Comet ISON, formally known as C/2012 S1, was a sungrazing comet discovered on 21 September 2012 by Vitali Nevski (Виталий Невский, Vitebsk, Belarus) and Artyom Novichonok (Артём Новичонок, Kondopoga, Russia). The discovery was made using the 0.4-meter (16 in) reflector of the International Scientific Optical Network (ISON) 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 Comet ISON's nucleus was around 5 kilometers (3 mi) in diameter. Later estimates were that the nucleus was only about 2 kilometers (1 mi) in diameter. Mars Reconnaissance Orbiter (MRO) observations suggested the nucleus was smaller than 0.8 kilometers (0.5 mi) in diameter.
Comet ISON was at first suspected to have disintegrated on 28 November 2013 (the day of perihelion passage) from the Sun's heat and tidal forces. However, later that day CIOC (NASA Comet ISON Observing Campaign) members discovered a coma-like feature, suggesting a small fragment of it may have survived perihelion. On 29 November 2013, the coma dimmed to an apparent magnitude of 5. By the end of 30 November 2013, the coma had further faded to below naked-eye visibility at magnitude 7. On 1 December, the coma continued to fade even further as it finished traversing the Solar and Heliospheric Observatory's view. By 2 December 2013, the CIOC announced that Comet ISON had fully disintegrated, though NASA continues to investigate the possibility that an inactive fragment could have survived. The Hubble Space Telescope's wide-field camera will attempt to recover fragments of ISON on 19 December 2013.