Comet Shoemaker–Levy 9 rarity: one post-impact study estimated that comets in diameter impact the planet once in approximately 500 years and those in diameter do so just once in every 6,000 years. There is very strong evidence that comets have previously been fragmented and collided with Jupiter and its satellites. During the Voyager missions to the planet, planetary scientists identified 13 crater chains on Callisto and three on Ganymede, the origin of which was initially a mystery. Crater chains seen on the Moon often radiate from large craters, and are thought to be caused by secondary impacts of the original ejecta, but the
Comet ISON 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, Comet ISON was named after the team that discovered it, rather than the individual discoverers. After it was discovered in 2012, some media sources called Comet ISON the "Comet of the Century" and speculated that it might outshine the full Moon. An "Astronomy Now" columnist wrote in September 2012 that "if predictions hold true then Comet ISON will