Comet of the year
C/2012 S1, also known as Comet ISON or Comet Nevski–Novichonok, is a sungrazing comet discovered on 21 September 2012 by Vitali Nevski and Artyom Novichonok .The discovery was made using the 0.4-meter (16 in) reflector of the International Scientific Optical Network near Kislovodsk, Russia. Data processing was carried out by automated asteroid-discovery program CoLiTec. Precovery images by the Mount Lemmon Survey from 28 December 2011 and by Pan-STARRS from 28 January 2012 were quickly located. Follow-up observations were made on 22 September by a team from Remanzacco Observatory in Italy using the iTelescope network. The discovery was announced by the Minor Planet Center on 24 September. Observations by Swift in January 2013 suggested that C/2012 S1's nucleus was around 5 kilometers (3 mi) in diameter.
The comet's formal designation is C/2012 S1. The "C" indicates that it is non-periodic, followed by the year of discovery. The "S" represents the half-month of discovery—in the case of C/2012 S1, the second half of September—and the number "1" shows that this was the first comet found in that half month. The comet is named "(ISON)" after the organization where its discovery was made, the Russia-based International Scientific Optical Network. The initial report of the object to the Central Bureau for Astronomical Telegrams identified the object as an asteroid, and it was listed on the Near Earth Objects Confirmation Page. Follow-up observations by independent teams were the first to report cometary features. Therefore, under the International Astronomical Union's comet-naming guidelines, the comet is named after the team that discovered it, rather than the individual discoverers.
On 1 October 2013, C/2012 S1
passed within 0.07 AU (10,000,000 km;
6,500,000 mi) of Mars. Between 29 September and 2 October, the Mars
Reconnaissance Orbiter (MRO) detected C/2012 S1.The twin STEREO
spacecraft began detecting C/2012 S1 in the second week of October.
October 2013 images of C/2012 S1 displayed a greenish tint, probably
attributable to the release of cyanogen and diatomic carbon. On 31 October 2013, C/2012 S1 was detected with 10×50 binoculars.
On 14 November 2013, C/2012 S1 was reported to be visible to the naked eye by experienced observers located at dark sites. ISON has an appearance similar to comet that is also visible to the naked eye. C/2012 S1 was not expected to reach the naked eye magnitude of 6 until mid-November, and may not be observable by the general public until it brightens to about magnitude 4.[ On 17–18 November, when C/2012 S1 was brighter and much closer to the morning twilight, it passed the bright star Spica in the constellation Virgo. But due to the full moon and glow of twilight, C/2012 S1 has not become bright enough to be seen without optical aid by the general public.
On 22 November C/2012 S1 started to drop below Mercury in the bright twilight. SOHO will be able to view C/2012 S1 starting 27 November. Around the time it reaches perihelion on 28 November, it may become extremely bright if it remains intact, probably reaching a negative magnitude. 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 media sources predicted it might become brighter than the full Moon, but based on more recent observations, it is only expected to reach around apparent magnitude −3 to −5, about the same brightness as Venus. In comparison, the brightest comet since 1935 was Comet Ikeya–Seki in 1965 at magnitude −10, which was much brighter than Venus.
C/2012 S1 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, C/2012 S1 will grow dimmer, but, assuming that it remains
intact, it will be visible from both hemispheres of Earth, possibly with
a long tail.C/2012 S1 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 8 January. Assuming it survives perihelion passage, ISON may be visible to the naked eye until early January 2014.
