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
Herschel Space Observatory had been delivered as a result of the collision of Comet Shoemaker–Levy 9 in 1994. On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet, Ceres, largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the "Herschel Space Observatory". The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids." On 29 April 2013, ESA announced that "Herschel" supply