C/2012 S1 (ISON) is a sungrazing comet discovered on 21 September 2012 by Vitali Nevski and Artyom Novichonok. The comet will come to perihelion (closest approach to the Sun) on 28 November 2013 at a distance of 0.012 AU (1,800,000 km; 1,100,000 mi) from the center point of the Sun.
Between 5 June and 29 August 2013, comet ISON will have an elongation less than 30 degrees from the Sun. The Spitzer Space Telescope may observe the comet on June 13 and help estimate carbon dioxide production. Around September 2013, it should become bright enough to be visible through small telescopes or binoculars. But the comet is not expected to reach the naked eye magnitude of 6 until November.Assuming it survives perihelion passage, it should be visible to the naked eye until early January 2014.
In October, the comet will pass through the constellation Leo, passing near Leo’s brightest star Regulus and then passing near Mars in the night sky, and these brighter objects might make the comet easier to locate. STEREO should be able to view ISON around 10 October. In November, when the comet is brighter, it will sweep past another bright star in our sky, Spica in the constellation Virgo, and another planet, Saturn. SOHO will be able to view ISON starting 27 November. Around the time the comet reaches its perihelion on 28 November, it may become extremely bright if it remains intact, probably reaching a negative magnitude. It may briefly become brighter than the full Moon.
It 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, the comet will be growing dimmer, but, assuming that it remains intact, it will be visible from both hemispheres of Earth, possibly with a long tail.