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 Shoemaker–Levy 9 Despite published predictions, astronomers had not expected to see the fireballs from the impacts and did not have any idea in advance how visible the other atmospheric effects of the impacts would be from Earth. Observers soon saw a huge dark spot after the first impact. The spot was visible even in very small telescopes, and was about (one Earth radius) across. This and subsequent dark spots were thought to have been caused by debris from the impacts, and were markedly asymmetric, forming crescent shapes in front of the direction of impact. Over the next six days, 21 distinct impacts