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
Erich Meyer Erich Meyer Erich Meyer (born August 6, 1951) is an Austrian engineer, amateur astronomer and discoverer of asteroids. Between 1996 and 1999, using the private observatory Meyer/Obermair in Davidschlag (municipality Kirchschlag bei Linz, Austria), he discovered a total of 21 asteroids. Seven of these asteroids were discovered together with his colleague Erwin Obermair and one together with Herbert Raab. Furthermore, he was involved in six other discoveries of asteroids between 1996 and 2005, which were assigned as site discoveries to the observatory Davidschlag by the International Astronomical Union. Meyer's most important observations include precisie astrometry of the comet Shoemaker–Levy 9,