A highly stark photograph of ISON by MESSENGER
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).[4] The discovery was made using the 0.4-meter (16 in) reflector of the International Scientific Optical Network (ISON) near Kislovodsk, Russia.[1] Data processing was carried out by automated asteroid-discovery program CoLiTec.[5] Precovery images by the Mount Lemmon Survey from 28 December 2011 and by Pan-STARRS from 28 January 2012 were quickly located.[6] Follow-up observations were made on 22 September by a team from Remanzacco Observatory in Italy using the iTelescope network.[1][7] The discovery was announced by the Minor Planet Center on 24 September.[6] Observations by Swift in January 2013 suggested that Comet ISON's nucleus was around 5 kilometers (3 mi) in diameter.[8] Later estimates were that the nucleus was only about 2 kilometers (1 mi) in diameter.[9] Mars Reconnaissance Orbiter (MRO) observations suggested the nucleus was smaller than 0.8 kilometers (0.5 mi) in diameter.[10]
Comet ISON was at first suspected to have disintegrated on 28 November 2013 (the day of perihelion passage)[11][12] 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.[11][12][13][14][15] On 29 November 2013, the coma dimmed to an apparent magnitude of 5.[16] By the end of 30 November 2013, the coma had further faded to below naked-eye visibility at magnitude 7.[17] On 1 December, the coma continued to fade even further as it finished traversing the Solar and Heliospheric Observatory's view.[18][19] By 2 December 2013, the CIOC announced that Comet ISON had fully disintegrated,[20] though NASA continues to investigate the possibility that an inactive fragment could have survived.[21] The Hubble Space Telescope's wide-field camera will attempt to recover fragments of ISON on 19 December 2013.[22]