You're likely referring to the Shoemaker-Levy 9 comet impact event in 1994. The fragments of this comet, which had broken apart due to Jupiter's gravity, hit the planet at extremely high speeds, estimated to be around 130,000 miles per hour (210,000 kilometers per hour).

This is roughly 60 times the speed of sound and more than five times faster than the speed of a spacecraft leaving Earth orbit.

The impact created massive fireballs, expanding shockwaves, and left lasting scars on Jupiter's atmosphere.