Shoemaker-Levy 9

Shoemaker-Levy 9 is a comet that looks like a 21 piece necklace which collided with Jupiter in 1994. Astronomers observed multiple explosions in the planet's atmosphere and realised that the lines of craters observed by Voyager 1 on the moons of Jupiter Callisto and Ganymede were chains caused by the impact of comet fragments similar to those of Shoemaker-Levy 9 on their surface.