong solar glare. NASA / JPL